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For example, if you declare the array to contain references of type Object , those references can refer to any type of object (including array objects) because a reference of type Object can be used to refer to any object.
You can do similar things using interface types. I will discuss interface types in a future module.
Often need to downcast to use an Object reference
If you store all of your references as type Object , you will often need to downcast thereferences to the true type before you can use them to access the instance variables and instance methods of the objects.
Doing the downcast no great challenge as long as you can decide what type to downcast them to.
The Vector class
There is a class named Vector that takes advantage of this capability. An object of type Vector is a self-expanding array of reference variables of type Object . You can use an object of type Vector to manage a group of objects of any type, either all of the same type, or mixed.
(Note that you cannot store primitive values in elements of a non-primitive or reference type. If you need to do that, you will need to wrapyour primitive values in an object of a wrapper class as discussed in an earlier module.)
A sample program using the Date class
The sample program, named array03 and shown in Listing 3 isn't quite that complicated. This program behaves as follows:
As you might expect from the name of the class, each object contains information about the date.
| Listing 3 . The program named Array03 . |
|---|
/*File array03.java Copyright 1997, R.G.Baldwin
Illustrates use of arrays with objects.Illustrates that "an array of objects" is not really an
array of objects, but rather is an array of referencesto objects. The objects are not stored in the array,
but rather are stored somewhere else in memory and thereferences in the array elements refer to them.
The output from running this program is:myArrayOfRefs contains
nullnull
nullmyArrayOfRefs contains
Sat Dec 20 16:56:34 CST 1997Sat Dec 20 16:56:34 CST 1997
Sat Dec 20 16:56:34 CST 1997**********************************************************/
import java.util.*;class array03 { //define the controlling class
Date[]myArrayOfRefs; //Declare reference to the array
array03() {//constructor//Instantiate the array of three reference variables
// of type Date. They will be initialized to null.myArrayOfRefs = new Date[3];//Display the contents of the array.
System.out.println( "myArrayOfRefs contains" );for(int cnt = 0; cnt<3; cnt++)
System.out.println(this.myArrayOfRefs[cnt]);
System.out.println();//Instantiate three objects and assign references to// those three objects to the three reference
// variables in the array.for(int cnt = 0; cnt<3; cnt++)
myArrayOfRefs[cnt]= new Date();
}//end constructor//-----------------------------------------------------//public static void main(String[] args){//main methodarray03 obj = new array03();
System.out.println( "myArrayOfRefs contains" );for(int cnt = 0; cnt<3; cnt++)
System.out.println(obj.myArrayOfRefs[cnt]);
System.out.println();}//end main
}//End array03 class. |
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