In this post we are covering some important java Interview Questions and Answers:
Q1. What is the difference between an Inner Class and a Sub-Class?
Ans: An Inner class is a class which is nested within another class. An Inner class has access rights for the class which is nesting it and it can access all variables and methods defined in the outer class.
A sub-class is a class which inherits from another class called super class. Sub-class can access all public and protected methods and fields of its super class.
Q2: How can you generate random numbers in Java?
Ans:
- Using Math.random() you can generate random numbers in the range greater than or equal to 0.1 and less than 1.0
- Using Random class in package java.util
Ans: Yes we can create an abstract class by using abstract keyword before class name even if it doesn’t have any abstract method. However, if a class has even one abstract method, it must be declared as abstract otherwise it will give an error.
Q4. What’s the difference between an Abstract Class and Interface in Java?
Ans: The primary difference between an abstract class and interface is that an interface can only possess declaration of public static methods with no concrete implementation while an abstract class can have members with any access specifiers (public, private etc) with or without concrete implementation.
Another key difference in the use of abstract classes and interfaces is that a class which implements an interface must implement all the methods of the interface while a class which inherits from an abstract class doesn’t require implementation of all the methods of its super class.
A class can implement multiple interfaces but it can extend only one abstract class.
Q5. Can we declare the main method of our class as private?
Ans: In java, main method must be public static in order to run any application correctly. If main method is declared as private, developer won’t get any compilation error however, it will not get executed and will give a runtime error.
Q6. Is it compulsory for a Try Block to be followed by a Catch Block in Java for Exception handling?
Ans: Try block needs to be followed by either Catch block or Finally block or both. Any exception thrown from try block needs to be either caught in the catch block or else any specific tasks to be performed before code abortion are put in the Finally block.
Q7. Is there any way to skip Finally block of exception even if some exception occurs in the exception block?
Ans: If an exception is raised in Try block, control passes to catch block if it exists otherwise to finally block. Finally block is always executed when an exception occurs and the only way to avoid execution of any statements in Finally block is by aborting the code forcibly by writing following line of code at the end of try block:
System.exit(0);
Q8. Can a class have multiple constructors?
Ans: Yes, a class can have multiple constructors with different parameters. Which constructor gets used for object creation depends on the arguments passed while creating the objects.
Q9. Why Strings in Java are called as Immutable?
Ans: In java, string objects are called immutable as once value has been assigned to a string, it can’t be changed and if changed, a new object is created.
In below example, reference str refers to a string object having value “Value one”.
String str = "Value One";
When a new value is assigned to it, a new String object gets created and the reference is moved to the new object.
str = "New Value";
Q10. How garbage collection is done in Java?
Ans: In java, when an object is not referenced any more, garbage collection takes place and the object is destroyed automatically. For automatic garbage collection java calls either System.gc() method or Runtime.gc() method.
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