Java’s functional interface is available since v1.8. It allows you to leverage lambda function’s possibilities.

We already talked about parametrized java function where we were also utilizing the lambdas. So to add a twist, we’re going to add some generics into the mix.

Generics

Java is a typed language, everything should have a type. Generics, or generic type is a generic class or interface that is parameterized over types.

Without going into details, you may have seen <T> in java like:

public class Box<T> {
    public T exampleMethod() { ...}
}

Where T stands for type and can be anything for example new Box<String>() is a Box object of a String (meaning all T inside box will be considered as String).

You may also have seen it in the Option〈T〉article, it’s the same T! … Mind blown 🤯 ? Not really, it’s just java.

Generics adds a bit of abstraction on the types allowing to make ( compile-time safe) methods and objects operating on various types.

Generics with Lambda

To work with lambda, you need functional interface because () -> T is not an accepted type in Java. So you need your functional interface to actually define and use your lambda in methods.

Create a Generic functional interface

You can define one like:

@FunctionalInterface
public interface GenericSupplier<T> {
    T doStuff();
}

Functional interfaces have only one method that can be called. Calling them will automatically call the lambda function.

Use your generic functional interface

Let’s say you have a very simple lambda method, you can define it like:

public GenericSupplier<Boolean> supplier=()->true;

Now you want to create a generic method that will take your generic functional interface:

public <T> T genericMethod(GenericSupplier<T> supplier) {
    return supplier.doStuff();
}

Note that you have <T> T at the beginning of the method:

• The first <T> means that it’s a generic method of type T that is used to define the T of the GenericSupplier<T>
• The second T is the return type and could be anything else like String or Optional<T>.

Test your generic lambda method

If you want to test it you can run it like (having the previous bits of code in a GenericLambda class):

@Test
public void genericLambdaMethod(){
    private final GenericLambda genericLambda=new GenericLambda();
    assertTrue(genericLambda.genericMethod(()->true));
    assertTrue(genericLambda.genericMethod(genericLambda.supplier));
}

See that you can pass the supplier as defined or directly the anonymous lambda within the method, and it will be triggered automatically. You can also be creative like:

@Test
public void genericLambdaMethodOptional(){
    GenericLambda.GenericSupplier<Optional<String>>supplier=()->Optional.of("Hello");
    Assertions.assertEquals("Hello",genericLambda.genericMethod(supplier).orElse("fail"));
}

Where the lambda function returns an Optional<String> that we can leverage.

Generics allow to be very flexible with our lambda and are a necessary fit in your refactoring toolbox. Functional interface made me think of typealias in Kotlin where you can simplify types with an alias, but this is mostly cosmetic as you can pass lambda as a type without an interface in Kotlin.