meet it

A brings its own type parameter.

A whole class does not need to be generic: one method can be. A generic method announces its type parameter in angle brackets before its return type: static <T> T lastOf(List<T> items).

When you call it, you never say what T is. Java looks at the list you pass in and works it out. This is . Pass a List<String> and a String comes back, with no casts.

Type parameters only accept class types, so a list of whole numbers is List<Integer>, never List<int>. Need two types? List them with commas: <K, V>.

import java.util.List;
public class Main {
    static <T> T lastOf(List<T> items) {
        return items.get(items.size() - 1);
    }
    public static void main(String[] args) {
        List<String> drinks = List.of("tea", "coffee", "cocoa");
        List<Integer> scores = List.of(12, 30, 18);
        String drink = lastOf(drinks);
        int score = lastOf(scores);
        System.out.println(drink + " " + (score + 2));
    }
}

List.of(...) builds a small list in one line. Later you will see why it cannot be changed.

guess first

One method, a fallback. What will this show?

Guess before you run it. Wrong guesses are how you learn, and nothing is marked.

import java.util.List;
public class Main {
    static <T> T pick(List<T> items, int index, T fallback) {
        if (index < items.size()) {
            return items.get(index);
        }
        return fallback;
    }
    public static void main(String[] args) {
        List<String> pets = List.of("cat", "dog");
        System.out.println(pick(pets, 1, "none"));
        System.out.println(pick(pets, 5, "none"));
    }
}

your turn

Your turn. Count the votes.

Announce the type parameter on line 3, in the gap before int. Then on line 15, count another meal from the list, like curry.

Your code

output

press run to see what happens

fix it

The lap timer will not compile. Can you fix it?

Broken code to fix

output

press run to see what happens

Java says unexpected type on line 7. Look at what sits inside the angle brackets.
Type parameters only take class types, and int is a plain value type.
Change List<int> to List<Integer> on line 7, then run.

nailed it

one method now fits every type. :D

A generic method puts <T> before its return type.
Type inference works out T from the arguments.
Use Integer, not int, inside angle brackets.

new words in your collection

generic method
type inference

This lesson isn’t saved. A free account remembers every lesson you finish.

A generic method brings its own type parameter.

One method, a fallback. What will this show?

Your turn. Count the votes.

The lap timer will not compile. Can you fix it?

one method now fits every type. :D

A brings its own type parameter.