Using Objects as Parameters

[Steve Jensen]

Hiya folks, well, just like the greatest rock and roll band in the world, ever - The WHO, i'm back, after an almighty almost 3 year absence. Close family bereavments, job changes and site secondments aside, i've decided to wind up the old Java elastic band, and dive back into it. Are the Java tutorials still being done [think i completed the 1 to 99 in words exercise].

I've got back into the books, and am currently on a chapter called "A closer look at classes and methods".

The topic of "Using Objects as Parameters" has just cropped up.

The code i'm slightly confused on, is the following:-

  
 
// objects may be passed to methods
class Test {
    int a, b;
    
    Test (int i, int j) {
        a = i;
        b = j;
    }
 
// return true if o is equal to the invoking object
    boolean equals (Test o) {
        if(o.a == a && o.b == b) return true;
        else return false;
    }
}
 
class PassOb {
    public static void main (String args[]) {
        Test ob1 = new Test(100,100);
        Test ob2 = new Test(100,22);
        Test ob3 = new Test(-1,-1);
        
        System.out.println("ob1 == ob2: " + ob1.equals(ob2));
        System.out.println("ob1 == ob3: " + ob1.equals(ob3));
    }
}
 
  

I basically understand what the program is doing, except for the lines

   
 
        System.out.println("ob1 == ob2: " + ob1.equals(ob2));
        System.out.println("ob1 == ob3: " + ob1.equals(ob3));
 

Could someone explain to me what is happening in these 2 lines?

Thanks in advance

Steve

[Ryan McGuire]

What part don't you understand?

The equals(Object) method is defined in the Object class, which is the ultimate base class for all other classes. The class Test overloads equals() so for cases where it's called with a Test object as an argument. equals() returns a boolean value, so those two System.out.println() lines will print the string and either "true" or "false". e.g. "obj1 == obj2: false".

Did that answer your question? You understand what the constructor and equals() method in the Test class are doing, right?

[Ray Stojonic]

Test ob1 = new Test(100,100);
Test ob2 = new Test(100,100);

System.out.println("ob1 == ob2: " + ob1.equals(ob2));

would make more sense as an example

here ob1 == ob2 is false where as ob1.equals(ob2) is true.

This is because == will compare the hashcodes (usually the memory location) of the 2 objects. They're distinct objects and therefore not in the same memory location.

.equals, on the other hand, compares the contents of the object, which, in this case, are equal.

[Gregg Bolinger]

System.out.println("ob1 == ob2: " + ob1.equals(ob2));

That's what was throwing me off

System.out.println("ob1 == ob2: " + ob1 == ob2);

That makes more sense. Or...

System.out.println("ob1 equals ob2: " + ob1.equals(ob2));

[Ray Stojonic]

Good point, mine should read
System.out.println("==: " + ob1 == ob2 + " : equals: " + ob1.equals(ob2));

[Tony Morris]

Looks like knowledge of "order of precedence of operators" is required for this discussion by looking at the last couple of posts.
Here's some fun:

String s1 = "s";
String s2 = "s";
System.out.println("s1 == s2: " + s1 == s2);
// a) s1 == s2: true
// b) s1 == s2: false
// c) exception
// d) fail compile
// e) something else __________________

[ August 28, 2006: Message edited by: Tony Morris ]