Java Interview Questions: Series 3

 27. Why there are no pointers in

Java?

In Java there are references instead of pointers. These references

point to objects in memory. But there is no direct access to these

memory locations. JVM is free to move the objects within VM

memory.

The absence of pointers helps Java in managing memory and

garbage collection effectively. Also it provides developers with

convenience of not getting worried about memory allocation and deallocation.

28. If there are no pointers in Java,

then why do we get

NullPointerException?

In Java, the pointer equivalent is Object reference. When we use a .

it points to object reference. So JVM uses pointers but

programmers only see object references.

In case an object reference points to null object, and we try to

access a method or member variable on it, then we get

NullPointerException.

29. What is the purpose of ‘super’

keyword in java?

‘super’ keyword is used in the methods or constructor of a child

class. It refers to immediate parent class of an object.

By using ‘super’ we can call a method of parent class from the

method of a child class.

We can also call the constructor of a parent class from the

constructor of a child class by using ‘super’ keyword.

30. Is it possible to use this() and

super() both in same constructor?

No, Java does not allow using both super() and this() in same

constructor. As per Java specification, super() or this() must be the

first statement in a constructor.

31. What is the meaning of object

cloning in Java?

Object.clone() method is used for creating an exact copy of the

object in Java. It acts like a copy constructor. It creates and returns

a copy of the object, with the same class and with all the fields

having same values as of the original object.

One disadvantage of cloning is that the return type is an Object. It

has to be explicitly cast to actual type.

Static

32. In Java, why do we use static

variable?

Whenever we want to have a common property for all objects of a

class, we use a class level variable i.e. a static variable.

This variable is loaded in memory only once at the time of class

loading. So it saves memory, since it is not defined per object in

Java.

33. Why it is not a good practice to

create static variables in Java?

Static variables are common to all the objects of a class. If a new

object is created, there is no need to test the value of static variable.

Any code that uses static variable can be in any state. It can be

within a new object or at a class level. So the scope of static

variable is open ended in a Java class.

If we want tighter control on scope, then variables should be

created at the object creation level.

Also defining static variables is not a good practice because they go

against the principles of Object Oriented Programming.

34. What is the purpose of static

method in Java?

Java provides the feature of static method to create behavior at the

class level. The static method is common to all the objects of a

class. We do not need to create any object of a class to call a static

method. So it provides convenience of not creating an object for

calling it.

Also a static method can access and modify static data members.

This also helps in keeping the behavior as well as state at the class

level.

35. Why do we mark main method as

static in Java?

The main method in Java is marked as static, so that JVM can call it

to start the program. If main method is not static, then which

constructor will be called by Java process?

As such it is a known as convention to mark main method static in

Java. But if we remove the static, then there will be ambiguity. Java

process may not know which method of a class to call to start the

program.

So this convention helps in Java process to identify the starting code

for a program in class that is passed as an argument to java process.

36. In what scenario do we use a static

block?

At times, there is a class that has static member variables. These

variables need some complicated initialization. At this time static

block helps as a tool to initialize complex static member variable

initialization.

The static block is executed even before the execution of main.

Sometimes, we can also replace static block with a static method of

class.

37. Is it possible to execute a program

without defining a main() method?

No, with Java 7 onwards, you need a main() method to execute a

program. In earlier versions of Java, there was a workaround

available to use static blocks for execution. But now this gap has

been closed.

38. What happens when static

modifier is not mentioned in the

signature of main method?

As per Java specification, main method has to be marked as static.

It needs only one argument that is an array of String.

A program can compile with a non-static method. But on execution

it will give NoSuchMethodError.

39. What is the difference between

static method and instance method in

Java?

Often, there is a need to define a behavior for a class that is not

dependent on member variables of an object. Such behavior is

captured in a static method. If there is a behavior dependent upon

the member variables of an object, then we do not mark it static, it

remains as instance method.

To call as static method, we do not need to create an object. We just

call it with class name. But to call an instance method, we need to

create/get an object first.

Instance member variables cannot be accessed by a static method.

But an instance method can call both instance variables and static

variables.

40. What is the other name of Method

Overloading?

Method Overloading is also known as Static Polymorphism.

41. How will you implement method

overloading in Java?

In Java, a class can have multiple methods with same name but

different arguments. It is called Method Overloading. To implement

method overloading we have to create two methods with same name

in a class and do one/more of the following:

1. Different number of parameters

2. Different data type of parameters

3. Different sequence of data type of parameters

42. What kinds of argument

variations are allowed in Method

Overloading?

Method Overloading allows two methods with same name to differ

in:

1. Number of parameters

2. Data type of parameters

3. Sequence of data type of parameters

43. Why it is not possible to do

method overloading by changing

return type of method in java?

If we change the return type of overloaded methods then it will lead

to ambiguous behavior. How will clients know which method will

return what type. Due to this different return type are not allowed in

overloaded methods.

44. Is it allowed to overload main()

method in Java?

Yes, Java allows users to create many methods with same name

‘main’. But only public static void main(String[] args) method is

used for execution.

45. How do we implement method

overriding in Java?

To override a method, we just provide a new implementation of a

method with same name in subclass. So there will be at least two

implementations of the method with same name. One

implementation is in parent class. And another implementation is in

child class.

46. Are we allowed to override a static

method in Java?

No. Java does not allow overriding a static method. If you create a

static method with same name in subclass, then it is a new method,

not an overridden method.

47. Why Java does not allow

overriding a static method?

To override a method, you need an instance of a class. Static method

is not associated with any instance of the class. So the concept of

overriding does not apply here.

Therefore, Java does not allow overriding a static method.

48. Is it allowed to override an

overloaded method?

Yes. You can override an overloaded method in Java.

49. What is the difference between

method overloading and method

overriding in Java?

Differences between method overloading and overriding are:

1. Method overloading is static polymorphism. Method

overriding is runtime polymorphism.

2. Method overloading occurs within the same class. Method

overriding happens in two classes with hierarchy

relationship.

3. Parameters must be different in method overloading.

Parameters must be same in method overriding.

4. Method overloading is a compile time concept. Method

overriding is a runtime concept.

50. Does Java allow virtual functions?

Yes. All instance methods in Java are virtual functions by default.

Only class methods and private instance methods are not virtual

methods in Java.