Top 50 Java J2EE interview questions

java j2ee interview questions and answers for experienced and java j2ee interview companion by arulkumaran and java j2ee interview questions ebook
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1 Learn Java/J2EE core concepts and design/coding issues With Java/J2EE Job Interview Companion By K.Arulkumaran Technical Reviewers Craig Malone Lara D’Albreo Stuart Watson Acknowledgements A. Sivayini R.Kumaraswamipillai Cover Design K. Arulkumaran A.Sivayini 2 Java/J2EE Job Interview Companion Copy Right 2005 K.Arulkumaran The author has made every effort in the preparation of this book to ensure the accuracy of the information. However, information in this book is sold without warranty either express or implied. The author will not be held liable for any damages caused or alleged to be caused either directly or indirectly by this book. 3 Outline SECTION DESCRIPTION What this book will do for you? Motivation for this book Key Areas index SECTION 1 Interview questions and answers on: Java ƒ Language Fundamentals ƒ Swing ƒ Applet ƒ Performance and memory Leaks. ƒ Personal SECTION 2 Interview questions and answers on: Enterprise Java ƒ J2EE ƒ Servlet ƒ JSP ƒ JDBC ƒ JNDI ƒ RMI ƒ EJB ƒ JMS ƒ XML ƒ SQL, Database tuning and O/R mapping ƒ RUP & UML ƒ Struts ƒ Web and Application servers. ƒ Best practices and performance considerations. ƒ Testing and deployment. ƒ Personal SECTION 3 Putting it all together section. How would you go about…? 1. How would you go about documenting your Java/J2EE application? 2. How would you go about designing a Java/J2EE application? 3. How would you go about identifying performance problems and/or memory leaks in your Java application? 4. How would you go about minimising memory leaks in your Java/J2EE application? 5. How would you go about improving performance of your Java/J2EE application? 6. How would you go about identifying any potential thread-safety issues in your Java/J2EE application? 7. How would you go about identifying any potential transactional issues in your Java/J2EE application? 8. How would you go about applying the Object Oriented (OO) design concepts in your Java/J2EE 4 application? 9. How would you go about applying the UML diagrams in your Java/J2EE project? 10. How would you go about describing the software development processes you are familiar with? 11. How would you go about applying the design patterns in your Java/J2EE application? 12. How would you go about determining the enterprise security requirements for your Java/J2EE application? 13. How would you go about describing the open source projects like JUnit (unit testing), Ant (build tool), CVS (version control system) and log4J (logging tool) which are integral part of most Java/J2EE projects? 14. How would you go about describing Web services? SECTION 4 Emerging Technologies/Frameworks ƒ Test Driven Development (TDD). ƒ Aspect Oriented Programming (AOP). ƒ Inversion of Control (IOC) (Also known as Dependency Injection). ƒ Annotations or attributes based programming (xdoclet etc). ƒ Spring framework. ƒ Hibernate framework. ƒ EJB 3.0. ƒ JavaServer Faces (JSF) framework. SECTION 5 Sample interview questions … ƒ Java ƒ Web Components ƒ Enterprise ƒ Design ƒ General GLOSSARY OF TERMS RESOURCES INDEX 5 Table of contents Outline_________________________________________________________________________________________ 3 Table of contents ________________________________________________________________________________ 5 What this book will do for you? ____________________________________________________________________ 7 Motivation for this book __________________________________________________________________________ 8 Key Areas Index ________________________________________________________________________________ 10 Java – Interview questions & answers _____________________________________________________________ 11 Java – Language Fundamentals ____________________________________________________________________________ 12 Java – Swing ____________________________________________________________________________________________ 44 Java – Applet____________________________________________________________________________________________ 48 Java – Performance and Memory leaks ______________________________________________________________________ 50 Java – Personal__________________________________________________________________________________________ 53 Java – Key Points ________________________________________________________________________________________ 56 Enterprise Java – Interview questions & answers ____________________________________________________ 59 Enterprise - J2EE ________________________________________________________________________________________ 60 Enterprise - Servlet_______________________________________________________________________________________ 69 Enterprise - JSP _________________________________________________________________________________________ 77 Enterprise - JDBC ________________________________________________________________________________________ 83 Enterprise – JNDI & LDAP _________________________________________________________________________________ 87 Enterprise - RMI _________________________________________________________________________________________ 90 Enterprise – EJB 2.x ______________________________________________________________________________________ 94 Enterprise - JMS ________________________________________________________________________________________ 110 Enterprise - XML ________________________________________________________________________________________ 114 Enterprise – SQL, Tuning and O/R mapping _________________________________________________________________ 119 Enterprise - RUP & UML__________________________________________________________________________________ 126 Enterprise - Struts_______________________________________________________________________________________ 133 Enterprise - Web and Application servers ___________________________________________________________________ 137 Enterprise - Best practices and performance considerations ___________________________________________________ 139 Enterprise – Logging, testing and deployment _______________________________________________________________ 141 Enterprise - Personal ____________________________________________________________________________________ 144 Enterprise – Software development process_________________________________________________________________ 144 Enterprise – Key Points __________________________________________________________________________________ 146 How would you go about…?_____________________________________________________________________ 151 Q 01: How would you go about documenting your Java/J2EE application? ____________________________________ 152 Q 02: How would you go about designing a Java/J2EE application? __________________________________________ 153 Q 03: How would you go about identifying performance and/or memory issues in your Java/J2EE application? _____ 156 Q 04: How would you go about minimising memory leaks in your Java/J2EE application? _______________________ 157 Q 05: How would you go about improving performance in your Java/J2EE application? _________________________ 157 Q 06: How would you go about identifying any potential thread-safety issues in your Java/J2EE application?_______ 158 Q 07: How would you go about identifying any potential transactional issues in your Java/J2EE application?_______ 159 Q 08: How would you go about applying the Object Oriented (OO) design concepts in your Java/J2EE application? _ 160 Q 09: How would you go about applying the UML diagrams in your Java/J2EE project? _________________________ 162 6 Q 10: How would you go about describing the software development processes you are familiar with? ____________163 Q 11: How would you go about applying the design patterns in your Java/J2EE application? _____________________165 Q 12: How would you go about determining the enterprise security requirements for yor Java/J2EE application? ____194 Q 13: How would you go about describing the open source projects like JUnit (unit testing), Ant (build tool), CVS (version control system) and log4J (logging tool) which are integral part of most Java/J2EE projects? ________________199 Q 14: How would you go about describing Web services? __________________________________________________206 Emerging Technologies/Frameworks… ____________________________________________________________210 Q 01: What is Test Driven Development (TDD)? ___________________________________________________________211 Q 02: What is the point of Test Driven Development (TDD)? _________________________________________________211 Q 03: What is aspect oriented programming? Explain AOP?_________________________________________________212 Q 04: What are the differences between OOP and AOP? ____________________________________________________214 Q 05: What are the benefits of AOP?_____________________________________________________________________214 Q 06: What is attribute or annotation oriented programming?________________________________________________215 Q 07: What are the pros and cons of annotations over XML based deployment descriptors?______________________215 Q 08: What is XDoclet? ________________________________________________________________________________216 Q 09: What is inversion of control (IOC) (also known as dependency injection)? ________________________________216 Q 10: What are the different types of dependency injections?________________________________________________217 Q 11: What are the benefits of IOC (aka Dependency Injection)? _____________________________________________217 Q 12: What is the difference between a service locator pattern and an inversion of control pattern? _______________217 Q 13: Why dependency injection is more elegant than a JNDI lookup to decouple client and the service? ___________218 Q 14: Explain Object-to-Relational (O/R) mapping? ________________________________________________________218 Q 15: Give an overview of hibernate framework? __________________________________________________________218 Q 16: Explain some of the pitfalls of Hibernate and explain how to avoid them? ________________________________220 Q 17: Give an overview of the Spring framework? _________________________________________________________221 Q 18: How would EJB 3.0 simplify your Java development compared to EJB 1.x, 2.x? ___________________________222 Q 19: Briefly explain key features of the JavaServer Faces (JSF) framework? __________________________________223 Q 20: How would the JSF framework compare with the Struts framework?_____________________________________225 Sample interview questions… ____________________________________________________________________226 Java___________________________________________________________________________________________________227 Web components________________________________________________________________________________________227 Enterprise______________________________________________________________________________________________227 Design_________________________________________________________________________________________________229 General ________________________________________________________________________________________________229 GLOSSARY OF TERMS__________________________________________________________________________230 RESOURCES __________________________________________________________________________________232 INDEX ________________________________________________________________________________________234 7 What this book will do for you? Have you got the time to read 10 or more books and articles to add value prior to the interview? This book has been written mainly from the perspective of Java/J2EE job seekers and interviewers. There are numerous books and articles on the market covering specific topics like Java, J2EE, EJB, Design Patterns, ANT, CVS, Multi-Threading, Servlets, JSP, emerging technologies like AOP (Aspect Oriented Programming), Test Driven Development (TDD), Inversion of Control (IoC) etc. But from an interview perspective it is not possible to brush up on all these books where each book usually has from 300 pages to 600 pages. The basic purpose of this book is to cover all the core concepts and design/coding issues which, all Java/J2EE developers, designers and architects should be conversant with to perform well in their current jobs and to launch a successful career by doing well at interviews. The interviewer can also use this book to make sure that they hire the right candidate depending on their requirements. This book contains a wide range of topics relating to Java/J2EE development in a concise manner supplemented with diagrams, tables, sample codes and examples. This book is also appropriately categorised to enable you to choose the area of interest to you. This book will assist all Java/J2EE practitioners to become better at what they do. Usually it takes years to understand all the core concepts and design/coding issues when you rely only on your work experience. The best way to fast track this is to read appropriate technical information and proactively apply these in your work environment. It worked for me and hopefully it will work for you as well. I was also at one stage undecided whether to name this book “Java/J2EE core concepts and solving design/coding issues” or “Java/J2EE Job Interview Companion”. The reason I chose “Java/J2EE Job Interview Companion” is because these core concepts and design/coding issues helped me to be successful in my interviews and also gave me thumbs up in code reviews. 8 Motivation for this book I started using Java in 1999 when I was working as a junior developer. During those two years as a permanent employee, I pro-actively spent many hours studying the core concepts behind Java/J2EE in addition to my hands on practical experience. Two years later I decided to start contracting. Since I started contracting in 2001, my career had a much- needed boost in terms of contract rates, job satisfaction, responsibility etc. I moved from one contract to another with a view of expanding my skills and increasing my contract rates. In the last 5 years of contracting, I have worked for 5 different organisations both medium and large on 8 different projects. For each contract I held, on average I attended 6-8 interviews with different companies. In most cases multiple job offers were made and consequently I was in a position to negotiate my contract rates and also to choose the job I liked based on the type of project, type of organisation, technology used, etc. I have also sat for around 10 technical tests and a few preliminary phone interviews. The success in the interviews did not come easily. I spent hours prior to each set of interviews wading through various books and articles as a preparation. The motivation for this book was to collate all this information into a single book, which will save me time prior to my interviews but also can benefit others in their interviews. What is in this book has helped me to go from just a Java/J2EE job to a career in Java/J2EE in a short time. It has also given me the job security that ‘I can find a contract/permanent job opportunity even in the difficult job market’. I am not suggesting that every one should go contracting but by performing well at the interviews you can be in a position to pick the permanent role you like and also be able to negotiate your salary package. Those of you who are already in good jobs can impress your team leaders, solution designers and/or architects for a possible promotion by demonstrating your understanding of the key areas discussed in this book. You can discuss with your senior team members about performance issues, transactional issues, threading issues (concurrency issues) and memory issues. In most of my previous contracts I was in a position to impress my team leads and architects by pinpointing some of the critical performance, memory, transactional and threading issues with the code and subsequently fixing them. Trust me it is not hard to impress someone if you understand the key areas. For example: ƒ Struts action classes are not thread-safe (Refer Q113 in Enterprise section). ƒ JSP variable declaration is not thread-safe (Refer Q34 in Enterprise section). ƒ Valuable resources like database connections should be closed properly to avoid any memory and performance issues (Refer Q45 in Enterprise section). ƒ Throwing an application exception will not rollback the transaction in EJB. (Refer Q77 in Enterprise section). The other key areas, which are vital to any software development, are a good understanding of some of key design concepts, design patterns, and a modelling language like UML. These key areas are really worthy of a mention in your resume and interviews. For example: ƒ Know how to use inheritance, polymorphism and encapsulation (Refer Q5, Q6, Q7, and Q8 in Java section.). ƒ Why use design patterns? (Refer Q5 in Enterprise section). ƒ Why is UML important? (Refer Q106 in Enterprise section). If you happen to be in an interview with an organization facing serious issues with regards to their Java application relating to memory leaks, performance problems or a crashing JVM etc then you are likely to be asked questions on these topics. Refer Q 63 – Q 65 in Java section and Q123, Q125 in Enterprise section. Another good reason why these key areas like transactional issues, design concepts, design patterns etc are vital are because solution designers, architects, team leads, and/or senior developers are usually responsible for conducting the technical interviews. These areas are their favourite topics because these are essential to any software development. Some interviewers request you to write a small program during interview or prior to getting to the interview stage. This is to ascertain that you can code using object oriented concepts and design patterns. So I have included a coding key area to illustrate what you need to look for while coding. 9 ƒ Apply OO concepts like inheritance, polymorphism and encapsulation: Refer Q08 in Java section. ƒ Program to interfaces not to implementations: Refer Q08, Q15 in Java section. ƒ Use of relevant design patterns: Refer Q11 in How would you go about… section. ƒ Use of Java collection API and exceptions correctly: Refer Q15, Q34, and Q35 in Java section. ƒ Stay away from hard coding values: Refer Q04 in Java section. L anguage F undam entals P erfo rm an ce H o w m a ny b o o k s d o I hav e to re a d to u nde rs ta nd and pu t to g e th er a ll th es e Issues k e y are a s? H o w m a ny y ears o f ex perienc e s hou ld I hav e to un ders tan d a ll th es e S p ecificatio n k e y are a s? E xcep tion F undam entals H a n d lin g W ill thes e k e y a rea s h e lp m e S o ftw a re p rog re s s in m y c a reer? D evelo p m ent P ro cess W ill thes e k e y a rea s h e lp m e c u t q uality c ode ? D esig n D esig n C oncepts SE cu rity P a tte rn s T ransactio nal Issu es B est C oncurrency S calab ility P ra c tic e s Issues Issu es M em o ry Issu es CO ding LF DC CI PI SE EH SD DP SF MI SI TI BP CO This book aims to solve the above dilemma. My dad keeps telling me to find a permanent job (instead of contracting), which in his view provides better job security but I keep telling him that in my view in Information Technology the job security is achieved only by keeping your knowledge and skills sharp and up to date. The 8 contract positions I held over the last 5.5 years have given me broader experience in Java/J2EE and related technologies. It also kept me motivated since there was always something new to learn in each assignment, and not all companies will appreciate your skills and expertise until you decide to leave. Do the following statements sound familiar to you when you hand in your resignation or decide not to extend your contract after getting another job offer? “Can I tempt you to come back? What can I do to keep you here?” etc. You might even think why you waited so long. The best way to make an impression in any organisations is to understand and proactively apply and resolve the issues relating to the Key Areas discussed in the next section. But be a team player, be tactful and don’t be critical of everything, do not act in a superior way and have a sense of humour. “Technical skills must be complemented with interpersonal skills.” Quick Read guide: It is recommended that you go through all the questions in all the sections but if you are pressed for time or would like to read it just before an interview then follow the steps shown below: 1. Read/Browse Popular Questions in Java and Enterprise Java sections. 2. Read/Browse Key Points in Java and Enterprise Java sections. 3. Read/Browse through “Emerging Technologies/Frameworks” section. 4. Read/Browse “How would you go about…” section excluding Q11 & Q13, which are discussed in detail. 10 Key Areas Index I have categorised the core concepts and issues into 14 key areas as listed below. These key areas are vital for any good software development. This index will enable you to refer to the questions based on key areas. Also note that each question has an icon next to it to indicate which key area or areas it belongs to. Additional reading is recommended for beginners in each of the key areas. Key Areas icon Question Numbers Java section Enterprise section How Emerging would you Technologies go /Frameworks about…? Language LF Q1-Q4, Q10-Q14, Q16 Q10, Q15, Fundamentals Q20, Q22-Q27, Q30- Q17, Q19 Q33, Q36-Q43, Q47-Q62 Specification - Q1-Q19, Q26-Q33, Q35- Q14 SF Fundamentals Q38, Q41, Q42, Q44, Q46- Q81, Q89-Q97, Q99, 102, Q110, Q112-Q115, Q118- Q119, Q121, Q126, Q127, Q128 Design Concepts Q5-Q9, Q10, Q13, Q22, Q2, Q3, Q19, Q20, Q21, Q02, Q08, Q3-Q9, Q11, DC Q49 Q31, Q45, Q98, Q106, Q09 Q13, Q14, Q107, Q108, Q109, 101, Q16, Q18, Q111 Q20 Design Patterns Q10, Q14, Q20, Q31, Q5, Q5, Q22, Q24, Q25, Q11 Q12 DP Q45, Q46, Q50, Q54, Q83, Q84, Q85, Q86, Q87, Q66 Q88, Q110, Q111, Q116 Transactional TI - Q43, Q71, Q72, Q73, Q74, Q7 Issues Q75, Q77, Q78, Q79 Concurrency Issues Q13, Q15, Q29, Q36, Q16, Q34, Q113 Q6 CI Q40, Q53 Performance Issues PI Q13, Q15 -Q22, Q40, Q10, Q16, Q43, Q45, Q46, Q3, Q5 Q53, Q63. Q72, Q83-Q88, Q97, Q98, Q100, Q102, Q123, Q125, Q128 Memory Issues MI Q22, Q29, Q32, Q33, Q45, Q93 Q3, Q4 Q36, Q45, Q64, Q65. Scalability Issues Q19, Q20 Q20, Q21, Q120, Q122 SI Exception Handling Q34,Q35 Q76, Q77 EH Security Q61 Q12, Q13, Q23, Q35, Q46, Q12 SE Q51, Q58, Q81 Best Practices Q15, Q21, Q34, Q63, Q10, Q16, Q39, Q40, Q46, BP Q64 Q82, Q124, Q125 Software SD - Q103-Q109, Q129, Q133, Q1, Q10, Q1, Q2 Development Q134, Q136 Q13 Process 1 Coding Q04, Q08, Q10, Q12, Q10, Q18, Q21, Q23, Q36, Q11 CO Q13, Q15, Q16, Q17, Q38, Q42, Q43, Q45, Q74, Q21, Q34, Q45, Q46 Q75, Q76, Q77, Q112, Q114, Q127, Q128 1 Some interviewers request you to write a small program during interview or prior to getting to the interview stage. This is to ascertain that you can code using object oriented concepts and design patterns. I have included a coding key area to illustrate what you need to look for while coding. Unlike other key areas, the CO is not always shown against the question but shown above the actual section of relevance within a question. Java 11 SECTION ONE Java – Interview questions & answers ƒ Language Fundamentals LF K ƒ Design Concepts DC E ƒ Design Patterns DP Y ƒ Concurrency Issues CI Aƒ Performance Issues PI R ƒ Memory Issues MI E ƒ Exception Handling EH A ƒ Security SE S ƒ Scalability Issues SI 1 ƒ Coding CO Popular Questions: Q01, Q04, Q07, Q08, Q13, Q16, Q17, Q18, Q19, Q25, Q27, Q29, Q32, Q34, Q40, Q45, Q46, Q50, Q51, Q53, Q54, Q55, Q63,Q64, Q66, Q67 1 Unlike other key areas, the CO is not always shown against the question but shown above the actual subsection of relevance within a question. Java 12 Java – Language Fundamentals Q 01: Give a few reasons for using Java? LF DC A 01: Java is a fun language. Let’s look at some of the reasons: ƒ Built-in support for multi-threading, socket communication, and memory management (automatic garbage collection). ƒ Object Oriented (OO). ƒ Better portability than other languages across operating systems. ƒ Supports Web based applications (Applet, Servlet, and JSP), distributed applications (sockets, RMI. EJB etc) and network protocols (HTTP, JRMP etc) with the help of extensive standardised APIs (Application Program Interfaces). Q 02: What is the main difference between the Java platform and the other software platforms? LF A 02: Java platform is a software-only platform, which runs on top of other hardware-based platforms like UNIX, NT etc. The Java platform has 2 components: ƒ Java Virtual Machine (JVM) – ‘JVM’ is a software that can be ported onto various hardware platforms. Byte codes are the machine language of the JVM. ƒ Java Application Programming Interface (Java API) - ++ Q 03: What is the difference between C and Java? LF A 03: Both C++ and Java use similar syntax and are Object Oriented, but: ƒ Java does not support pointers. Pointers are inherently tricky to use and troublesome. ƒ Java does not support multiple inheritances because it causes more problems than it solves. Instead Java supports multiple interface inheritance, which allows an object to inherit many method signatures from different interfaces with the condition that the inheriting object must implement those inherited methods. The multiple interface inheritance also allows an object to behave polymorphically on those methods. Refer Q 8 and Q 10 in Java section. ƒ Java does not support destructors but rather adds a finalize() method. Finalize methods are invoked by the garbage collector prior to reclaiming the memory occupied by the object, which has the finalize() method. This means you do not know when the objects are going to be finalized. Avoid using finalize() method to release non-memory resources like file handles, sockets, database connections etc because Java has only a finite number of these resources and you do not know when the garbage collection is going to kick in to release these resources through the finalize() method. ƒ Java does not include structures or unions because the traditional data structures are implemented as an object oriented framework (Java collection framework – Refer Q14, Q15 in Java section). Java 13 ƒ All the code in Java program is encapsulated within classes therefore Java does not have global variables or functions. ƒ C++ requires explicit memory management, while Java includes automatic garbage collection. Refer Q32 in Java section. Q 04: Explain Java class loaders? Explain dynamic class loading? LF A 04: Class loaders are hierarchical. Classes are introduced into the JVM as they are referenced by name in a class that is already running in the JVM. So how is the very first class loaded? The very first class is specially loaded with the help of static main() method declared in your class. All the subsequently loaded classes are loaded by the classes, which are already loaded and running. A class loader creates a namespace. All JVMs include at least one class loader that is embedded within the JVM called the primordial (or bootstrap) class loader. Now let’s look at non-primordial class loaders. The JVM has hooks in it to allow user defined class loaders to be used in place of primordial class loader. Let us look at the class loaders created by the JVM. CLASS LOADER reloadable? Explanation Bootstrap No Loads JDK internal classes, java. packages. (as defined in the sun.boot.class.path (primordial) system property, typically loads rt.jar and i18n.jar) Extensions No Loads jar files from JDK extensions directory (as defined in the java.ext.dirs system property – usually lib/ext directory of the JRE) System No Loads classes from system classpath (as defined by the java.class.path property, which is set by the CLASSPATH environment variable or –classpath or –cp command line options) JVM class loaders Bootstrap (primordial) Classes loaded by Bootstrap class loader have no visibility into classes (rt.jar, i18.jar) loaded by its descendants (ie Extensions and Systems class loaders). The classes loaded by system class loader have visibility into classes loaded Extensions by its parents (ie Extensions and Bootstrap class loaders). (lib/ext) If there were any sibling class loaders they cannot see classes loaded by each other. They can only see the classes loaded by their parent class System loader. For example Sibling1 class loader cannot see classes loaded by (-classpath) Sibling2 class loader Both Sibling1 and Sibling2 class loaders have visibilty into classes loaded Sibling1 Sibling2 by their parent class loaders (eg: System, Extensions, and Bootstrap) classloader classloader Class loaders are hierarchical and use a delegation model when loading a class. Class loaders request their parent to load the class first before attempting to load it themselves. When a class loader loads a class, the child class loaders in the hierarchy will never reload the class again. Hence uniqueness is maintained. Classes loaded by a child class loader have visibility into classes loaded by its parents up the hierarchy but the reverse is not true as explained in the above diagram. Important: Two objects loaded by different class loaders are never equal even if they carry the same values, which mean a class is uniquely identified in the context of the associated class loader. This applies to singletons too, where each class loader will have its own singleton. Refer Q45 in Java section for singleton design pattern Explain static vs. dynamic class loading? Static class loading Dynamic class loading Classes are statically loaded with Java’s Dynamic loading is a technique for programmatically invoking the functions of a “new” operator. class loader at run time. Let us look at how to load classes dynamically. class MyClass Class.forName (String className); //static method which returns a Class public static void main(String args) Car c = new Car(); The above static method returns the class object associated with the class name. The string className can be supplied dynamically at run time. Unlike the static loading, the dynamic loading will decide whether to load the class Car or the class Jeep at runtime based on a properties file and/or other runtime Java 14 conditions. Once the class is dynamically loaded the following method returns an instance of the loaded class. It’s just like creating a class object with no arguments. class.newInstance (); //A non-static method, which creates an instance of a class (i.e. creates an object). Jeep myJeep = null ; //myClassName should be read from a properties file or Constants interface. //stay away from hard coding values in your program. CO String myClassName = "au.com.Jeep" ; Class vehicleClass = Class.forName(myClassName) ; myJeep = (Jeep) vehicleClass.newInstance(); myJeep.setFuelCapacity(50); A NoClassDefFoundException is A ClassNotFoundException is thrown when an application tries to load in a thrown if a class is referenced with class through its string name using the following methods but no definition for the Java’s “new” operator (i.e. static loading) class with the specified name could be found: but the runtime system cannot find the referenced class. ƒ The forName(..) method in class - Class. ƒ The findSystemClass(..) method in class - ClassLoader. ƒ The loadClass(..) method in class - ClassLoader. What are “static initializers” or “static blocks with no function names”? When a class is loaded, all blocks that are declared static and don’t have function name (i.e. static initializers) are executed even before the constructors are executed. As the name suggests they are typically used to initialize static fields. CO public class StaticInitilaizer public static final int A = 5; public static final int B; //Static initializer block, which is executed only once when the class is loaded. static if(A == 5) B = 10; else B = 5; public StaticInitilaizer() // constructor is called only after static initializer block The following code gives an Output of A=5, B=10. public class Test System.out.println("A =" + StaticInitilaizer.A + ", B =" + StaticInitilaizer.B); Q 05: What are the advantages of Object Oriented Programming Languages (OOPL)? DC A 05: The Object Oriented Programming Languages directly represent the real life objects like Car, Jeep, Account, Customer etc. The features of the OO programming languages like polymorphism, inheritance and encapsulation make it powerful. Tip: remember pie which, stands for Polymorphism, Inheritance and Encapsulation are the 3 pillars of OOPL Q 06: How does the Object Oriented approach improve software development? DC A 06: The key benefits are: ƒ Re-use of previous work: using implementation inheritance and object composition. ƒ Real mapping to the problem domain: Objects map to real world and represent vehicles, customers, products etc: with encapsulation. ƒ Modular Architecture: Objects, systems, frameworks etc are the building blocks of larger systems. Java 15 The increased quality and reduced development time are the by-products of the key benefits discussed above. If 90% of the new application consists of proven existing components then only the remaining 10% of the code have to be tested from scratch. Q 07: How do you express an ‘is a’ relationship and a ‘has a’ relationship or explain inheritance and composition? What is the difference between composition and aggregation? DC A 07: The ‘is a’ relationship is expressed with inheritance and ‘has a’ relationship is expressed with composition. Both inheritance and composition allow you to place sub-objects inside your new class. Two of the main techniques for code reuse are class inheritance and object composition. Inheritance is a Vs Composition has a is a House is a Building has a House has a Bathroom is a Building class Building class House has a ....... Bathroom room = new Bathroom() ; .... public void getTotMirrors() class House extends Building room.getNoMirrors(); Bathroom ......... .... House Inheritance is uni-directional. For example House is a Building. But Building is not a House. Inheritance uses extends key word. Composition: is used when House has a Bathroom. It is incorrect to say House is a Bathroom. Composition simply means using instance variables that refer to other objects. The class House will have an instance variable, which refers to a Bathroom object. Which one to use? The guide is that inheritance should be only used when subclass ‘is a’ superclass. ƒ Don’t use inheritance just to get code reuse. If there is no ‘is a’ relationship then use composition for code reuse. Overuse of implementation inheritance (uses the “extends” key word) can break all the subclasses, if the superclass is modified. ƒ Do not use inheritance just to get polymorphism. If there is no ‘is a’ relationship and all you want is polymorphism then use interface inheritance with composition, which gives you code reuse (Refer Q8 in Java section for interface inheritance). What is the difference between aggregation and composition? Aggregation Composition Aggregation is an association in which one class Composition is an association in which one class belongs to a belongs to a collection. This is a part of a whole collection. This is a part of a whole relationship where a part relationship where a part can exist without a whole. cannot exist without a whole. If a whole is deleted then all parts are For example a line item is a whole and product is a deleted. For example An order is a whole and line items are parts. part. If a line item is deleted then corresponding If an order deleted then all corresponding line items for that order product need not be deleted. So aggregation has a should be deleted. So composition has a stronger relationship. weaker relationship. Q 08: What do you mean by polymorphism, inheritance, encapsulation, and dynamic binding? DC A 08: Polymorphism – means the ability of a single variable of a given type to be used to reference objects of different types, and automatically call the method that is specific to the type of object the variable references. In a nutshell, polymorphism is a bottom-up method call. The benefit of polymorphism is that it is very easy to add new classes of derived objects without breaking the calling code (i.e. getTotArea() in the sample code shown below) that uses the polymorphic classes or interfaces. When you send a message to an object even though you don’t know what specific type it is, and the right thing happens, that’s called polymorphism. The process used by object- oriented programming languages to implement polymorphism is called dynamic binding. Let us look at some sample code to demonstrate polymorphism: CO Java 16 Sample code: abstract //client or calling code For example: given a base Shape double dim = 5.0; //ie 5 meters radius or width class/interface Shape, List listShapes = new ArrayList(20); polymorphism allows the +area() : double Shape s = new Circle(); programmer to define listShapes.add(s); //add circle different area(double dim1) methods for any Circle s = new Square(); Square listShapes.add(s); //add square number of derived classes such as Circle, Square etc. +area() : double getTotArea (listShapes,dim); //returns 78.5+25.0=103.5 +area() : double No matter what shape an object is, applying the area //Later on, if you decide to add a half circle then define method to it will return the HalfCircle //a HalfCircle class, which extends Circle and then provide an //area(). method but your called method getTotArea(...) remains right results. //same. +area() : double Later on HalfCicle can be s = new HalfCircle(); added without breaking listShapes.add(s); //add HalfCircle your called code i.e. getTotArea (listShapes,dim); //returns 78.5+25.0+39.25=142.75 «interface» method getTotalArea(...) Shape / called method: method which adds up areas of various +area() : double shapes supplied to it. Depending on what the / shape is, appropriate public double getTotArea(List listShapes, double dim) area(double dim) method Circle Square Iterator it = listShapes.iterator(); gets called and calculated. double totalArea = 0.0; //loop through different shapes +area() : double +area() : double Circle Æ area is 78.5sqm while(it.hasNext()) Square Æ area is 25sqm Shape s = (Shape) it.next(); HalfCircle Æ area is 39.25 totalArea += s.area(dim); //polymorphic method call HalfCircle sqm return totalArea ; +area() : double Inheritance – is the inclusion of behaviour (i.e. methods) and state (i.e. variables) of a base class in a derived class so that they are accessible in that derived class. The key benefit of Inheritance is that it provides the formal mechanism for code reuse. Any shared piece of business logic can be moved from the derived class into the base class as part of refactoring process to improve maintainability of your code by avoiding code duplication. The existing class is called the superclass and the derived class is called the subclass. Inheritance can also be defined as the process whereby one object acquires characteristics from one or more other objects the same way children acquire characteristics from their parents. There are two types of inheritances: 1. Implementation inheritance (aka class inheritance): You can extend an applications’ functionality by reusing functionality in the parent class by inheriting all or some of the operations already implemented. In Java, you can only inherit from one superclass. Implementation inheritance promotes reusability but improper use of class inheritance can cause programming nightmares by breaking encapsulation and making future changes a problem. With implementation inheritance, the subclass becomes tightly coupled with the superclass. This will make the design fragile because if you want to change the superclass, you must know all the details of the subclasses to avoid breaking them. So when using implementation inheritance, make sure that the subclasses depend only on the behaviour of the superclass, not on the actual implementation. For example in the above diagram the subclasses should only be concerned about the behaviour known as area() but not how it is implemented. 2. Interface inheritance (aka type inheritance): This is also known as subtyping. Interfaces provide a mechanism for specifying a relationship between otherwise unrelated classes, typically by specifying a set of common methods each implementing class must contain. Interface inheritance promotes the design concept of program to interfaces not to implementations. This also reduces the coupling or implementation dependencies between systems. In Java, you can implement any number of interfaces. This is more flexible than implementation inheritance because it won’t lock you into specific implementations which make subclasses difficult to maintain. So care should be taken not to break the implementing classes by modifying the interfaces. Which one to use? Prefer interface inheritance to implementation inheritance because it promotes the design concept of coding to an interface and reduces coupling. Interface inheritance can achieve code reuse with the help of object composition. If you look at Gang of Four (GoF) design patterns, you can see that it favours interface inheritance to implementation inheritance. CO Java 17 Implementation inheritance Interface inheritance Let’s assume that savings account and term deposit Let’s look at an interface inheritance code sample, which makes account have a similar behaviour in terms of depositing use of composition for reusability. In the following example the and withdrawing money, so we will get the super class to methods deposit(…) and withdraw(…) share the same piece of code implement this behaviour and get the subclasses to reuse in AccountHelper class. The method calculateInterest(…) has its this behaviour. But saving account and term deposit specific implementation in its own class. account have specific behaviour in calculating the interest. public interface Account public abstract class Account public abstract void deposit(double amount); public abstract void withdraw(double amount); public void deposit(double amount) public abstract int getAccountType(); //deposit logic public interface SavingsAccount extends Account public void withdraw(double amount) public abstract double calculateInterest(double amount); //withdraw logic public interface TermDepositAccount extends Account public abstract double calculateInterest(double amount); public abstract double calculateInterest(double amount); The classes SavingsAccountImpl, TermDepositAccountImpl public class SavingsAccount extends Account should implement the methods declared in its interfaces. The class AccountHelper implements the methods deposit(…) and public double calculateInterest(double amount) withdraw(…) //calculate interest for SavingsAccount public class SavingsAccountImpl implements SavingsAccount private int accountType = 1; public class TermDepositAccount extends Account //helper class which promotes code reuse through composition AccountHelper helper = new AccountHelper(); public double calculateInterest(double amount) //calculate interest for TermDeposit public void deposit(double amount) helper.deposit(amount, getAccountType()); public void withdraw(double amount) The calling code can be defined as follows for illustration helper.withdraw(amount, getAccountType()); purpose only: public double calculateInterest(double amount) //calculate interest for SavingsAccount public class Test public static void main(String args) public int getAccountType() Account acc1 = new SavingsAccount(); return accountType; acc1.deposit(5.0); acc1.withdraw(2.0); Account acc2 = new TermDepositAccount(); public class TermDepositAccountImpl implements acc2.deposit(10.0); TermDepositAccount acc2.withdraw(3.0); private int accountType = 2; acc1.calculateInterest(500.00); //helper class which promotes code reuse through composition acc2.calculateInterest(500.00); AccountHelper helper = new AccountHelper(); public void deposit(double amount) helper.deposit(amount, getAccountType()); public void withdraw(double amount) helper.withdraw(amount, getAccountType()); public double calculateInterest(double amount) //calculate interest for TermDeposit public int getAccountType() return accountType; The calling code can be defined as follows for illustration purpose only: public class Test public static void main(String args) int ge tM arks () setN am e (St ring na me ) Java 18 Account acc1 = new SavingsAccountImpl(); acc1.deposit(5.0); Account acc2 = new TermDepositAccountImpl(); acc2.deposit(10.0); if (acc1.getAccountType() == 1) ((SavingsAccount) acc1).calculateInterest(500.00); if (acc2.getAccountType() == 2) ((TermDepositAccount) acc2).calculateInterest(500.00); Encapsulation – refers to keeping all the related members (variables and methods) together in an object. Specifying members as private can hide the variables and methods. Objects should hide their inner workings from the outside view. Good encapsulation improves code modularity by preventing objects interacting with each other in an unexpected way, which in turn makes future development and refactoring efforts easy. Being able to encapsulate members of a class is important for security and integrity. We can protect variables from unacceptable values. The sample code below describes how encapsulation can be used to protect the MyMarks object from having negative values. Any modification to member variable “vmarks” can only be carried out through the setter method setMarks(int mark). This prevents the object “MyMarks” from having any negative values by throwing an exception. CO Sample code Class MyMarks private int vmarks = 0; Member private String name; variables are encapsulated, public void setMarks(int mark) so that they throws MarkException can only be if(mark 0) accessed via this.vmarks = mark; encapsulating else private int vmarks; methods. throw new MarkException("No negative private String name; Values"); public int getMarks() return vmarks; //getters and setters for attribute name goes here. Q 09: What is design by contract? Explain the assertion construct? DC A 09: Design by contract specifies the obligations of a calling-method and called-method to each other. Design by contract is a valuable technique, which should be used to build well-defined interfaces. The strength of this programming methodology is that it gets the programmer to think clearly about what a function does, what pre and post conditions it must adhere to and also it provides documentation for the caller. Java uses the assert statement to implement pre- and post-conditions. Java’s exceptions handling also support design by contract especially checked exceptions (Refer Q34 in Java section for checked exceptions). In design by contract in addition to specifying programming code to carrying out intended operations of a method the programmer also specifies: 1. Preconditions – This is the part of the contract the calling-method must agree to. Preconditions specify the conditions that must be true before a called method can execute. Preconditions involve the system state and the arguments passed into the method at the time of its invocation. If a precondition fails then there is a bug in the calling-method or calling software component. k) ar nt m ks(i ar etM s ) e( am etN g g trin SJava 19 On public methods On non-public methods Preconditions on public methods are enforced by explicit checks You can use assertion to check the parameters of the that throw particular, specified exceptions. You should not use non-public methods. assertion to check the parameters of the public methods but can use for the non-public methods. Assert is inappropriate private void setCalculatedRate(int rate) because the method guarantees that it will always enforce the assert (rate 0 && rate MAX_RATE) : rate; argument checks. It must check its arguments whether or not //calculate the rate and set it. assertions are enabled. Further, assert construct does not throw an exception of a specified type. It can throw only an AssertionError. Assertions can be disabled, so programs must not assume that assert construct will be always executed: public void setRate(int rate) //Wrong: if assertion is disabled, CarpenterJob never if(rate = 0 rate MAX_RATE) //Get removed throw new IllegalArgumentException(“Invalid rate Æ ” + rate); assert jobsAd.remove(PilotJob); setCalculatedRate(rate); //Correct: boolean pilotJobRemoved = jobsAd.remove(PilotJob); assert pilotJobRemoved; 2. Postconditions – This is the part of the contract the called-method agrees to. What must be true after a method completes successfully. Postconditions can be used with assertions in both public and non-public methods. The postconditions involve the old system state, the new system state, the method arguments and the method’s return value. If a postcondition fails then there is a bug in the called-method or called software component. public double calcRate(int rate) if(rate = 0 rate MAX_RATE) throw new IllegalArgumentException(“Invalid rate ”); //logic to calculate the rate and set it goes here assert this.evaluate(result) 0 : this; //this Æ message sent to AssertionError on failure return result; 3. Class invariants - what must be true about each instance of a class? A class invariant as an internal invariant that can specify the relationships among multiple attributes, and should be true before and after any method completes. If an invariant fails then there could be a bug in either calling-method or called-method. There is no particular mechanism for checking invariants but it is convenient to combine all the expressions required for checking invariants into a single internal method that can be called by assertions. For example if you have a class, which deals with negative integers then you define the isNegative() convenient internal method: class NegativeInteger Integer value = new Integer (-1); //invariant //constructor public NegativeInteger(Integer int) //constructor logic goes here assert isNegative(); //rest of the public and non-public methods goes here. public methods should call assert isNegative(); prior to its return //convenient internal method for checking invariants. Returns true if the integer value is negative private boolean isNegative() return value.intValue() 0 ; The isNegative() method should be true before and after any method completes, each public method and constructor should contain the following assert statement immediately prior to its return. assert isNegative(); Explain the assertion construct? The assertion statements have two forms as shown below: assert Expression1; Java 20 assert Expression1 : Expression2; Where: ƒ Expression1 Æ is a boolean expression. If the Expression1 evaluates to false, it throws an AssertionError without any detailed message. ƒ Expression2 Æ if the Expression1 evaluates to false throws an AssertionError with using the value of the Expression2 as the errors’ detailed message. Note: If you are using assertions (available from JDK1.4 onwards), you should supply the JVM argument to enable it by package name or class name. Java -ea:packagename...:classname or Java -enableassertions:packagename...:classname Java –ea:Account Q 10: What is the difference between an abstract class and an interface and when should you use them? LF DP DC A 10: In design, you want the base class to present only an interface for its derived classes. This means, you don’t want anyone to actually instantiate an object of the base class. You only want to upcast to it (implicit upcasting, which gives you polymorphic behaviour), so that its interface can be used. This is accomplished by making that class abstract using the abstract keyword. If anyone tries to make an object of an abstract class, the compiler prevents it. The interface keyword takes this concept of an abstract class a step further by preventing any method or function implementation at all. You can only declare a method or function but not provide the implementation. The class, which is implementing the interface, should provide the actual implementation. The interface is a very useful and commonly used aspect in OO design, as it provides the separation of interface and implementation and enables you to: ƒ Capture similarities among unrelated classes without artificially forcing a class relationship. ƒ Declare methods that one or more classes are expected to implement. ƒ Reveal an object's programming interface without revealing its actual implementation. ƒ Model multiple interface inheritance in Java, which provides some of the benefits of full on multiple inheritances, a feature that some object-oriented languages support that allow a class to have more than one superclass. Diamond problem & use of interface Interface Shape ShapeIF Circle Square CircleOnSquare Circle Square Interface Interface CircleIF SquareIF CircleOnSquare Multiple interface inheritance in JAVA No multiple inheritance in JAVA Abstract class Interface Have executable methods and abstract methods. Have no implementation code. All methods are abstract. Can only subclass one abstract class. A class can implement any number of interfaces. Can have instance variables, constructors and any Cannot have instance variables, constructors and can have visibility: public, private, protected, none (aka package). only public and none (aka package) visibility. When to use an abstract class?: In case where you want to use implementation inheritance then it is usually provided by an abstract base class. Abstract classes are excellent candidates inside of application frameworks. Abstract classes let you define some default behaviour and force subclasses to provide any specific behaviour. Care should be taken not to overuse implementation inheritance as discussed in Q8 in Java section.

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