Generic method

Learn generic methods with <T>.

Generic method

A generic method is a method that declares its own type parameter. You write static <T> T identity(T value) { return value; }; the <T> comes before the return type. You do not need to pass the type at the call site; the compiler infers it from the argument.

For example identity("Hello") returns String, while identity(42) returns Integer. The same method works type-safely for different types. Generic methods can live in non-generic classes too.

Type inference

The Java compiler infers the type from the argument at a generic method call. When you write identity("Hello") the compiler decides T is String; for identity(42) it becomes Integer.

You can also be explicit: Main.<String>identity("Hello"). Most of the time this is unnecessary. Thanks to inference the code stays short and readable; passing the wrong type still gives a compile error.

Exercise Write the identity generic method

Define an identity generic method: static <T> T identity(T value) whose body is return value;. In main, call it with String s = identity("Hello"); and print it with System.out.println(s);.

Show example solution

public class Main {
    static <T> T identity(T value) {
        return value;
    }
    public static void main(String[] args) {
        String s = identity("Hello");
        System.out.println(s);
    }
}

Test yourself

Match the generic method concepts

<T> T identity(T)

Generic method signature
T identity(T)

Non-generic signature
return value

Return value
identity("Hello")

Call with type inference
Where does <T> go in a generic method?
- At the end of the method body
- After the class name
- Before the return type
- Below the parameter list
Show answer

Before the return type — <T> comes before the return type; e.g. static <T> T identity(T value).

Complete the generic method declaration

static ____ T identity(T value) {
    return value;
}

Show answer

static <T> T identity(T value) {
    return value;
}

Next lesson: Bounded type

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