// Class definition
class Thing {
    int x;  // instance variable
    public Thing(int i) { x = i; } // constructor method
}

// Driver
public class CopyDemo {
    public static void main(String[] argv) {

        // Primitive variables 
	int i = 0, j = 1;
	System.out.println("1. i is " + i + "  j is " + j + 
			   "       i == j is " + (i == j));
	j = i; // assign primitive variable
	System.out.println("2. i is " + i + "  j is " + j + 
			   "       i == j is " + (i == j));
	i = i + 1; // note j does not change
	System.out.println("3. i is " + i + "  j is " + j +
			   "       i == j is " + (i == j));
	j = j + 1;
	System.out.println("4. i is " + i + "  j is " + j +
			   "       i == j is " + (i == j));

        // Reference variables
	Thing a = new Thing(0); Thing b = new Thing(1);
	System.out.println("5. a.x is " + a.x + "  b.x is " + b.x +
			   "   a == b is " + (a == b));
	b = a; // assign reference variable
	System.out.println("6. a.x is " + a.x + "  b.x is " + b.x + 
			   "   a == b is " + (a == b));
	a.x = a.x + 1; // note b.x also changes
	System.out.println("7. a.x is " + a.x + "  b.x is " + b.x +
			   "   a == b is " + (a == b));

	Thing c = new Thing(1); // but a and c are not == 
	System.out.println("8. a.x is " + a.x + "  c.x is " + c.x +
			   "   a == c is " + (a == c));
    }
}

-----------------

$ java CopyDemo
1. i is 0  j is 1       i == j is false
2. i is 0  j is 0       i == j is true
3. i is 1  j is 0       i == j is false
4. i is 1  j is 1       i == j is true
5. a.x is 0  b.x is 1   a == b is false
6. a.x is 0  b.x is 0   a == b is true
7. a.x is 1  b.x is 1   a == b is true
8. a.x is 1  c.x is 1   a == c is false