Length and loops

An array knows its own size: hours.length.

int[] hours = new int[7];
System.out.println("The week has " + hours.length + " days");

The week has 7 days

length has no parentheses. It isn't a method but a field: a value stored with the array. That's easy to mix up, because each kind of collection says it differently:

With parentheses, javac looks for a method called length and doesn't find one:

public class Main {
    public static void main(String[] args) {
        int[] hours = new int[7];
        System.out.println(hours.length());
    }
}

Main.java:4: error: cannot find symbol
        System.out.println(hours.length());
                                ^
  symbol:   method length()
  location: variable hours of type int[]
1 error

Looping over the indexes

The indexes go from 0 to length - 1, so the same for loop you used with lists goes through all of them. Here one loop fills the array and a second one prints it:

int[] squares = new int[5];
for (int i = 0; i < squares.length; i++) {
    squares[i] = i * i;
}
for (int i = 0; i < squares.length; i++) {
    System.out.println(i + " squared is " + squares[i]);
}

0 squared is 0
1 squared is 1
2 squared is 4
3 squared is 9
4 squared is 16

Because both loops use squares.length instead of the number 5, changing new int[5] is all it takes to make the program work with a bigger array.

for-each

When you need the values but not the indexes, a for-each loop works on an array just as it does on a list:

int[] squares = new int[5];
for (int i = 0; i < squares.length; i++) {
    squares[i] = i * i;
}
int sum = 0;
for (int square : squares) {
    sum += square;
}
System.out.println("Sum: " + sum);

Sum: 30

The loop variable square gets a copy of each element in turn. Assigning to it doesn't change the array, so filling or changing an array takes the loop over the indexes.

One past the end

An index that isn't in the array crashes the program, as it does with a list. The usual cause is <= where < belongs:

public class Main {
    public static void main(String[] args) {
        int[] squares = new int[3];
        for (int i = 0; i <= squares.length; i++) {
            squares[i] = i * i;
            System.out.println(squares[i]);
        }
    }
}

0
1
4

Exception in thread "main" java.lang.ArrayIndexOutOfBoundsException: Index 3 out of bounds for length 3
	at Main.main(Main.java:5)

The message names the index that failed and the array's length. An array of length 3 has the indexes 0, 1 and 2, so index 3 doesn't exist. The loop that visits every index is for (int i = 0; i < array.length; i++).