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In this lesson we start our investigation of objects and classes by looking at arrays and how to use them. in Java the predefined data type Array object is used for array manipulation. Investigation of this object will give us an insight into using classes before we start to create our own.

Array Definition Top

An array in Java is an object that contains a collection of values which can be a collection of primitive data types or a collection of reference variable types.

  • An array of primitives data types is a collection of values that constitute the primitive values themselves.
  • An array of reference variable types is actually a collection of pointer values which point to the memory address where each object is stored on the Heap.

Whichever variable type the array holds, primitive or reference, the array itself is still an object. in Java you can define a one dimensional array which is an object that refers to a collection of values repeated one or more times, or multi dimensional arrays which are a collection of array objects (arrays of arrays). Multi dimensional arrays may contain the same number of elements in each row or column and are known as regular arrays or an uneven number of elements in each row or column which are known as irregular arrays.


array diagram
Figure 1. Array types.

Array Creation Top

Array creation is a three step process as outlined below and can be achieved using separate statements or combined into a single statement.

  1. Array declaration
  2. Array allocation
  3. Array element initialization

The following table shows the different forms of the array construct that can be used.

Construct Description
One dimensional array examples
int intArray1[];
int[] intArray2;
String strArray1[];
String[] strArray2;		Array declaration.
intArray1 = new int[4];
strArray1 = new String[5];		Array allocation.

Array elements initialized to their default value.

The new keyword is mandatory for multiple statement array creation.
for (int i=0; i<intArray1.length; i++) {
    intArray1[i] = someAssignmentValue;
}

strArray[0] = "one";
...
strArray[4] = "five";		 Array element initialization using a loop.



Array element initialization using a single element.
int intArray1[] = {0, 1, 5, 8};
int[] intArray2 = {0, 8};
int intArray3[] = new int[]{5, 8};
int[] intArray4 = new int[]{5, 8, 23};

String strArray1[] = {"one", "aa", "c", "rt", "je"};
String[] strArray2 = {"one", "aa", "c", "rt", "je"};
String strArray3[] = new String[]{"one", "rt", "je"};
String[] strArray4 = new String[]{"one", "rt", "je"}; 		Single statement array declaration

The size of the array is calculated by the number of values that are assigned to the array and should not be specfied for single statement array creation.
Two dimensional array examples
int intArray1[][];
int[][] intArray2;
int[] intArray3[];
String strArray1[][];
String[][] strArray2;
String[] strArray3[];		Array declaration.
intArray1 = new int[2][4];
intArray2 = new int[2][];
strArray1 = new String[5][];
strArray2 = new String[5][2];		Array allocation.

Array elements initialized to their default value.

The new keyword is mandatory for multiple statement array creation.
for (int i=0; i<intArray1.length; i++) {
    for (int j=0; j<intArray1[i].length; j++) {
        intArray1[i][j] = someAssignmentValue;
    }
}

strArray[0][0] = "aa";
...
strArray[4][1] = "dd";		Array element initialization using a loop.





Array element initialization using a single element.
int intArray1[][] = { {6, 2}, {5, 22, 67} };
int[][] intArray2 = { {65, 33}, {0}, {5, 14, 71} };
int[] intArray3[] = { {65, 33}, {0}, {5, 14, 71} };

int intArray1[][] = new int[][]{ {6, 2}, {5, 22, 67} };
int[][] intArray2 = new int[][]{ {5, 33}, {0}, {5, 4, 71} };
int[] intArray3[] = new int[][]{ {1, 7}, {0, 6}, {1, 1, 4} };		Single statement array declaration

The size of the array is calculated by the number of values that are assigned to the array and should not be specfied for single statement array creation.
Multi dimensional array
type arrayName[][]...[] = new type[size1][size2]...[sizeN]Same rules as one and two dimensional arrays.

Array Notes Top

As you can see from the table above there is a lot of ways to create arrays in Java. There are several points about arrays shown in the table above that we will highlight again here before we go into some code examples:

  • Array indexes are zero-based.
  • When declaring an array the square brackets can appear after the type, after the array name or in the case of multi dimensional arrays a combination of both.
  • After allocation of an array, each element of the array is initialized with the default for the array type:
    • object - null
    • boolean - false
    • char - /u0000
    • integer types (byte, short, int and long) - 0
    • floating-point types (float, double) - 0.0
  • For multiple statement array creation the new keyword is mandatory.
  • For single statement array creation the size of the array is calculated by the number of values that are assigned to the array and should not be specfied.

Java Documentation Top

As mentioned in the first section Java comes with very rich documentation. The following link will take you to the Oracle online version of documentation for the Java™ Platform, Standard Edition 8 API Specification. Take a look at the documentation for the Array class which you can find by scrolling down the lower left pane and clicking on Array. Take a look through the class so you get a feel for the documentation style and the Array class itself. You will go back to this documentation time and time again so if you haven't done so already I suggest adding this link to your browser's favourites toolbar for fast access.

Related Quiz

Objects & Classes Quiz 1 - Arrays

Lesson 1 Complete

In this lesson we looked at the various types of array for use in Java.

What's Next?

In the next lesson we look at some examples of arrays and some of the exceptions that can occur when using them.