Using Objects as Parameters

[Mubeen Shaik]

Hi All,
I am not understanding the Concept of Passing objects as parameters.I have copied the exmaple from the following book (Java2 Complete Reff by Patrick Naughton).
Please explain me more about this code.

  
//Objects 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)  //I am confused here.
  return true;
  else
  return false;
} 
}
class PassOb
{
   public static void main(String args[])
   {
     Test Ob1 = new Test(100,22);
     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));
   }  
}

Thanks in advance,
Mubeen.

[Layne Lund]

boolean equals(Test o)
{
  if(o.a == a && o.b == b)  //I am confused here.
    return true;
  else
   return false;
}

I've copied the above function since this is where you say you are confused. I'll discuss some of the issues involved, but if I'm unclear on any point or don't hit something you want to know more about, please feel free to ask for clarifiction.
The first thing I think you should understand is that this equals() method "belongs" to the Test class. When you call the method, there is always one implied parameter that is invisibly passed to the method. This parameter is called "this" and is a reference to the object which you use to call the method. For example,

ob1.equals(ob2));

When equals begins execution, "this" refers to the same object as ob1.
Next, the equals() method also defines an explicit parameter, called o, which refers to a Test object. Using the above example, o will refer to the same object as ob2.
Now let's look at this line of code:

if(o.a == a && o.b == b)

o.a refers to the member variable a in the object named o. (Remember that o refers to the object that was explicitly passed as a parameter -- ob2 in our example.) a refers to the member a of the object named this. If you specified this.a, it means the same thing. In fact, I prefer using this.a when refering to a member variable because it avoids confusion with local variables.
The same concepts apply to o.b and b.
I hope this clears up some things for you. If you have any more questions, please come back to ask them.
Keep coding!
Layne

[David Peterson]

I must admit the code does look a bit confusing. "o" isn't a particularly good character to use for a variable name because it looks a bit like a zero. Using the "this" keyword to show explicitly that the references to "a" and "b" are in the current object might also make it clearer. And there's no real reason to have an if.. else construct.
You could rewrite the function more clearly as:

boolean equals(Test t) {
   return (t.a == this.a) && (t.b == this.b);
}

David

[Mubeen Shaik]

Hi Layne,
Thanks for that Detailed explanation. Please correct me if i am wrong here. I added some comments based on my understanding. If my JAVA terminology OR my concept is wrong please correct me.

 
class Test
{
  int a,b;
  Test(int i, int j)
  {
    a = i;
  
    b = j;
  }
 
// return true of  o is equal to the invoking object
boolean equals(Test o)
{
 //if(o.a == a && o.b == b) 
                   // I am Confused here - Now i think i am OK
         // Can i change the code like this?
            
            if(this.a == a && this.b == b)    
      // The other variable refers to the object which was passed to 
      // Test class,In our example it is Ob2.   
  return true;
  else
  return false;
} 
}

class PassOb
{
   public static void main(String args[])
   {
     Test Ob1 = new Test(100,22);  
          // So the values of Ob1.a = 100  and Ob1.b = 22
          // This is in Test calss - this.a = 100 and this.b = 22
     Test Ob2 = new Test(-1,-2);   // I have changed values here
   
     System.out.println(" ob1 == ob2:" +ob1.equals(ob2));
        //When we invoke equals method of Test class with an object Ob2,
        //It is sending Ob2.a as -1 an  and Ob2.b as -2.                
   }  
}

Thanks in advance,
Mubeen.

[Mubeen Shaik]

Hi Layne,
I got it now.Is this correct ?

  
     boolean equals(Test PassedObj)
{
          //if(o.a == a && o.b == b) - Changed below.

  if(PassedObj.a == this.a && PassedObj.b == this.b)
  return true;
  else
  return false;
} 

Thanks,
Mubeen.

[David Peterson]

Yep.
Minor point: You shouldn't start your variable names with capital letters. It is a Java coding convention that variables start with lower-case letters - i.e. your "PassedObj" should be "passedObj" instead.
David

[Jim Yingst]

Mubeen - is this method intended to replace the equals(Obejct) method inherited from the Object class? Have you learned about overriding and overloading yet?

[Mubeen Shaik]

Jim,
Not yet. I think when i start that i will post more questions in this forum.
Thanks,
Mubeen.

[Mubeen Shaik]

Hi All,
Can any one suggest some Weblink on Java Style guide(Java naming conventions and Rules to follow while coding).I know one is in this forum itself.
Thanks,
Mubeen.

[Layne Lund]

It looks like you are getting it. Keep coming back with more questions.
The official Java Coding Conventions can be found at http://java.sun.com/docs/codeconv/. They are available online or for download.
[ February 19, 2004: Message edited by: Layne Lund ]