Java Interview Question & Answer Set-4

Q:	What are Checked and UnChecked Exception?
A:	A checked exception is some subclass of Exception (or Exception itself), excluding class RuntimeException and its subclasses. Making an exception checked forces client programmers to deal with the possibility that the exception will be thrown. eg, IOException thrown by java.io.FileInputStream's read() method· Unchecked exceptions are RuntimeException and any of its subclasses. Class Error and its subclasses also are unchecked. With an unchecked exception, however, the compiler doesn't force client programmers either to catch the exception or declare it in a throws clause. In fact, client programmers may not even know that the exception could be thrown. eg, StringIndexOutOfBoundsException thrown by String's charAt() method· Checked exceptions must be caught at compile time. Runtime exceptions do not need to be. Errors often cannot be.

Q:	What is Overriding?
A:	When a class defines a method using the same name, return type, and arguments as a method in its superclass, the method in the class overrides the method in the superclass. When the method is invoked for an object of the class, it is the new definition of the method that is called, and not the method definition from superclass. Methods may be overridden to be more public, not more private.

Q:	What are different types of inner classes?
A:	Nested  -level classes, Member classes, Local classes, Anonymous classes Nested  -level classes- If you declare a class within a class and specify the static modifier, the compiler treats the class just like any other  -level class. Any class outside the declaring class accesses the nested class with the declaring class name acting similarly to a package. eg, outer.inner.  -level inner classes implicitly have access only to static variables.There can also be inner interfaces. All of these are of the nested  -level variety. Member classes - Member inner classes are just like other member methods and member variables and access to the member class is restricted, just like methods and variables. This means a public member class acts similarly to a nested  -level class. The primary difference between member classes and nested  -level classes is that member classes have access to the specific instance of the enclosing class. Local classes - Local classes are like local variables, specific to a block of code. Their visibility is only within the block of their declaration. In order for the class to be useful beyond the declaration block, it would need to implement a more publicly available interface.Because local classes are not members, the modifiers public, protected, private, and static are not usable. Anonymous classes - Anonymous inner classes extend local inner classes one level further. As anonymous classes have no name, you cannot provide a constructor. Q: Are the imports checked for validity at compile time? e.g. will the code containing an import such as java.lang.ABCD compile? A: Yes the imports are checked for the semantic validity at compile time. The code containing above line of import will not compile. It will throw an error saying,can not resolve symbol symbol : class ABCD location: package io import java.io.ABCD; Q: Does importing a package imports the subpackages as well? e.g. Does importing com.MyTest.* also import com.MyTest.UnitTests.*? A: No you will have to import the subpackages explicitly. Importing com.MyTest.* will import classes in the package MyTest only. It will not import any class in any of it's subpackage. Q: What is the difference between declaring a variable and defining a variable? A: In declaration we just mention the type of the variable and it's name. We do not initialize it. But defining means declaration + initialization. e.g String s; is just a declaration while String s = new String ("abcd"); Or String s = "abcd"; are both definitions. Q: What is the default value of an object reference declared as an instance variable? A: null unless we define it explicitly. Q: Can a   level class be private or protected? A: No. A   level class can not be private or protected. It can have either "public" or no modifier. If it does not have a modifier it is supposed to have a default access.If a   level class is declared as private the compiler will complain that the "modifier private is not allowed here". This means that a   level class can not be private. Same is the case with protected. Q: What type of parameter passing does Java support? A: In Java the arguments are always passed by value . Q: Primitive data types are passed by reference or pass by value? A: Primitive data types are passed by value. Q: Objects are passed by value or by reference? A: Java only supports pass by value. With objects, the object reference itself is passed by value and so both the original reference and parameter copy both refer to the same object . Q: What is serialization? A: Serialization is a mechanism by which you can save the state of an object by converting it to a byte stream. Q: How do I serialize an object to a file? A: The class whose instances are to be serialized should implement an interface Serializable. Then you pass the instance to the ObjectOutputStream which is connected to a fileoutputstream. This will save the object to a file. Q: Which methods of Serializable interface should I implement? A: The serializable interface is an empty interface, it does not contain any methods. So we do not implement any methods. Q: How can I customize the seralization process? i.e. how can one have a control over the serialization process? A: Yes it is possible to have control over serialization process. The class should implement Externalizable interface. This interface contains two methods namely readExternal and writeExternal. You should implement these methods and write the logic for customizing the serialization process. Q: What is the common usage of serialization? A: Whenever an object is to be sent over the network, objects need to be serialized. Moreover if the state of an object is to be saved, objects need to be serilazed. Q: What is Externalizable interface? A: Externalizable is an interface which contains two methods readExternal and writeExternal. These methods give you a control over the serialization mechanism. Thus if your class implements this interface, you can customize the serialization process by implementing these methods. Q: When you serialize an object, what happens to the object references included in the object? A: The serialization mechanism generates an object graph for serialization. Thus it determines whether the included object references are serializable or not. This is a recursive process. Thus when an object is serialized, all the included objects are also serialized alongwith the original obect. Q: What one should take care of while serializing the object? A: One should make sure that all the included objects are also serializable. If any of the objects is not serializable then it throws a NotSerializableException. Q: What happens to the static fields of a class during serialization? A: There are three exceptions in which serialization doesnot necessarily read and write to the stream. These are 1. Serialization ignores static fields, because they are not part of ay particular state state. 2. Base class fields are only hendled if the base class itself is serializable. 3. Transient fields.
	Q: Does Java provide any construct to find out the size of an object? A: No there is not sizeof operator in Java. So there is not direct way to determine the size of an object directly in Java. Q: Give a simplest way to find out the time a method takes for execution without using any profiling tool? A: Read the system time just before the method is invoked and immediately after method returns. Take the time difference, which will give you the time taken by a method for execution. To put it in code... long start = System.currentTimeMillis (); method (); long end = System.currentTimeMillis (); System.out.println ("Time taken for execution is " + (end - start)); Remember that if the time taken for execution is too small, it might show that it is taking zero milliseconds for execution. Try it on a method which is big enough, in the sense the one which is doing considerable amout of processing. [ Received from Sandesh Sadhale] Q: What are wrapper classes? A: Java provides specialized classes corresponding to each of the primitive data types. These are called wrapper classes. They are e.g. Integer, Character, Double etc. Q:Why do we need wrapper classes? A: It is sometimes easier to deal with primitives as objects. Moreover most of the collection classes store objects and not primitive data types. And also the wrapper classes provide many utility methods also. Because of these resons we need wrapper classes. And since we create instances of these classes we can store them in any of the collection classes and pass them around as a collection. Also we can pass them around as method parameters where a method expects an object. Q: What are checked exceptions? A: Checked exception are those which the Java compiler forces you to catch. e.g. IOException are checked Exceptions. Q: What are runtime exceptions? A: Runtime exceptions are those exceptions that are thrown at runtime because of either wrong input data or because of wrong business logic etc. These are not checked by the compiler at compile time. Q: What is the difference between error and an exception? A: An error is an irrecoverable condition occurring at runtime. Such as OutOfMemory error. These JVM errors and you can not repair them at runtime. While exceptions are conditions that occur because of bad input etc. e.g. FileNotFoundException will be thrown if the specified file does not exist. Or a NullPointerException will take place if you try using a null reference. In most of the cases it is possible to recover from an exception (probably by giving user a feedback for entering proper values etc.). Q: How to create custom exceptions? A: Your class should extend class Exception, or some more specific type thereof. Q: If I want an object of my class to be thrown as an exception object, what should I do? A: The class should extend from Exception class. Or you can extend your class from some more precise exception type also. Q: If my class already extends from some other class what should I do if I want an instance of my class to be thrown as an exception object? A: One can not do anytihng in this scenarion. Because Java does not allow multiple inheritance and does not provide any exception interface as well. Q: How does an exception permeate through the code? A: An unhandled exception moves up the method stack in search of a matching When an exception is thrown from a code which is wrapped in a try block followed by one or more catch blocks, a search is made for matching catch block. If a matching type is found then that block will be invoked. If a matching type is not found then the exception moves up the method stack and reaches the caller method. Same procedure is repeated if the caller method is included in a try catch block. This process continues until a catch block handling the appropriate type of exception is found. If it does not find such a block then finally the program terminates. Q: What are the different ways to handle exceptions? A: There are two ways to handle exceptions, 1. By wrapping the desired code in a try block followed by a catch block to catch the exceptions. and 2. List the desired exceptions in the throws clause of the method and let the caller of the method hadle those exceptions. Q: What is the basic difference between the 2 approaches to exception handling. 1> try catch block and 2> specifying the candidate exceptions in the throws clause? When should you use which approach? A: In the first approach as a programmer of the method, you urself are dealing with the exception. This is fine if you are in a best position to decide should be done in case of an exception. Whereas if it is not the responsibility of the method to deal with it's own exceptions, then do not use this approach. In this case use the second approach. In the second approach we are forcing the caller of the method to catch the exceptions, that the method is likely to throw. This is often the approach library creators use. They list the exception in the throws clause and we must catch them. You will find the same approach throughout the java libraries we use. Q: Is it necessary that each try block must be followed by a catch block? A: It is not necessary that each try block must be followed by a catch block. It should be followed by either a catch block OR a finally block. And whatever exceptions are likely to be thrown should be declared in the throws clause of the method. Q: If I write return at the end of the try block, will the finally block still execute? A: Yes even if you write return as the last statement in the try block and no exception occurs, the finally block will execute. The finally block will execute and then the control return. Q: If I write System.exit (0); at the end of the try block, will the finally block still execute? A: No in this case the finally block will not execute because when you say System.exit (0); the control immediately goes out of the program, and thus finally never executes.