Java Object confusion

Question

I am working on a classroom exercise that is producing unexpected output. Hopefully someone can explain what I am missing.

I am fine up to the last 4 lines of Class1, then I'm a bit confused.

public class Class1 {
public static void main(String[] args){
    Class2 val1 = new Class2 ();
    Class2 val2 = new Class2 ();
    val1.set (3);
    val2.set (4);

    for (int num = 11; num>=1; num -=4){
        val1.change (num);
        val2.change(val1.get());

        System.out.println("Val1 " + val1);
        System.out.println("Val2 " + val2);
    }
    val1 = val2;

    val1.change(3);
    val2.change(val1.get());
    System.out.println(val1.get());
    System.out.println(val2.get());


    }
} // end Class1

Here is Class2

public class Class2 {
private int num1;
private int num2;

public void set (int inValue){
    num1 = inValue;
    num2 = inValue + 2;
}

public void change (int inValue){
    num1++;
    num2 += inValue;
}

public int get (){
    return num1 + num2;
}

public String toString (){
    return num1 + " = " + num2;
}
}

The output I am expecting from the code is as follows:

Val1 4 = 16

Val2 5 = 26

Val1 5 = 23

Val2 6 = 54

Val1 6 = 26

Val2 7 = 86

97

191

However the output the code produces when I run it is:

Val1 4 = 16

Val2 5 = 26

Val1 5 = 23

Val2 6 = 54

Val1 6 = 26

Val2 7 = 86

195

I don't understand where the 195 is coming from.

Before the last 4 lines, we set val1 equal to val2, so now both are (num1 = 7, num2 = 86) In the last 4 lines of Class1:

val1.change(3);    // should change val1 to (num1 = 8, num2 = 89)
val2.change(val1.get());  // val.get() should return (8+89)=97,

pass that in as a parameter to val2.change() should result in val2(num1 = 8, num2 = 183)

System.out.println(val1.get()); // should produce (8+89)=97
System.out.println(val2.get()); // should produce (8+183)=191

Yet neither of these numbers are what is output. What am I missing?

This is a perfect opportunity to familiarize yourself with a debugger. Rather than just looking at the code and trying to figure out what it's going to do (especially with strange, intentionally confusing contrived example code like this), you can step through the code while it executes, line by line, and observe exactly what it's doing. See the changes made to the variables. See how results are calculated. Good debugging skills are your most valuable tool in software development. — David
– David, Commented Apr 18, 2015 at 1:39
@David came here to write the exact same thing - glad to see that I'm not the only one who thinks that debugging is the most valuable tool in software development. — DominikAngerer
– DominikAngerer, Commented Apr 18, 2015 at 1:47

RockOnRockOut · Accepted Answer · 2015-04-18 01:44:54Z

2

Since this is a homework assignment, I second David's suggestion to learn how to use a debugger. However, I will give you a hint - your problem happens when you do this: val1 = val2; - both your variables will now point to the same object.

Think of it this way:

Class2 val1 = new Class2 (); //now val1 points to instance #1 of Class2
Class2 val2 = new Class2 (); //now val2 points to instance #2 of Class2
val1 = val2; //now val1 also points to instance #2 of Class2

answered Apr 18, 2015 at 1:44

RockOnRockOut

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1 Comment

Ross Satchell Over a year ago

Ah! I didn't realize that the would be linked from there on forwards.

jriccio · Accepted Answer · 2015-04-18 01:42:50Z

0

It looks like at the end of your main method you are assigning val2 to val1, did you mean to do this? From that point forward val2 and val1 will both be the same.

answered Apr 18, 2015 at 1:42

jriccio

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