Java Part 4: Objects and Information Hiding - Objects and "this"

(Page 2 of 5 )

In the last article, we used the Number class to represent a simple object (remember, an object represents an instance of a class). In the class’s main() function, only one instance of the class was created:

public static void main( String[] args ){

Number number = new Number(); // single instance of Number class

number.addTo( 5 );

number.divideBy( 5 );

System.out.println( +number.getVal() );

System.exit(1);

}

We could create more instances of the Number class like so:

Number number2 = new Number( 4 );

This would create a new Number object called number2, with its “value” data member being set to 4. If we then used number2’s addTo() method,

number2.addTo( 5 );

then number2’s value would increase to 9, and number’s value would remain unchanged, at 1.

As you can see from the examples above, we can have multiple instances of a single class that are entirely independent. This can be quite handy, and, if needed, we can pass on instance of a class to another. Let’s demonstrate this by writing another method for the Number class:

void addTwoNumbers( Number num ){

this.value += num.value;

}

This method works on the object that it is a member of. The “this” statement represents the object that this method is a part of. Now, let’s demonstrate this function:

number.addTwoNumbers( number2 );

would perform the operation:

number.value = number.value + number2.value;

Now that I’ve described how to create a method for a class that will operate on itself, try writing similar methods for multiplying, subtracting and dividing from another Number class.

Next: Enforcing information hiding >>
 

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