Question

I understand the following Java outputs.

public class EchoTestDrive {
    public static void main(String[] args){
        Echo e1 = new Echo();
        Echo e2 = new Echo();
        int x = 0;
        while (x<4){
            e1.hello();
            e1.count = e1.count + 1;
            if (x==3){
                e2.count = e2.count +1;
            }
            if(x>0){
                e2.count = e2.count + e1.count;
            }
            x = x+1;
            System.out.println("e1.count is " + e1.count);
            System.out.println("e2.count is " + e2.count);
        }
        System.out.println(e2.count);
    }
}

class Echo {
    int count = 0;
    void hello (){
        System.out.println ("helloooooooo..");
    }
}

Outputs

helloooooooo..
e1.count is 1
e2.count is 0
helloooooooo..
e1.count is 2
e2.count is 2
helloooooooo..
e1.count is 3
e2.count is 5
helloooooooo..
e1.count is 4
e2.count is 10
10

However when I change Echo e2 = new Echo () to Echo e2 = e1, I don’t understand why the outputs is so.

public class EchoTestDrive {
    public static void main(String[] args){
        Echo e1 = new Echo();
        Echo e2 = e1;
        int x = 0;
        while (x<4){
            e1.hello();
            e1.count = e1.count + 1;
            if (x==3){
                e2.count = e2.count +1;
            }
            if(x>0){
                e2.count = e2.count + e1.count;
            }
            x = x+1;
            System.out.println("e1.count is " + e1.count);
            System.out.println("e2.count is " + e2.count);
        }
        System.out.println(e2.count);
    }
}

class Echo {
    int count = 0;
    void hello (){
        System.out.println ("helloooooooo..");
    }
}

Output

helloooooooo..
e1.count is 1
e2.count is 1
helloooooooo..
e1.count is 4
e2.count is 4
helloooooooo..
e1.count is 10
e2.count is 10
helloooooooo..
e1.count is 24
e2.count is 24
24

To me when x = 0, e1.count is 1 and e2.count is 0.
When x = 1, e1.count is e1.count is 2 and e2.count is 2. etc.

I am hoping someone explains it.

Thanks in advance.

Answer 1

When you have Echo e2=e1; you make it so both e1 and e2 point to the same memory location. Thus whenever you add to e2 it adds to that memory location so e1 has the same value and vis versa. Specifically

When x = 0

e1.hello();
        e1.count = e1.count + 1;   //adds 1 to the memory location
        if (x==3){  // x is 0 so doesn't go in
            e2.count = e2.count +1;
        }
        if(x>0){  // x is 0 so doesn't go in
            e2.count = e2.count + e1.count;
        }
        x = x+1;
        System.out.println("e1.count is " + e1.count);
        System.out.println("e2.count is " + e2.count);
    }
    System.out.println(e2.count);
}

Thus the memory location equals 1 and both e1 and e2 are 1

When x = 1

e1.hello();
        e1.count = e1.count + 1;   
           //adds 1 to the memory location which was already 1 from last time and now equals 2
        if (x==3){  // x is 1 so doesn't go in
            e2.count = e2.count +1;
        }
        if(x>0){  // x is 1 so goes in as 1 is greater than 0
            e2.count = e2.count + e1.count;  // adds e2 and e1 = 2 + 2 from previous line above = 4
        }
        x = x+1;
        System.out.println("e1.count is " + e1.count);
        System.out.println("e2.count is " + e2.count);
    }
    System.out.println(e2.count);
}

Thus the memory location equals 4 and both e1 and e2 are 4