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.
This tutorial 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:
|
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.
or
double
myList[]; // works but
not preferred way.
|
Creating
Arrays:
You can create an array by using the new operator with the
following syntax:
|
arrayRefVar =
new dataType[arraySize];
|
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:
|
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 of 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 would produce following result:
|
1.9
2.9
3.4
3.5
Total is 11.7
Max is 3.5
|
The
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:
|
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 (double element: myList) {
System.out.println(element);
}
}
}
|
This would produce following result:
|
1.9
2.9
3.4
3.5
|
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:
|
public static
void printArray(int[] array) {
for
(int i = 0; i < array.length; i++) {
System.out.print(array[i] + " ");
}
}
|
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:
|
printArray(new
int[]{3, 1, 2, 6, 4, 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:
|
public static
int[] reverse(int[] list) {
int[]
result = new int[list.length];
for
(int i = 0; i = result.length - 1;
i < list.length; i++, j--) {
result[j] = list[i];
}
result
result;
}
|
The
Arrays Class:
The java.util.Arrays class contains various static methods for
sorting and searching arrays, comparing arrays, and filling array elements.
These methods are overloaded for all primitive types.
|
SN
|
Methods with Description
|
|
1
|
public static
int binarySearch(Object[] a, Object key)
Searches the specified array of Object ( Byte, Int , double etc) for the specified value using the binary search algorithm. The array must be sorted prior to making this call. This returns index of the search key, if it is contained in the list; otherwise, (-(insertion point + 1). |
|
2
|
public static
boolean equals(long[] a, long[] a2)
Returns true if the two specified arrays of longs are equal to one another. Two arrays are considered equal if both arrays contain the same number of elements, and all corresponding pairs of elements in the two arrays are equal. This returns true if the two arrays are equal. Same method could be used by all other premitive data types ( Byte, short, Int etc.) |
|
3
|
public static
void fill(int[] a, int val)
Assigns the specified int value to each element of the specified array of ints. Same method could be used by all other premitive data types ( Byte, short, Int etc.) |
|
4
|
public static
void sort(Object[] a)
Sorts the specified array of objects into ascending order, according to the natural ordering of its elements. Same method could be used by all other premitive data types ( Byte, short, Int etc.) |
No comments:
Post a Comment