Interview Questions in core Java

1) Q: What is the purpose of the toolkit in the Abstract Window Toolkit (AWT)? How does AWT

work ?

A: The AWT toolkit is an interface between the abstract window layer and a specific windowing

implementation.

2) Q: What is layout manager ? How does it work ?

A: A layout manager is an object that positions and resizes the components in a Container according to

some algorithm; for example, the FlowLayout layout manager lays out components from left to right until itruns out of room and then continues laying out components below that row.

3) Q: Advantages and disadvantages of layout manager ?

4) Q: Compare SWING components to standard AWT.

A: Swing is an extension of, and not a replacement for the AWT. There is some overlap between AWT and Swing (for example a Swing JButton component might be viewed as an improved functional replacement for an AWT Button component.) One of the advantages of Swing components is that because the components are not rendered on the screen by the operating system, the look and feel of a component does not change as the application or applet is executed on different platforms running under different operating systems. Furthermore, it is possible to cause Swing components to mimic the look and feel of a specific platform no matter what platform the program is running on. This is known as pluggable look and feel. Swing components sup+port the JDK 1.1

Delegation Event Model. From an event handling viewpoint, Swing components operate the same as

AWT components (except that Swing provides a number of new event types). Many Swing components

don't have an AWT counterpart. A number of new and exciting components are included in the Swing

library that don't exist in the AWT (tooltips, progress bars, trees, etc.)

5) Q: What is Java Beans ?

A: According to JavaSoft, "A Java Bean is a reusable software component that can be manipulated visually in a builder tool."

6) Q: What you know about Corba implementation in Java ?

A: Java 1.2 promises full CORBA IDL support.

7) Q: What do you know about networking support in Java ?

A: Java supports "low-level" and "high-level" classes. "Low-level" classes provide support for socket

programming: Socket, DatagramSocket, and ServerSocket classes. "High-level" classes provide "Web

programming": URL, URLEncoder, and URLConnection classes. Networking programming classes ease the programming of network applications, but do not substitute your knowledge of networking. Java networking like anything else in Java is platform-independent.

8) Q: What is it object serialization ?

A: Serialization is a way to convert objects (including complex data structures such as lists and

trees) into a stream of bytes.

9) Q: How to make application thread-safe ?

A: You should use the word synchronized to mark the critical section of code. You may also use other

methods of thread synchronization (see wait(), notify(), notifyAll() etc.

10) Q: What is it reflection (introspection) ? Why is reflection possible in the Java language?

A: Reflection (introspection) is querying a class about its properties, and operating on methods and

fields by the name for a given object instance. Reflection is possible in the Java language because of late binding.

11) Q: Why are Java ARchive (JAR) files important?

A: JAR files bundle .class files and optimize applet downloads.

12) Describe what happens when an object is created in Java

Several things happen in a particular order to ensure the object is constructed properly:

1. Memory is allocated from heap to hold all instance variables and implementation-specific data of the object and its superclasses. Implemenation-specific data includes pointers to class and method data.

2. The instance variables of the objects are initialized to their default values.

3. The constructor for the most derived class is invoked. The first thing a constructor does is call the

consctructor for its superclasses. This process continues until the constrcutor for java.lang.Object is called, as java.lang.Object is the base class for all objects in java.

4. Before the body of the constructor is executed, all instance variable initializers and initialization blocks are executed. Then the body of the constructor is executed. Thus, the constructor for the base class completes first and constructor for the most derived class completes last.

13) In Java, You can create a String object as below : String str = "abc"; & String str = new

String("abc");

Why cant a button object be created as : Button bt = "abc" Why is it compulsory to create a button object

as: Button bt = new Button("abc"); Why this is not compulsory in String's case.

The main reason you cannot create a button by

Button bt1= "abc";

is because "abc" is a literal string (something slightly different than a String object, by-the-way) and bt1 is a Button object. That simple. The only object in Java that can be assigned a literal String is java.lang.String. Important to not that you are NOT calling a java.lang.String constuctor when you type String s = "abc";

For example

String x = "abc";

String y = "abc";

refer to the same object. While

String x1 = new String("abc");

String x2 = new String("abc");

refer to two different objects.

14) What are the main differences between Java and C++?

Everything is an object in Java( Single root hierarchy as everything gets derived from java.lang.Object)

Java does not have all the complicated aspects of C++ ( For ex: Pointers, templates, unions, operator

overloading, structures etc..) The Java language promoters initially said "No pointers!", but when many programmers questioned how you can work without pointers, the promoters began saying "Restricted pointers." You can make up your mind whether it’s really a pointer or not. In any event, there’s no pointer arithmetic. There are no destructors in Java. (automatic garbage collection)

Java does not support conditional compile (#ifdef /#ifndef type). Thread support is built into java but not in C++.Java does not support default arguments. There’s no scope resolution operator :: in Java. Java uses the dot for everything, but can get away with it since you can define elements only within a class. Even the method definitions must always occur within a class, so there is no need for scope

resolution there either. There’s no "goto " statement in Java. Java doesn’t provide multiple inheritance (MI), at least not in the same sense that C++ does. Exception handling in Java is different because there are no destructors. Java has method overloading, but no operator overloading. The String class does use the + and += operators to concatenate strings and String expressions use automatic type conversion, but that’s a special built-in case. Java is interpreted for the most part and hence platform independent.

15) What are interfaces?

Interfaces provide more sophisticated ways to organize and control the objects in your system.

The interface keyword takes the abstract concept one step further. You could think of it as a “pure” abstract class. It allows the creator to establish the form for a class: method names, argument lists, and return types, but no method bodies. An interface can also contain fields, but The interface keyword takes the abstract concept one step further. You could think of it as a “pure” abstract class. It allows the creator to establish the form for a class: method names, argument lists, and return types, but no method bodies. An interface can also contain fields, but An interface says: “This is what all classes that implement this particular interface will look like.” Thus, any code that uses a particular interface knows what methods might be called for that interface, and that’s all. So the interface is used to establish a “protocol” between classes. (Some object-oriented programming languages have a keyword called protocolto do the same thing.)

15) How can you achieve Multiple Inheritance in Java?

Java's interface mechanism can be used to implement multiple inheritance, with one important difference from c++ way of doing MI: the inherited interfaces must be abstract. This obviates the need to choose between different implementations, as with interfaces there are no implementations.

16) What is the difference between StringBuffer and String class?

A string buffer implements a mutable sequence of characters. A string buffer is like a String, but can be

modified. At any point in time it contains some particular sequence of characters, but the length and content of the sequence can be changed through certain method calls. The String class represents character strings. All string literals in Java programs, such as "abc" are constant and implemented as instances of this class; their values cannot be changed after they are created.

17) Describe, in general, how java's garbage collector works?

The Java runtime environment deletes objects when it determines that they are no longer being used. This process is known as garbage collection. The Java runtime environment supports a garbage collector that periodically frees the memory used by objects that are no longer needed. The Java garbage collector is a mark-sweep garbage collector that scans Java's dynamic memory areas for objects, marking those that are referenced. After all possible paths to objects are investigated, those objects that are not marked (i.e. are not referenced) are known to be garbage and are collected.

18) What's the difference between == and equals method?

The equals method can be considered to perform a deep comparison of the value of an object, whereas the == operator performs a shallow comparison. The equals() method compares the characters inside a string object. == operator compares two object references to check whether they refer to the same instances or not.

19) What are abstract classes, abstract methods?

Simply speaking a class or a method qualified with "abstract" keyword is an abstract class or abstract

method. You create an abstract class when you want to manipulate a set of classes through a common interface. All derived-class methods that match the signature of the base-class declaration will be called using the dynamic binding mechanism. An abstract method is an incomplete method. It has only a declaration and no method body. Here is the syntax for an abstract method declaration: abstract void f();

20) How can you force all derived classes to implement a method present in the base class?

Creating and implementing an interface would be the best way for this situation. Just create an interface

with empty methods which forces a programmer to implement all the methods present under it.

Another way of achieving this task is to declare a class as abstract with all its methods abstract.

21) What is the difference between an Applet and an Application?

1. Applets can be embedded in HTML pages and downloaded over the Internet whereas Applications have no special support in HTML for embedding or downloading.

2. Applets can only be executed inside a java compatible container, such as a browser or appletviewer

whereas Applications are executed at command line by java.exe or jview.exe.

3. Applets execute under strict security limitations that disallow certain operations(sandbox model security) whereas Applications have no inherent security restrictions.

4. Applets don't have the main() method as in applications. Instead they operate on an entirely different

mechanism where they are initialized by init(),started by start(),stopped by stop() or destroyed by destroy().

22) Java says "write once, run anywhere". What are some ways this isn't quite true?

Any time you use system calls specific to one operating system and do not create alternative calls for

another operating system, your program will not function correctly.Solaris systems and Intel systems order the bits of an integer differently. (You may have heard of little  endian vs. big endian) If your code uses bit shifting, or other binary operators, they will not work on systems that have opposide endianism.

23) Describe java's security model.

Java's security model is one of the most interesting and unique aspects of the language. For the most part it's broken into two pieces: the user adjustable security manager that checks various API operations like file access, and the byte code verifier that asserts the validity of compiled byte code.

public abstract class SecurityManager java.lang.SecurityManager is an abstract class which different

applications subclass to implement a particular security policy. It allows an application to determine

whether or not a particular operation will generate a security exception.

24) What is the difference between a Vector and an Array. Discuss the advantages and disadvantages

of both?

The vector container class generalizes the concept of an ordinary C array. Like an array, a vector is an

indexed data structure, with index values that range from 0 to one less than the number of elements

contained in the structure. Also like an array, values are most commonly assigned to and extracted from the vector using the subscript operator. However, the vector differs from an array in the following important respects: The size of the vector can change dynamically. New elements can be inserted on to the end of a vector, or into the middle. It is important to note, however, that while these abilities are provided, insertion into the middle of a vector is not as efficient as insertion into the middle of a list.

A vector has more "self-knowledge" than an ordinary array. In particular, a vector can be queried about its size, about the number of elements it can potentially hold (which may be different from its current size), and so on. A vector can only hold references to objects and not primitive types.

Vector Implementaions are usually slower then array because of all the functionality that comes with them. As implemented in Java, vector is a thread-safe class and hence all methods are synchronous methods, which makes them considerably slow.

25) How many different types of JDBC drivers are present? Discuss them.

Type 1: JDBC-ODBC Bridge plus ODBC Driver:

The first type of JDBC driver is the JDBC-ODBC Bridge. It is a driver that provides JDBC access to

databases through ODBC drivers. The ODBC driver must be configured on the client for the bridge to

work. This driver type is commonly used for prototyping or when there is no JDBC driver available for a particular DBMS.

Type 2: Native-API partly-Java Driver:

The Native to API driver converts JDBC commands to DBMS-specific native calls. This is much like therestriction of Type 1 drivers. The client must have some binary code loaded on its machine. These drivers do have an advantage over Type 1 drivers because they interface directly with the database.

Type 3: JDBC-Net Pure Java Driver:

The JDBC-Net drivers are a three-tier solution. This type of driver translates JDBC calls into a databaseindependent network protocol that is sent to a middleware server. This server then translates this DBMSindependent protocol into a DBMS-specific protocol, which is sent to a particular database. The results are then routed back through the middleware server and sent back to the client. This type of solution makes it possible to implement a pure Java client. It also makes it possible to swap databases without affecting the client.

Type 4: Native-Protocol Pur Java Driver

These are pure Java drivers that communicate directly with the vendor's database. They do this by

converting JDBC commands directly into the database engine's native protocol. This driver has no

additional translation or middleware layer, which improves performance tremendously.

26) What does the keyword "synchronize" mean in java. When do you use it? What are the

disadvantages of synchronization?

Synchronize is used when u want to make ur methods thread safe. The disadvantage of synchronise is it

will end up in slowing down the program. Also if not handled properly it will end up in dead lock.

1. Only use (and minimize it's use)synchronization when writing multithreaded code as there is a speed (up to five to six time slower, depending on the execution time of the synchronized/non-synchronized method ) cost associated with its use.

2. In case of syncronized method modifier, the byte code generated is the exact same as non-syncronized method. The only difference is that a flag called ACC_SYNCRONIZED property flag in method's method_info structure is set if the syncronized method modifier is present.

3. Also, syncronized keyword can make the code larger in size if used in the body of the method as

bytecode for monitorenter/monitorexit is generated in addition to any exception handling.

27) What are native methods? How do you use them?

Native methods are methods that are defined as public static methods within a java class, but whose

implementation is provided in another programming language such as C.

28) What is RMI?

RMI stands for Remote Method Invocation. Traditional approaches to executing code on other machines across a network have been confusing as well as tedious and error-prone to implement. The nicest way to think about this problem is that some object happens to live on another machine, and that you can send a message to the remote object and get a result as if the object lived on your local machine. This simplification is exactly what Java Remote Method Invocation (RMI) allows you to do.

29) What is JDBC? Describe the steps needed to execute a SQL query using JDBC.

The JDBC is a pure Java API used to execute SQL statements. It provides a set of classes and interfaces that can be used by developers to write database applications.

The steps needed to execute a SQL query using JDBC:

1. Open a connection to the database.

2. Execute a SQL statement.

3. Process th results.

4. Close the connection to the database.

30) Access specifiers: "public", "protected", "private", nothing?

Public – any other class from any package can instantiate and execute the classes and methods

Protected – only subclasses and classes inside of the package can access the classes and methods

Private – the original class is the only class allowed to executed the methods.

31) What does the "final" keyword mean in front of a variable? A method? A class?

FINAL for a variable : value is constant

FINAL for a method : cannot be overridden

FINAL for a class : cannot be derived

32) Does Java have "goto"?

no

33) Why "bytecode"? Can you reverse-engineer the code from bytecode?

34) What synchronization constructs does Java provide? How do they work?

35) Are constructors inherited? Can a subclass call the parent's class constructor? When?

You cannot inherit a constructor. That is, you cannot create a instance of a subclass using a constructor of one of it's superclasses. One of the main reasons is because you probably don't want to overide the

superclasses constructor, which would be possible if they were inherited. By giving the developer the

ability to override a superclasses constructor you would erode the encapsulation abilities of the language.

36) Does Java have destructors?

No garbage collector does the job working in the background

37) What does the "abstract" keyword mean in front of a method? A class?

Abstract keyword declares either a method or a class. If a method has a abstract keyword in front of it,it is called abstract method.Abstract method hs no body.It has only arguments and return type.Abstract methods act as placeholder methods that are implemented in the subclasses. Abstract classes can't be instantiated.If a class is declared as abstract,no objects of that class can be

created.If a class contains any abstract method it must be declared as abstract

38) Name four methods every Java class will have.

public String toString();

public Object clone();

public boolean equals();

public int hashCode();

39) Given a text file, input.txt, provide the statement required to open

this file with the appropriate I/O stream to be able to read and process this file.

40) Discuss the differences between creating a new class, extending a class and implementing an

interface; and when each would be appropriate.

*Creating a new class is simply creating a class with no extensions and no

implementations. The signature is as follows

public class MyClass()

{

}

*Extending a class is when you want to use the functionality of another class or

classes. The extended class inherits all of the functionality of the previous class. An

example of this when you create your own applet class and extend from

java.applet.Applet. This gives you all of the functionality of the java.applet.Applet class.

The signature would look like this

public class MyClass extends MyBaseClass

{

}

*Implementing an interface simply forces you to use the methods of the interface

implemented. This gives you two advantages. This forces you to follow a standard

(forces you to use certain methods) and in doing so gives you a channel for

polymorphism. This isn’t the only way you can do polymorphism but this is one of the

ways.

public class Fish implements Animal

{

}

40) What's the difference between the == operator and the equals() method? What test does

Object.equals() use, and why?

The == operator would be used, in an object sense, to see if the two objects were actually the same object. This operator looks at the actually memory address to see if it actually the same object. The equals() method is used to compare the values of the object respectively. This is used in a higher level to see if the object values are equal. Of course the the equals() method would be overloaded in a meaningful way for whatever object that you were working with.

41) why do you create interfaces, and when MUST you use one.

You would create interfaces when you have two or more functionalities talking to each other. Doing it this way help you in creating a protocol between the parties involved.

42) What is the difference between instanceof and isInstance?

instanceof is used to check to see if an object can be cast into a specified type without throwing a cast class exception. isInstance()

Determines if the specified Object is assignment-compatible with the object represented by this Class. This method is the dynamic equivalent of the Java language instanceof operator. The method returns true if the specified Object argument is non-null and can be cast to the reference type represented by this Class object without raising a ClassCastException. It returns false otherwise.

43) How many methods do u implement if implement the Serializable Interface?

The Serializable interface is just a "marker" interface, with no methods of its own to implement.

Are there any other 'marker' interfaces?

java.rmi.Remote

java.util.EventListener

44) *What are the advantages of developing an n-tiered system?

45) *Why is it often difficult to separate the business layer from the data access layer?

46) . Diff between ArrayList and Vector

47) Variable shadowing with example

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