Tricky Java interview questions and answers


Q. Can interfaces contain inner classes?
A. Yes, but not recommended as it can compromise on clarity of your code.

The interface with an inner class for demo purpose only

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package innerstatic;
 
public interface InterfaceWithInnerClass {
     
     
    public static final Util UTIL = new Util( );
     
 
    public String getName(); // next node in this list
 
    public final static InterfaceWithInnerClass FIELD =
      //anonymous inner class as the interface field declaration 
      new InterfaceWithInnerClass() {
          public String getName() {
            return "Peter";
        }
    };
 
     
 //static inner class
    public static class Util {
        public String getDetail(){
            return FIELD.getName() + " age 25";
        }
    }
}


The class that implemnts the above interface.


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package innerstatic;
 
public class Test implements InterfaceWithInnerClass {
 
    @Override
    public String getName() {
        return FIELD.getName() + " : " + UTIL.getDetail();
    }
     
    public static void main(String[] args) {
        System.out.println(new Test().getName());
    }
}


The output is:

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Peter : Peter age 25


Q. What happens if you pass a primitive int value to a method that accepts

a) long primitive
b) float primitive
c) Float object
d) Number object

A. 

a) A widening conversion takes place from int to long. So, no compile or run time error.
b) A widening conversion takes place from int to float. So, no compile or run time error.
c) compile-time error. primitive type int can be auto-boxed to type Integer, but type Integer and Float are derived from type Number, and don't have the parent child relationship. So, they cannot be implicityly or exlplicitly cast to each other.
d) primitive type int can be auto-boxed to type Integer, and then implicitly cast to type Number as Number and Integer have the parent child relationship. So, no compile or run time error.



Q. What is the purpose of the intern( ) method in the String class?
A. It is used for moving the String objects created with the "new" operator to the internal string pool.



Q. Why is the String class in Java is final immutable object?
A.

Security: the system can pass on sensitive info like password, credit card number, etc as read-only information without worrying that it will be altered.

Immutable objects are thread-safe. Two or more threads can  work on an immutable object at the same time without any possibility of conflict. Immutable String objects are great to be used as keys in a map without worrying about the key getting modified.

To keep things simple. Since final, you can't subclass it to have two different String objects that have the same value. It is also easier to implement some of the string methods like substring because of its immutability.


Q. Is the finally block is always guaranteed to be execute?
A. No. The finally block is not executed under circumstances like

  • System.exit( ) in try or catch block.
  • If the thread executing the try or catch block is killed or interrupted. 
  • If an exception is thrown from the finally block and not handled, then remaining code in the finally block is ignored.

Q. What is the purpose of 'const' reserved keyword in Java?
A. It is reserved, but not currently used in Java. 

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