4.3 Array Creation and Access

1. What an array is

An array stores multiple values of the same type 4.3.A.1. Instead of six separate variables for six scores, you use one array variable with six elements. The values can be primitive values (such as int, double or boolean) or object references (such as String objects) 4.3.A.1.

The type of an array is written with square brackets after the element type:

int[] goals;          // an array of int values
double[] prices;      // an array of double values
String[] players;     // an array of String references

Declaring the variable does not create the array yet. An array is an object, so it is created with new or with an initializer list (sections 2 and 3). This page is about one-dimensional (1D) arrays: a single row of elements.

Watch out: every element must match the declared type.

int[] scores = {90, 85.5, 77};

Compiler error:

error: incompatible types: possible lossy conversion from double to int

2. Creating an array with new

When you create an array with the keyword new, you give its length, and every element is set to the default value for its type 4.3.A.3:

Element type	Default value
int	0
double	0.0
boolean	false
any reference type (String, a class)	null

int[] goals = new int[4];
double[] prices = new double[3];
boolean[] present = new boolean[2];
String[] players = new String[3];

System.out.println(goals[0] + " " + prices[0] + " " + present[0] + " " + players[0]);
System.out.println("goals has " + goals.length + " elements");

Output:

0 0.0 false null
goals has 4 elements

The length of an array is established when it is created and cannot be changed 4.3.A.2. You read it with the length attribute 4.3.A.2. Notice: no parentheses. length is an attribute of the array, not a method.

int[] goals = new int[4];
int n = goals.length();

Compiler error:

error: cannot find symbol

And because the length cannot change, you cannot assign to it:

int[] goals = new int[4];
goals.length = 10;

Compiler error:

error: cannot assign a value to final variable length

3. Initializer lists

An initializer list creates an array and fills it with the values you list, all in one step 4.3.A.4. The length is the number of values in the list.

String[] days = {"Mon", "Tue", "Wed", "Thu", "Fri"};
int[] laps = {62, 58, 60};

System.out.println(days.length);
System.out.println(laps.length);

Output:

5
3

Here is how days looks in memory. The index row is what you use to reach each element.

index	0	1	2	3	4
days	“Mon”	“Tue”	“Wed”	“Thu”	“Fri”

4. Accessing and modifying elements

Square brackets [] with an index access an element, and the same brackets on the left of = modify it 4.3.A.5.

int[] scores = {7, 4, 9, 6};

System.out.println(scores[2]);     // read index 2

scores[1] = 10;                    // change index 1
scores[3] = scores[3] + 2;         // read index 3, add 2, store it back
scores[0]++;                       // works like any int variable

System.out.println(scores[0] + " " + scores[1] + " " + scores[2] + " " + scores[3]);

Output:

9
8 10 9 8

index	0	1	2	3
before	7	4	9	6
after	8	10	9	8

The index can be any int expression, such as a variable or scores.length - 1:

int[] scores = {7, 4, 9, 6};
int i = 1;
System.out.println(scores[i + 1]);
System.out.println(scores[scores.length - 1]);   // always the last element

Output:

9
6

5. Valid indexes and ArrayIndexOutOfBoundsException

The valid index values for an array are 0 through one less than the length of the array, inclusive 4.3.A.6. For an array of length 4, that means 0, 1, 2 and 3. Using any index outside that range results in an ArrayIndexOutOfBoundsException 4.3.A.6.

The code compiles, because the compiler does not check index values. The error happens at run time, and the program stops.

int[] scores = {7, 4, 9, 6};
System.out.println(scores[3]);
System.out.println(scores[4]);
System.out.println("done");

Output:

6
Exception in thread "main" java.lang.ArrayIndexOutOfBoundsException: Index 4 out of bounds for length 4

Real view: this is the first line the console shows. “done” is never printed, because the exception stops the program. A negative index such as scores[-1] causes the same exception.

Watch out: scores[scores.length] is always out of bounds. The last element is scores[scores.length - 1].

6. Arrays of object references

An array of a reference type stores references, not the objects themselves 4.3.A.1. After new, every element is null, so you must assign objects before calling methods on them.

String[] team = new String[3];
team[0] = "Ana";
team[2] = "Leo";

System.out.println(team[0].length());
System.out.println(team[1]);
System.out.println(team[1].length());

Output:

3
null
Exception in thread "main" java.lang.NullPointerException: Cannot invoke "String.length()" because "team[1]" is null

index	0	1	2
team	“Ana”	null	“Leo”

Printing a null element is fine; it prints null. Calling a method on it, such as team[1].length(), throws a NullPointerException. (The exact wording after the colon depends on your Java version and settings; on the AP® exam you only need the exception name.)