English 312.04

 

To:  Computer Science Professional

From:  John Sharkey

Subject:  The declaration of arrays while programming in Java

Date:  September 16, 1998

 

The declaration of an array of primitive types

     An array is an object, so when the array declaration is given, no memory is yet allocated to store the array. The declaration example that I am going to use is:

 

int [ ] array1;

 

array1 is a reference for an array, and at this point is null. To declare 100 ints, we issue a new command. An example of this is:

 

array1 = new int [ 100 ];

 

Now array1 references an array of 100 ints.  There are other ways to declare arrays in Java. Some examples of this are:

 

int [ ] array1 = new int [ 100 ];

or

int [ ] array1 = { 1, 2, 3, 4 };

 

In Java, the brackets that are used can go before or after the array name. Putting the brackets before the array name (as shown above) is a better programming style.

 

The declaration of an array of objects

     To declare an array of objects uses the same syntax. However, when programmers allocate an array of objects, each object initially stores a null reference. Each must be set to reference a constructed object.

An example of an array of 5 buttons is constructed as follows:

 

Button [ ] arrayOfButtons;

arrayOfButtons = new Button [5];

for (int j = 0; j < arrayOfButtons.length; j ++ )

   arrayOfButtons [ j ] = new Button( );

 

This is a prefect example of declaring an array of objects in Java. Above, arrayOfButtons is an array of type Button. The Button is set to have 5 buttons and set to variable name arrayOfButtons. Next is the for loop (which uses j as its loop variable) which runs from 0 to the length of arrayOfButtons (which is 5).