Java Arrays
What is an Array?
So
far, you have been working with variables that hold only one value. 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.
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 −
Syntax
dataType[]
arrayRefVar; // preferred way.
or
dataType
arrayRefVar[]; // works but not
preferred way.
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 −
double[]
myList;
// preferred way.
// preferred way.
or
double
myList[];
// works but not preferred way.
// works but not preferred way.
Creating Arrays
You
can create an array by using the new operator with the following syntax −
Syntax
arrayRefVar
= new dataType[arraySize];
The
above statement does two things −
Ø It
creates an array using new dataType[arraySize].
Ø 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 −
dataType[]
arrayRefVar = new dataType[arraySize];
Alternatively
you can create arrays as follows −
dataType[]
arrayRefVar = {value0, value1, ..., valuek};
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 −
double[]
myList = new double[10];
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 showing how to create, initialize, and process arrays −
public class
TestArray {
public static void main(String[] args) {
double[] myList = {1.9, 2.9, 3.4, 3.5};
// Print all the array elements
for (int i = 0; i < myList.length;
i++) {
System.out.println(myList[i] + "
");
}
// Summing all elements
double total = 0;
for (int i = 0; i < myList.length;
i++) {
total += myList[i];
}
System.out.println("Total is "
+ total);
// Finding the largest element
double max = myList[0];
for (int i = 1; i < myList.length;
i++) {
if (myList[i] > max) max =
myList[i];
}
System.out.println("Max is " +
max);
}
}
This will produce the
following result −
Output
1.9
2.9
3.4
3.5
Total is 11.7
Max is 3.5
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