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Java Part 4: Objects and Information Hiding


In part three of this ten part tutorial, Chris discussed some of the primitive data types available in Java. He also went through a basic Java class. In this article, he will explore that class further and discuss some aspects of objects, as well as information hiding.

Author Info:
By: Chris Noack
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December 12, 2001
TABLE OF CONTENTS:
  1. · Java Part 4: Objects and Information Hiding
  2. · Objects and "this"
  3. · Enforcing information hiding
  4. · The private and public declarations
  5. · Conclusion

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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 classís 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 ďvalueĒ data member being set to 4. If we then used number2ís addTo() method,

number2.addTo( 5 );

then number2ís value would increase to 9, and numberís 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. Letís 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 ďthisĒ statement represents the object that this method is a part of. Now, letís demonstrate this function:

number.addTwoNumbers( number2 );

would perform the operation:

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

Now that Iíve 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.
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