A lambda expression provides a concise implementation of a functional interface. A functional interface has one abstract method.

In this example, you will use a lambda expression to greet someone by name.

Source code

Save the following code as LambdaExample.java:

public class LambdaExample {
    void main() {
        Greeting greeting = name -> "Hello, " + name + "!";

        IO.println(greeting.greet("Alice"));
        IO.println(greeting.greet("Bob"));
    }
}

@FunctionalInterface
interface Greeting {
    String greet(String name);
}

Explanation

The Greeting interface declares one abstract method:

@FunctionalInterface
interface Greeting {
    String greet(String name);
}

The method accepts a name and returns a greeting.

The @FunctionalInterface annotation lets the compiler check that the interface meets the requirements of a functional interface.

Creating the lambda expression

The following statement assigns a lambda expression to a Greeting variable:

Greeting greeting = name -> "Hello, " + name + "!";

Each part has a purpose:

Part	Purpose
name	Receives the method argument
->	Separates the parameter from the body
"Hello, " + name + "!"	Produces the returned value

The compiler knows that name is a String because the greet() method declares a String parameter.

For a single parameter with an inferred type, parentheses are optional. You can also write:

Greeting greeting = (name) -> "Hello, " + name + "!";

Returning a value

This lambda has an expression body:

name -> "Hello, " + name + "!"

The expression’s result is returned automatically. You do not need braces or a return statement.

If you use a block body instead, you must return the value explicitly:

Greeting greeting = name -> {
    return "Hello, " + name + "!";
};

Both forms provide the same behavior.

Calling the implementation

Call the interface method through the variable:

greeting.greet("Alice")

This passes "Alice" to the lambda and returns "Hello, Alice!". The program then prints the returned string using IO.println().

Run the example

Compile and run the file:

javac LambdaExample.java
java LambdaExample

Output

Hello, Alice!
Hello, Bob!

A lambda expression needs a target functional interface, such as Greeting in this example. You can also use lambdas with standard functional interfaces from java.util.function, including Predicate, Function, and Consumer.