4.1 Arrays and Indexes

An array stores multiple values of the same type in one ordered block. Java array indexes start at 0.

int[] scores = {88, 91, 76, 95, 83};

System.out.println(scores[0]);  // 88
System.out.println(scores[4]);  // 83

An array of length 5 has valid indexes from 0 through 4. Java protects arrays at runtime: if you read scores[5], the program throws an ArrayIndexOutOfBoundsException instead of silently reading random memory.

Declaring arrays

The most common Java style puts the brackets after the type:

int[] values;
double[] temperatures;
String[] names;

You can create an array with literal values:

int[] scores = {88, 91, 76, 95, 83};

Or create a fixed-size array first, then fill it:

int[] counts = new int[4];
counts[0] = 12;
counts[1] = 9;
counts[2] = 15;
counts[3] = 6;

The size of a Java array is fixed after creation. new int[4] creates exactly four slots. You can change the values inside the slots, but you cannot make that same array longer.

Default values

When Java creates a new array, every slot gets a default value:

• Numeric arrays start with 0 or 0.0.

• boolean[] starts with false.

• char[] starts with the null character '\0'.

• Object arrays such as String[] start with null.

int[] counts = new int[3];
System.out.println(counts[0]);  // 0

Array length

Arrays use the field .length, not a method call:

int[] scores = {88, 91, 76, 95, 83};
System.out.println(scores.length);  // 5

Notice the difference:

• Array length: scores.length

• String length: name.length()

This difference feels small, but it matters. Arrays expose a length field; strings provide a length() method.

Processing arrays with loops

Arrays are often processed with loops:

for (int i = 0; i < scores.length; i++) {
    System.out.println(scores[i]);
}

The loop condition uses < scores.length, not <= scores.length, because the last legal index is scores.length - 1.

If you only need values and not indexes, use the enhanced for loop:

for (int score : scores) {
    System.out.println(score);
}

Use the index loop when you need to update an element or refer to its position. Use the enhanced for loop when you only need to read values.