Java WebDynpro Interview Questions
What is the purpose of garbage collection in Java, and when is it used?
The purpose of garbage collection is to identify and discard objects that are no longer needed by a program so that their resources can be reclaimed and reused. A Java object is subject to garbage collection when it becomes unreachable to the program in which it is used.
What if the main method is declared as private?
The program compiles properly but at runtime it will give "Main method not public." message.
What if I write static public void instead of public static void?
Program compiles and runs properly.
What if I do not provide the String array as the argument to the method?
Program compiles but throws a runtime error "NoSuchMethodError".
What is the first argument of the String array in main method?
The String array is empty. It does not have any element. This is unlike C/C++ where the first element by default is the program name
How can one prove that the array is not null but empty using one line of code?
Print args.length. It will print 0. That means it is empty. But if it would have been null then it would have thrown a Null Pointer Exception on attempting to print args.length.
What are Checked and Unchecked Exception?
A checked exception is some subclass of Exception (or Exception itself), excluding class Runtime Exception 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 Runtime Exception 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 char At () method· Checked exceptions must be caught at compile time. Runtime exceptions do not need to be. Errors often cannot be.
What are different types of inner classes?
Nested top-level classes, Member classes, Local classes, Anonymous classes Nested top-level classes- If you declare a class within a class and specify the static modifier, the compiler treats the class just like any other top-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. Top-level inner classes implicitly have access only to static variables. There can also be inner interfaces. These are of the nested top-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 top-level class. The primary difference between member classes and nested top-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. 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.
What is the difference between an Interface and an Abstract class?
An abstract class can have instance methods that implement a default behavior. An Interface can only declare constants and instance methods, but cannot implement default behavior and all methods are implicitly abstract. An interface has all public members and no implementation. An abstract class is a class which may have the usual flavors of class members (private, protected, etc.), but has some abstract methods.
Describe synchronization in respect to multithreading?
With respect to multithreading, synchronization is the capability to control the access of multiple threads to shared resources. Without synchronization, it is possible for one thread to modify a shared variable while another thread is in the process of using or updating same shared variable. This usually leads to significant errors.
What are pass by reference and pass by value?
Pass by Reference means the passing the address itself rather than passing the value. Passby Value means passing a copy of the value to be passed.
What is HashMap and Map?
Map is Interface and Hashmap is class that implements that.
Difference between HashMap and HashTable?
The HashMap class is roughly equivalent to Hashtable, except that it is unsynchronized and permits nulls. (HashMap allows null values as key and value whereas Hashtable doesnt allow). HashMap does not guarantee that the order of the map will remain constant over time. HashMap is unsynchronized and Hashtable is synchronized.
Difference between Vector and ArrayList?
Vector is synchronized whereas arraylist is not.
Difference between Swing and Awt?
AWT are heavy-weight components. Swings are light-weight components. Hence swing works faster than AWT.
What is the difference between a constructor and a method?
A constructor is a member function of a class that is used to create objects of that class. It has the same name as the class itself, has no return type, and is invoked using the new operator. A method is an ordinary member function of a class. It has its own name, a return type (which may be void), and is invoked using the dot operator.
What is an Iterator?
Some of the collection classes provide traversal of their contents via a java.util.Iterator interface. This interface allows you to walk through a collection of objects, operating on each object in turn. Remember when using Iterators that they contain a snapshot of the collection at the time the Iterator was obtained; generally, it is not advisable to modify the collection itself while traversing an Iterator.
What is an abstract class?
Abstract class must be extended/subclassed (to be useful). It serves as a template. A class that is abstract may not be instantiated (ie, you may not call its constructor), abstract class may contain static data. Any class with an abstract method is automatically abstract itself, and must be declared as such. A class may be declared abstract even if it has no abstract methods. This prevents it from being instantiated.
What is static in java?
Static means one per class, not one for each object no matter how many instance of a class might exist. This means that you can use them without creating an instance of a class.Static methods are implicitly final, because overriding is done based on the type of the object, and static methods are attached to a class, not an object. A static method in a superclass can be shadowed by another static method in a subclass, if the original method was not declared final. However, you can't override a static method with a non-static method. In other words, you can't change a static method into an instance method in a subclass.
What is final?
A final class can't be extended ie., final class may not be subclasses. A final method can't be overridden when its class is inherited. You can't change value of a final variable (is a constant)
Are the imports checked for validity at compile time? e.g. will the code containing an import such as java.lang.ABCD compile?
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, cannot resolve symbol
symbol: class ABCD
location: package io
What is the difference between declaring a variable and defining a variable?
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.
Objects are passed by value or by reference?
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.
What is serialization?
Serialization is a mechanism by which you can save the state of an object by converting it to a byte stream.
How do I serialize an object to a file?
The class whose instances are to be serialized should implement an interface Serializable. Then you pass the instance to the Object Output Stream which is connected to a file output stream. This will save the object to a file.
Which methods of Serializable interface should I implement?
The serializable interface is an empty interface; it does not contain any methods. So, we do not implement any methods.
What is the common usage of serialization?
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.
What is Externalizable interface?
Externalizable is an interface which contains two methods read External and write External. 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.
What one should take care of while serializing the object?
One should make sure that all the included objects are also serializable. If any of the objects is not serializable then it throws a Not Serializable Exception.
What happens to the static fields of a class during serialization?
There are three exceptions in which serialization does not necessarily read and write to the stream.
-Serialization ignores static fields, because they are not part of any state.
-Base class fields are only handled if the base class itself is serializable.
What are the different ways to handle exceptions?
There are two ways to handle exceptions:
By wrapping the desired code in a try block followed by a catch block to catch the exceptions.
List the desired exceptions in the throws clause of the method and let the caller of the method handle those exceptions.
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?
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 its 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.