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Assigning one array to another array -- be careful
Java allows you to assign one array to another. You mustbe aware, however, that when you do this, you are simply making another copy of the reference to the same data in memory.
Then you simply have two references to the same data in memory, which is often not a good idea. This is illustrated in the program named array02 shown in Listing 2 .
| Listing 2 . The program named array02 . |
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/*File array02.java Copyright 1997, R.G.Baldwin
Illustrates that when you assign one array to anotherarray, you end up with two references to the same array.
The output from running this program is:firstArray contents
0 1 2secondArray contents
0 1 2Change a value in firstArray and display both again
firstArray contents0 10 2
secondArray contents0 10 2
**********************************************************/class array02 { //define the controlling class
int[]firstArray;
int[]secondArray;
array02() {//constructorfirstArray = new int[3];for(int cnt = 0; cnt<3; cnt++) firstArray[cnt] = cnt;secondArray = new int[3];secondArray = firstArray;
}//end constructorpublic static void main(String[] args){//main methodarray02 obj = new array02();
System.out.println( "firstArray contents" );for(int cnt = 0; cnt<3; cnt++)
System.out.print(obj.firstArray[cnt]+ " " );
System.out.println();System.out.println( "secondArray contents" );
for(int cnt = 0; cnt<3; cnt++)
System.out.print(obj.secondArray[cnt]+ " " );
System.out.println();System.out.println(
"Change value in firstArray and display both again");obj.firstArray[1] = 10;System.out.println( "firstArray contents" );
for(int cnt = 0; cnt<3; cnt++)
System.out.print(obj.firstArray[cnt]+ " " );
System.out.println();System.out.println( "secondArray contents" );
for(int cnt = 0; cnt<3; cnt++)
System.out.print(obj.secondArray[cnt]+ " " );
System.out.println();}//end main
}//End array02 class. |
An array of objects really isn't an array of objects
There is another subtle issue that you need to come to grips with before we leave our discussion of arrays. In particular, when you create anarray of objects, it really isn't an array of objects.
Rather, it is an array of object references (or null) . When you assign primitive values to the elements in an array object, the actual primitivevalues are stored in the elements of the array.
However, when you assign objects to the elements in an array , the actual objects aren't actually stored in the array elements. Rather, the objects are storedsomewhere else in memory. The elements in the array contain references to those objects.
All the elements in an array of objects need not be of the same actual type
The fact that the array is simply an array of reference variables has some interesting ramifications. For example, it isn't necessary that all the elements in the array be of the same type, provided the reference variables are of a type that will allow them to refer to all the different types of objects.
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