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:
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:
1. It creates an array using new dataType[arraySize];
2. 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.
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 the following result:
1.9
2.9
3.4
3.5
Total is 11.7
ax 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 the 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, j = result.length - 1; i < list.length; i++, j--) {
result[j] = list[i];
}
return result;
}
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:
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:
1. It creates an array using new dataType[arraySize];
2. 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.
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 the following result:
1.9
2.9
3.4
3.5
Total is 11.7
ax 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 the 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, j = result.length - 1; i < list.length; i++, j--) {
result[j] = list[i];
}
return result;
}
