Looping over a list

To go through every value in a list, loop over the indexes, from 0 to the last one. With a while loop:

ArrayList<String> chores = new ArrayList<>();
chores.add("sweep");
chores.add("dust");
chores.add("mop");

int index = 0;
while (index < chores.size()) {
    System.out.println(chores.get(index));
    index++;
}

sweep
dust
mop

A for loop does the same with the counter in one line, and it's the usual way to write it:

ArrayList<String> chores = new ArrayList<>();
chores.add("sweep");
chores.add("dust");
chores.add("mop");

for (int i = 0; i < chores.size(); i++) {
    System.out.println((i + 1) + ". " + chores.get(i));
}

1. sweep
2. dust
3. mop

• The condition is i < chores.size(), with <, because the last index is size() - 1. With <=, the loop would ask for one index too many and crash.
• With size() instead of a fixed number, the loop works for a list of any length. For an empty list it doesn't run at all.
• i is the index, so chores.get(i) is this round's value, and i + 1 its place as people count it.

Other ranges

The loop's three parts can pick any part of the list. Starting at 1 skips the first value, i += 2 takes every second one, and starting at the last index and counting down goes through the list backwards:

ArrayList<Integer> numbers = new ArrayList<>();
numbers.add(10);
numbers.add(20);
numbers.add(30);
numbers.add(40);
numbers.add(50);

for (int i = 1; i < numbers.size(); i += 2) {
    System.out.print(numbers.get(i) + " ");
}
System.out.println();

for (int i = numbers.size() - 1; i >= 0; i--) {
    System.out.print(numbers.get(i) + " ");
}
System.out.println();

20 40
50 40 30 20 10

Going backwards, the loop starts at size() - 1, the last index, and runs as long as i >= 0, so index 0 is included.

The next module shows the for-each loop: a shorter way to visit every value, for when you don't need the index.