Output of Java Program | Set 6

Difficulty Level: Intermediate

Predict the Output of the Following Java Programs.

Program 1: Constructor Chaining

Java

class First
{
    public First() {  
        System.out.println("a"); 
    }
}
 
class Second extends First
{
    public Second()  {  
        System.out.println("b"); 
    }
}
 
class Third extends Second
{
    public Third()   {  
        System.out.println("c"); 
    }
}
 
public class MainClass
{
    public static void main(String[] args)
    {
        Third c = new Third();
    }
}

Output

a
b
c

Explanation: When a "Third" object is created, before calling the default constructor of Third class, the default constructor of super class i.e., Second class is called and then again before the default constructor of super class, default constructor of First class is called. And hence, the output is in the order a, b, c.

Program 2: Variable Initialization and Constructor Execution

Java

class First
{
    int i = 10;
 
    public First(int j)
    {
        System.out.println(i); 
        this.i = j * 10;
    }
}
 
class Second extends First
{
    public Second(int j)
    {
        super(j); 
        System.out.println(i); 
        this.i = j * 20;
    }
}
 
public class MainClass
{
    public static void main(String[] args)
    {
        Second n = new Second(20); 
        System.out.println(n.i);
    }
}

Output

10
200
400

Explanation: Since in "Second" class it doesn't have its own 'i', the variable is inherited from the super class. Also, the constructor of parent is called when we create an object of Second.

Program 3: instanceof with Iterators

Java

import java.util.*; 
class Geeks 
{
    public static void main (String[] args) 
    {
        Object i = new ArrayList().iterator(); 
        System.out.print((i instanceof List) + ", "); 
        System.out.print((i instanceof Iterator) + ", "); 
        System.out.print(i instanceof ListIterator); 
    } 
}

Output

false, true, false

Explanation: The iterator() method returns an iterator over the elements in the list in proper sequence, it doesn't return a List or a ListIterator object. A ListIterator can be obtained by invoking the listIterator method.

Program 4: Thread Start and Stop Exception

Java

class ThreadEx extends Thread
{
    public void run()
    {
        System.out.print("Hello...");
    }
    public static void main(String args[])
    {
        ThreadEx T1 = new ThreadEx();
        T1.start();
        T1.stop();
        T1.start();
    }
}

Output:

Run Time Exception

Explanation: A thread cannot be started more than once. Calling start() method on a stopped thread causes IllegalThreadStateException.

Output of Java Program | Set 6

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