Java Inteerview Question Set I

Explain Final keyword in java?
 

Final keyword in java is used to restrict usage of variable, class and method.

Variable: Value of Final variable is constant, you can not change it.
Method: you can’t override a Final method.
Class: you can’t inherit from Final class.

When is the super keyword used?

super keyword is used to refer:

• immediate parent class constructor,
• immediate parent class variable,
• immediate parent class method.

What is the difference between StringBuffer and String?
 

String is an Immutable class, i.e. you can not modify its content once created. While StringBuffer is a mutable class, means you can change its content later. Whenever we alter content of String object, it creates a new string and refer to that,it does not modify the existing one. This is the reason that the performance with StringBuffer is better than with String.

  Why multiple inheritance is not supported in java?
 

Java supports multiple inheritance but not through classes, it supports only through its interfaces. The reason for not supporting multiple inheritance is to avoid the conflict and complexity arises due to it and keep Java a Simple Object Oriented Language. If we recall this in C++, there is a special case of multiple inheritance (diamond problem) where you have a multiple inheritance with two classes which have methods in conflicts. So, Java developers decided to avoid such conflicts and didn’t allow multiple inheritance through classes at all.

  Can a top level class be private or protected?
 

Top level classes in java can’t be private or protected, but inner classes in java can. The reason for not making a top level class as private is very obvious, because nobody can see a private class and thus they can not use it. Declaring a class as protected also doesn’t make any sense. The only difference between default visibility and protected visibility is that we can use it in any package by inheriting it. Since in java there is no such concept of package inheritance, defining a class as protected is no different from default.

  What is the difference between ‘throw’ and ‘throws’ in Java Exception Handling?
 

Following are the differences between two:

• throw keyword is used to throw Exception from any method or static block whereas throws is used to indicate that which Exception can possibly be thrown by this method
• If any method throws checked Exception, then caller can either handle this exception(using try catch block )or can re throw it by declaring another ‘throws’ clause in method declaration.
• throw clause can be used in any part of code where you feel a specific exception needs to be thrown to the calling method

E.g. throw
throw new Exception(“You have some exception”)
throw new IOException(“Connection failed!!”) throws
throws IOException, NullPointerException, ArithmeticException

  What is finalize() method?

Unlike c++ , we don’t need to destroy objects explicitly in Java. ‘Garbage Collector‘ does that automatically for us. Garbage Collector checks if no references to an object exist, that object is assumed to be no longer required, and the memory occupied by the object can be freed. Sometimes an object can hold non-java resources such as file handle or database connection, then you want to make sure these resources are also released before object is destroyed. To perform such operation Java provide protected void finalize() in object class. You can override this method in your class and do the required tasks. Right before an object is freed, the java run time calls the finalize() method on that object.

  Difference in Set and List interface?
 

Set and List both are child interface of Collection interface. There are following two main differences between them

• List can hold duplicate values but Set doesn’t allow this.
• In List interface data is present in the order you inserted but in the case of Set insertion order is not preserved.

 

What will happen if you put System.exit(0) on try or catch block? Will finally block execute?

 

By Calling System.exit(0) in try or catch block, we can skip the finally block. System.exit(int) method can throw a SecurityException. If Sysytem.exit(0) exits the JVM without throwing that exception then finally block will not execute. But, if System.exit(0) does throw security exception then finally block will be executed.

Can you achieve Runtime Polymorphism by data members?

No.

What is the difference between static binding and dynamic binding?

In case of static binding type of object is determined at compile time whereas in dynamic binding type of object is determined at runtime.

Can we Overload or Override static methods in java ?

• Overriding : Overriding is related to run-time polymorphism. A subclass (or derived class) provides a specific implementation of a method in superclass (or base class) at runtime.
  
• Overloading: Overloading is related to compile time (or static) polymorphism. This feature allows different methods to have same name, but different signatures, especially number of input parameters and type of input paramaters.
  
• Can we overload static methods?   The answer is ‘Yes’. We can have two ore more static methods with same name, but differences in input parameters.
  
• Can we Override static methods in java?  We can declare static methods with same signature in subclass, but it is not considered overriding as there won’t be any run-time polymorphism. Hence the answer is ‘No’. Static methods cannot be overridden because method overriding only occurs in the context of dynamic (i.e. runtime) lookup of methods. Static methods (by their name) are looked up statically (i.e. at compile-time).

Why the main method is static in java?

 
The method is static because otherwise there would be ambiguity: which constructor should be called? Especially if your class looks like this:

public class JavaClass { protected JavaClass(int x) { } public void main(String[] args) { } }

Should the JVM call new JavaClass(int)? What should it pass for x? If not, should the JVM instantiate JavaClass without running any constructor method? because that will special-case your entire class – sometimes you have an instance that hasn’t been initialized, and you have to check for it in every method that could be called. There are just too many edge cases and ambiguities for it to make sense for the JVM to have to instantiate a class before the entry point is called. That’s why main is static.

What happens if you remove static modifier from the main method?

 
Program compiles successfully . But at runtime throws an error “NoSuchMethodError”.

What is the scope of variables in Java in following cases? 

• Member Variables (Class Level Scope) : The member variables must be declared inside class (outside any function). They can be directly accessed anywhere in class.
  
• Local Variables (Method Level Scope) : Variables declared inside a method have method level scope and can’t be accessed outside the method.
  
• Loop Variables (Block Scope) : A variable declared inside pair of brackets “{” and “}” in a method has scope withing the brackets only.

What is “this” keyword in java?

 
Within an instance method or a constructor, this is a reference to the current object — the object whose method or constructor is being called. You can refer to any member of the current object from within an instance method or a constructor by using this.
Usage of this keyword

• Used to refer current class instance variable.
• To invoke current class constructor.
• It can be passed as an argument in the method call.
• It can be passed as argument in the constructor call.
• Used to return the current class instance.
• Used to invoke current class method (implicitly)

What is an abstract class? How abstract classes are similar or different in Java from C++?
Abstract classes are classes that contain one or more abstract methods. An abstract method is a method that is declared, but contains no implementation. Abstract classes may not be instantiated, and require subclasses to provide implementations for the abstract methods.

• Like C++, in Java, an instance of an abstract class cannot be created, we can have references of abstract class type though.
• Like C++, an abstract class can contain constructors in Java. And a constructor of abstract class is called when an instance of a inherited class is created
• In Java, we can have an abstract class without any abstract method. This allows us to create classes that cannot be instantiated, but can only be inherited.
• Abstract classes can also have final methods (methods that cannot be overridden). For example, the following program compiles and runs fine.

Which class is the superclass for every class ?
Object class

Can we overload main() method?
The main method in Java is no extra-terrestrial method. Apart from the fact that main() is just like any other method & can be overloaded in a similar manner, JVM always looks for the method signature to launch the program.

• The normal main method acts as an entry point for the JVM to start the execution of program.
• We can overload the main method in Java. But the program doesn’t execute the overloaded main method when we run your program, we need to call the overloaded main method from the actual main method only.

What is object cloning?
Object cloning means to create an exact copy of the original object. If a class needs to support cloning, it must implement java.lang.Cloneable interface and override clone() method from Object class. Syntax of the clone() method is :

protected Object clone() throws CloneNotSupportedException

If the object’s class doesn’t implement Cloneable interface then it throws an exception ‘CloneNotSupportedException’ .

How is inheritance in C++  different from Java?

1. In Java, all classes inherit from the Object class directly or indirectly. Therefore, there is always a single inheritance tree of classes in Java, and Object class is root of the tree.
2. In Java, members of the grandparent class are not directly accessible. See this G-Fact for more details.
3. The meaning of protected member access specifier is somewhat different in Java. In Java, protected members of a class “A” are accessible in other class “B” of same package, even if B doesn’t inherit from A (they both have to be in the same package).
4. Java uses extends keyword for inheritance. Unlike C++, Java doesn’t provide an inheritance specifier like public, protected or private. Therefore, we cannot change the protection level of members of base class in Java, if some data member is public or protected in base class then it remains public or protected in derived class. Like C++, private members of base class are not accessible in derived class.
   Unlike C++, in Java, we don’t have to remember those rules of inheritance which are combination of base class access specifier and inheritance specifier.
5. In Java, methods are virtual by default. In C++, we explicitly use virtual keyword. See this G-Fact for more details.
6. Java uses a separate keyword interface for interfaces, and abstract keyword for abstract classes and abstract functions.
7. Unlike C++, Java doesn’t support multiple inheritance. A class cannot inherit from more than one class. A class can implement multiple interfaces though.
8. In C++, default constructor of parent class is automatically called, but if we want to call parametrized constructor of a parent class, we must use Initializer list. Like C++, default constructor of the parent class is automatically called in Java, but if we want to call parameterized constructor then we must use super to call the parent constructor.

Why method overloading is not possible by changing the return type in java?
In C++ and Java, functions can not be overloaded if they differ only in the return type . The return type of functions is not a part of the mangled name which is generated by the compiler for uniquely identifying each function. The No of arguments, Type of arguments & Sequence of arguments are the parameters which are used to generate the unique mangled name for each function. It is on the basis of these unique mangled names that compiler can understand which function to call even if the names are same(overloading).

Can we override private methods in Java?

No, a private method cannot be overridden since it is not visible from any other class.