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- Java Arrays
Java provides a data structure, the array, which stores a fixed-size sequential collection of elements of the same type. An array is used to store a collection of data, but it is often more useful to think of an array as a collection of variables of the same type.
Instead of declaring individual variables, such as number0, number1, ..., and number99, you declare one array variable such as numbers and use numbers[0], numbers[1], and ..., numbers[99] to represent individual variables. We will introduces how to declare array variables, create arrays, and process arrays using indexed variables.
Declaring Array Variables
To use an array in a program, you must declare a variable to reference the array, and you must specify the type of array the variable can reference. Here is the syntax for declaring an array variable:
Note:The style dataType[] arrayRefVar is preferred. The style dataType arrayRefVar[] comes from the C/C++ language and was adopted in Java to accommodate C/C++ programmers.
Example
The following code snippets are examples of this syntax:
Creating Array
You can create an array by using the new operator with the following syntax:
The above statement does two things:
- It creates an array using new dataType[arraySize];
- It assigns the reference of the newly created array to the variable arrayRefVar.
Declaring an array variable, creating an array, and assigning the reference of the array to the variable can be combined in one statement, as shown below:
Alternatively you can create arrays as follows:
The array elements are accessed through the index. Array indices are 0-based; that is, they start from 0 to arrayRefVar.length-1.
Example
Following statement declares an array variable, myList, creates an array of 10 elements of double type, and assigns its reference to myList.:
Following picture represents array myList. Here myList holds ten double values and the indices are from 0 to 9.
Processing Arrays
When processing array elements, we often use either for loop or foreach loop because all of the elements in an array are of the same type and the size of the array is known.
Example
Here is a complete example of showing how to create, initialize and process arrays:
Output
foreach Loops
JDK 1.5 introduced a new for loop, known as foreach loop or enhanced for loop, which enables you to traverse the complete array sequentially without using an index variable.
Example
The following code displays all the elements in the array myList:
Output
Passing Arrays to Methods:
Just as you can pass primitive type values to methods, you can also pass arrays to methods. For example, the following method displays the elements in an int array:
You can invoke it by passing an array. For example, the following statement invokes the printArray method to display 3, 1, 2, 6, 4, and 2:
Returning an Array from a Method:
A method may also return an array. For example, the method shown below returns an array that is the reversal of another array:
Arrays Class
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