Arrays in Java:

It is a common requirement when programming to have to process a collection of items of the same type. For example, one may have to add up a collection of numbers or check the due dates on a collection of library books.

It is convenient to be able to refer to such a collection as a group and to be able to iterate over the collection (that is, to process the items in the collection one at a time, re­ using some code). Because this is such a common requirement, most languages provide a data structure, the array, to support this.

In this part you will learn how to create and access array structures in Java, and in the next section you will study the Java constructs for iteration.

Array indexing

An array is a collection of items of the same type. Figure shows a simple array.As usual, we are depicting a primitive memory location as a rectangle. In showing these items side by side, we are indicating that they are part of a collection. They can be numbered as indicated above each box. We call this numbering an index.

This array contains seven values, all of which are integers. The first item in the array (at location zero) is 12, the second is 9, and so on.

In Java, the index that identifies the first value in an array is zero and the last is n - 1, where there are n items in the array.

Array declaration

In Java, array variables are references, not primitives. Because arrays are objects, they have instance variables we can access and we can invoke methods on them.

Already we have seen that integers can be declared by means of a statement such as:

int intValue;

An array can be declared using a similar syntax. For example, an array of integers could be declared as follows:

int []holder;

The type of the array in this case is written int []and the array can contain only elements of the primitive type int. You can create an array of any primitive type; for example, other types of arrays of primitives are boolean [], char [] and double [].

One may also write:

int holder [];

but this departs from the normal syntax of having the type appear at the beginning of the declaration, so we prefer to adopt the previous style.

One may also have an array of a reference type; for example, we could have an array of strings of type String [].

As with other reference types, declaration does not create an object of the type; it only creates a reference, in this case to an array type object.

Array initializers

Java provides a quick way of creating and initializing an array by assigninga list of initial values in curly brackets to an array type variable. For example, the declaration and assignment:

int []holder = {1, 1, 1, 1, 1};

declares an integer array with five elements, with the initial value of these elements being 1. The expression on the right of the assignment sign is called an array initializer.

In creating an array you reserve enough memory for the collection of values in the array initializer.

This is obviously not a feasible way of creating very large arrays, because it relies on us listing all the elements by hand.

It would be  illegal to write the following:

int []holder2 = {false, true}

because the array initializer in this case is of type boolean [], while holder2 is of type

int [], and  these types are  not compatible.