Introduction
Class Definition
Data Fields
Methods
Constructors
Instantiation
  Demo 1
  Demo 2
Static Members
  Demo 3
Value&Reference
  Demo 4
Overloading
   Demo 5
Wrappers 
  Demo 6
Autobox/Unbox
   Demo 7
Arrays
  Demo 8
Exceptions
Exercises

Primitive data type operations deal only with value. For example, the following code shows that assigning primitive data variable i to another named j simply passes a copy of the value in i into j. That is, a data value passes and not a reference or pointer to a data location.

The following shows code on top that creates an instance of AClass and invokes the aMethod with an int argument. The code on the bottom shows the AClass definition and the aMethod.

Modify Reference Variables

You can always assign a new object to a reference variable. For example, in the following code, we create two instances of the AClass, "a" and "b", and then assign new values to the "j" variable for each object:

Note that assigning a new object to the "a" variable orphans the original object that "a" pointed to previously. If an object is no longer referenced by any other variables, then the garbage collector will eventually reclaim the memory that it occupies.

References in Method Arguments

The argument list of a method can include object references. The methods and data of that object can be accessed and modified by the method.

However, the reference itself passes by value. That is, the JVM makes a copy of the internal memory pointer and the copy of the pointer goes into the argument value.

So if the reference variable in the method is set to a new object, that will have no affect on the reference variable in the calling method.

For example, in the following code, a reference to a BClass object passes to the method of a AClass object. In that method, the argument variable is assigned to a new instance of the BClass. However, when the process returns to the calling code, the originial reference remains unaffected.

Since reference variables only hold pointers, the test

       if (a == b)...

simply tests whether a and b point to the same object, not if the referenced objects have equal data values.

If you need to test whether two objects hold the same data values, many classes provide an "equals()" method such that

       a.equals(b)

returns true if the object b data matchs that in object a.

Latest update: Nov.8, 2006

           Tech
OOP in Tech Apps
Complex Number
Histogram Class
  Demo
More Wrappers

           Physics
OOP in Physics
Particle Class
Root Finding
  Demo 1
Newton Methods
  Demo 2
Exercises