Arrays

Now that you have an understanding of the primitive types, let's turn our attention to one of the reference types. Recall the three reference types in Java:

• Classes

• Interfaces

• Arrays

Classes and interfaces are complex enough to earn their own chapters, but arrays are comparatively simple, so they are covered here with the primitive types.

An array is simply a way to maintain a sequence of items. Arrays are objects that hold a fixed number of variables of the same type. Arrays can hold primitive types, other objects, or even references to other arrays. If you have data that can be easily indexed and the number of data elements is known in advance, you should definitely consider using an array. Alternatives to arrays are described in Chapter 10, "Data Structures and Java Utilities."

As an example of array usage, suppose you collect a rainfall measurement at five different locations around a city and need to manipulate those values. An array would work well in this case. An example might be

 float rainfall[] = { 0.01F, 0.012F, 0.0F, 0.222F, 0.5F} ; 

In this example, an array of floating point variables has been created and initialized with values corresponding to the rainfall measurements at the five locations.

Array elements are accessed using the indexing operator ([]) to specify an index into the array. As in the case of C and C++, Java array indices start at 0 rather than 1. The next line shows an example of accessing the measurement taken at the third location (index 2):

 float thirdMeasurement = rainfall[2]; 

The result of this statement is that the value 0.0F is assigned to thirdMeasurement.

Arrays in Java are different than those in most other languages because they are treated as objects. This might be confusing at first, but it does provide advantages. For example, every array has a length attribute that can be checked to determine its allocated size . In the example, the following would set numMeasurements to 5 :

 int numMeasurements = rainfall.length; 

The length attribute is especially useful when looping through the elements of an array. The maximum array index is always the value of the length attribute minus 1. (Remember that the index of an array has an origin of 0.) Although the length attribute can be used to get the size of an array, it cannot be set to change the size of an array. An array's size is determined when it is created and it cannot be changed.

Because Java arrays are reference types, there are several steps to creating an array and assigning its elements:

1. Declare the array. An array declaration must contain a pair of brackets, but the compiler allows you to choose between placing the brackets after the data type or after the identifier that names the array. The following two lines produce the same result:

    int[] myIntArray; int myIntArray[]; 

   Although both forms are allowed, you should pick one syntax and use it consistently. Most programmers opt for the form that places the brackets after the data type. In the preceding example, this form makes it instantly clear that the variable is an integer array.

   Also notice from these declarations that a Java array must contain elements that are of a single type only. Here, myIntArray cannot contain anything other than int primitives.

2. Create space for the array and define its length. To do this, you must use the keyword new , followed by the variable type and the number of elements:

    myIntArray = new int[500]; 

   The new operator is discussed in Chapter 7. Its purpose is to instantiate the objects associated with reference type variables. Before space is allocated for an array in this manner, it has a length of zero.

   Caution

   Forgetting to use the new operator before you attempt to assign a primitive or an object to an array element results in a NullPointerException. This is because an array of length zero cannot hold anything.

   
   

   There is a slight variation of using the new operator with arrays that allows you to create an anonymous array . These are called anonymous because they are created without being declared with a name . For example, the following expression creates an anonymous array of integers that could be used in an assignment statement or as a parameter in a method call:

    new int[] {  1, 2, 3 } 

3. Place data in the array. For arrays of primitive types (such as those in this chapter), the array values are all initialized to , false , or '\ u0000' as appropriate. The next line shows how to set the fifth element in the array:

    myIntArray[4] = 467; 

In the rainfall example, you might have noticed that you were able to create the five-element array and assign it values at the same time. This example took advantage of a shortcut.

You can declare the initial values of the array by placing the values between braces ( { ,} ) on the initial declaration line. The number of values provided determines the number of array elements allocated. This bulk assignment of values can only be done as part of an array's declaration. After an array has been declared, attempting to assign values this way causes a compiler error.

In addition to the preceding example that demonstrated the declaration of an array of primitives, you can also initialize an array of object references when you declare it. An example of this is shown in the following:

 String dayAbbreviation[] = { "Sun", "Mon", "Tues", "Wed",   "Thurs", "Fri", "Sat"} ; 

The preceding declaration allocates an array of seven String references and assigns references to the array elements using a list of String literals.

Array declarations are composed of the following parts :

Array modifiers	Optional	The keywords public , protected , private , or synchronized (see Chapter 7)
Type name	Required	The name of the type or class being arrayed
Brackets	Required	[]
Initialization	Optional	See Chapter 7 for more details about initialization
Semicolon	Required	;

Java supports multidimensional arrays, which are simply arrays of arrays. The declaration of a multidimensional array includes a set of brackets for each dimension. When you create space for a multidimensional array using new , you do not have to specify the size for each dimension, only the leftmost one (and any to the left of any other dimensions you specify). The following declarations are all valid:

 int[][] xyData1 = new int[5][10]; int[][] xyData2 = new int[5][]; int[][][] xyzData1 = new int[5][10][15]; int[][][] xyzData2 = new int[5][10][]; int[][][] xyzData3 = new int[5][][]; 

A unique aspect of multidimensional arrays in Java is that they can be nonrectangular. If you continue to think of a multidimensional array as an array of arrays, this makes sense. Each element of the main array references an array whose size is independent of the arrays referenced by the other elements. Consider the following example:

 int[][] pyramid = { {0}, {1, 2}, {3, 4, 5} }; System.out.println(pyramid[0][0]);  // 0 System.out.println(pyramid[1][0]);  // 1 System.out.println(pyramid[1][1]);  // 2 System.out.println(pyramid[2][0]);  // 3 System.out.println(pyramid[2][1]);  // 4 System.out.println(pyramid[2][2]);  // 5 

Listing 3.3 shows several more examples of using arrays.

Listing 3.3 Examples of Declaring Arrays

 long primes[] = new long[1000000];     // Declare an array and assign                                        // some memory to hold it. long[] evenPrimes = new long[1];       // Either way, it's an array. evenPrimes[0] = 2;                     // Populate the array. // Now declare an array with an implied 'new'and populate. long fibonacci[] = { 1,1,2,3,5,8,13,21,34,55,89,144} ; long perfects[] = { 6, 28} ;           // Creates a two element array. // Declare a two-dimensional array and populate it. long towerOfHanoi[][]={ { 10,9,8,7,6,5,4,3,2,1} ,{ } ,{ } } ; long[][][] threeDTicTacToe;            // Uninitialized 3D array. 

There are several additional points about arrays that you need to know:

• Array indexes must either be type int (32-bit integer) or be able to be cast as an int (if you use a short , byte , or char , the compiler promotes it to an int automatically). As a result, the largest possible array size is 2,147,483,647. Most Java installations would fail with arrays anywhere near that size, but that is the maximum defined by the language.

• Java automatically provides runtime bounds checking for arrays. If a statement attempts to access an invalid array index, it causes an ArrayIndexOutOfBoundsException.

Troubleshooting Tip

If you have trouble accessing array elements, see " ArrayIndexOutOfBoundsException, " in the "Troubleshooting" section at the end of this chapter.

• The System.arraycopy method provides the means to copy the elements of one array into another. If you assign one array variable to another, you have not created two copies of the array elements, only two references to the same set of elements.