contains and indexOf

The last module searched a list with a loop. Two list methods do that searching for you:

• list.contains(value) is true when the value is in the list, and false when it isn't.
• list.indexOf(value) gives the index of the value, or -1 when it isn't in the list. If the value is there more than once, you get the first index.

ArrayList<String> colors = new ArrayList<>();
colors.add("red");
colors.add("green");
colors.add("blue");
colors.add("green");

System.out.println(colors.contains("blue"));
System.out.println(colors.contains("pink"));
System.out.println(colors.indexOf("green"));
System.out.println(colors.indexOf("pink"));

true
false
1
-1

Both compare with equals, the way you compare Strings yourself, so they also find text the user typed in. The comparison is exact: "Green" with a capital G is not in this list.

They work on number lists too: numbers.contains(7) and numbers.indexOf(7) look for the value 7.

Searching in an if

contains gives a boolean, so the call can be the condition of an if. With indexOf, check for -1 before you use the index:

ArrayList<String> queue = new ArrayList<>();
queue.add("Ines");
queue.add("Tomas");
queue.add("Yuki");

if (queue.contains("Tomas")) {
    System.out.println("Tomas is in the queue");
}

String name = "Yuki";
int index = queue.indexOf(name);
if (index == -1) {
    System.out.println(name + " is not in the queue");
} else {
    System.out.println(name + " has " + index + " people ahead");
}

Tomas is in the queue
Yuki has 2 people ahead

Yuki is at index 2, and the index is exactly the number of people before her.