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:
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dataType[] arrayRefVar; // preferred way.
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------------------------------------------------- or
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------------------------------------------------- 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:
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double[] myList; // preferred way.
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------------------------------------------------- or
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------------------------------------------------- double myList[]; // works but not preferred way.
Creating Arrays:
You can create an array by using the new operator with the following syntax:
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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:
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dataType[] arrayRefVar = new dataType[arraySize];
Alternatively you can create arrays as follows:
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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:
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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:
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public class TestArray {
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------------------------------------------------- public static void main(String[] args) {
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double[] myList = {1.9, 2.9, 3.4, 3.5};
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------------------------------------------------- // Print all the array elements
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for (int i = 0; i < myList.length; i++) {
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System.out.println(myList[i] + " ");
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}
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// Summing all elements
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double total = 0;
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for (int i = 0; i < myList.length; i++) {
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total += myList[i];
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}
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System.out.println("Total is " + total);
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// Finding the largest element
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double max = myList[0];
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for (int i = 1; i < myList.length; i++) {
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if (myList[i] > max) max = myList[i];
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}
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System.out.println("Max is " + max);
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}
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}
This would produce the following result:
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1.9
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2.9
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3.4
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3.5
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Total is 11.7
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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:
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public class TestArray {
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------------------------------------------------- public static void main(String[] args) {
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double[] myList = {1.9, 2.9, 3.4, 3.5};
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------------------------------------------------- // Print all the array elements
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for (double element: myList) {
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System.out.println(element);
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}
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}
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}
This would produce the following result:
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1.9
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2.9
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3.4
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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:
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public static void printArray(int[] array) {
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for (int i = 0; i < array.length; i++) {
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System.out.print(array[i] + " ");
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}
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}
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:
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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:
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public static int[] reverse(int[] list) {
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int[] result = new int[list.length];
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------------------------------------------------- for (int i = 0, j = result.length - 1; i < list.length; i++, j--) {
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result[j] = list[i];
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}
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return result;
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}
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 primitive 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 primitive 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 primitive data types ( Byte, short, Int, etc.) |

Creating a Two-Dimensional Array | |
The most basic multidimensional array is made of two dimensions. This is referred to as two-dimensional. To create a two-dimensional array, inside of one pair of square brackets, use two pairs (of square brackets). The formula you would use is: DataType[][] VariableName;
Although the pairs of brackets are empty, they will contain some numbers. There are various ways you can initialize a two-dimensional array. If you are declaring the array variable but are not ready to initialize it, use the following formula: DataType[][] VariableName = new DataType[Number1][Number2];
In Java, you can place the first square brackets on the right side of the name of the variable: DataType VariableName[][] = new DataType[Number1][Number2];
In the square brackets on the right side of the = operator, enter an integer in each. Here is an example: public class Exercise { public static void main(String[] args) throws Exception { String[][] members = new String[2][4]; } }
In our declaration, the members variable contains two lists. Each of the two lists contains 4 elements. This means that the first list contains 4 elements and the second list contains 4 elements. Therefore, the whole list is made of 8 elements (2 * 4 = 8). Because the variable is declared as aString, each of the 8 items must be a string.
You can also create a two-dimensional array that takes more than two lists, such as 3, 4, 5 or more. Here is an example: public class Exercise { public static void main(String[] args) throws Exception { String[][] members = new String[2][8]; } }
This time, the variable is a 2-dimensional (2 lists) array where each list contains 8 components. This means that the whole array contains 2 * 8 = 16 components.
If you want to declare the array variable using the first formula where you do not know the sizes of the lists, you must use the data type of the array and you can omit the right square brackets. Here is an example: public class Exercise { public static void main(String[] args) throws Exception { String members[][]; } } Initializing a Two-Dimensional Array | |
There are various ways you can initialize a multidimensional array. You can initialize an array variable when declaring it. To do this, on the right side of the declaration, do not include the values in the square brackets: public class Exercise { public static void main(String[] args) throws Exception { String[][] Members = new String[][] . . . ; } }
Before the closing semi-colon, type an opening and a closing curly brackets. Inside of the brackets, include a pair of an opening and a closing curly brackets for each internal array. Then, inside of a pair of curly brackets, provide a list of the values of the internal array, just as you would do for a one-dimensional array. Here is an example: public class Exercise { public static void main(String[] args) throws Exception { String[][] Members = new String[][] { { "Celeste", "Mathurin", "Alex", "Germain" }, // First List { "Jeremy", "Mathew", "Anselme", "Frederique" } // Second List }; } } Practical Learning: Creating a Two-Dimensional Array | | 1. To create and use a two-dimensional array, change the file as follows: package departmentstore5; public class Main { public static void main(String[] args) throws Exception { long itemID = 0; double price = 0.00d; String description = "Unknown"; // The first list contains women's items // The other contains non-women items long[][] itemNumbers = new long[][] { { 947783, 934687, 973947, 987598, 974937 }, { 739579, 367583, 743937, 437657, 467945 } }; String[][] itemNames = new String[][] { { "Women Double-faced wool coat", "Women Floral Silk Tank Blouse", "Women Push Up Bra", "Women Chiffon Blouse", "Women Bow Belt Skirtsuit" }, { "Men Cotton Polo Shirt", "Children Cable-knit Sweater ", "Children Bear Coverall Cotton", "Baby three-piece Set ", "Girls Jeans with Heart Belt " } }; double[][] unitPrices = new double[][] { { 275.25D, 180.05D, 50.00D, 265.35D, 245.55D }, { 45.55D, 25.65D, 28.25D, 48.55D, 19.95D } }; System.out.println("Item Identification"); System.out.println("Item Number: " + itemID); System.out.println("Description: " + description); System.out.printf("Unit Price: %.2f\n", price); } } |