Parameterized lambda

Learn parameterized lambdas with (x) -> ....

Parameterized lambda

A lambda can take parameters: (String s) -> s.length(). The type is usually inferred; (s) -> s.length() also works. With a single parameter the parentheses are optional: s -> s.length().

Multiple parameters are separated by commas: (a, b) -> a + b. When the body is a single expression, no return is needed; Java returns the result automatically. With a block body return is required: (a, b) -> { return a + b; }.

Block lambda and return

When a lambda body needs multiple statements, you write a block with curly braces. Inside the block, a return statement returns the result. Example: (int a, int b) -> { int sum = a + b; return sum * 2; }.

Even with a single-line block body, return is required: (a, b) -> { return a + b; }. This differs from an expression body, where return is omitted. Mixing the two styles by mistake causes a compile error.

Exercise A two-number adder lambda

Define java.util.function.BiFunction<Integer, Integer, Integer> add = (a, b) -> a + b;. Call it with int result = add.apply(3, 4);. Print the result with System.out.println(result);.

Show example solution

import java.util.function.BiFunction;

public class Main {
    public static void main(String[] args) {
        BiFunction<Integer, Integer, Integer> add = (a, b) -> a + b;
        int result = add.apply(3, 4);
        System.out.println(result);
    }
}

Test yourself

Complete the single-parameter lambda

java.util.function.Function<String, Integer> len = ____ -> s.length();

Show answer

java.util.function.Function<String, Integer> len = s -> s.length();

What does (a, b) -> a + b return?
- A Runnable instance
- Throws an error
- The sum of a and b
- Prints a and b
Show answer

The sum of a and b — An expression body returns the result automatically; the sum is returned.
Match the lambda body types

() -> ...

No parameters
s -> ...

Single parameter
(a, b) -> ...

Two parameters
{ return ...; }

Block body

Next lesson: Method reference

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