Using Lambda Expressions in your Java Code.

Ajesh Kalayil
3 min readOct 6, 2021

All about making clearer and flexible code

Photo by Lagos Techie on Unsplash

As we know, JDK 1.8 is the game-changer in the Java ecosystem, which is shipped with a lot of improvements, a new rich set of functionalities in the existing java development system as wells as introduced new game-changing APIs.That’s Streams and Lambdas.

Throughout this article let us walk through the functional programming in java using a lambda expression.

Before going further let’s understand key things.

What is a lambda expression?

Lambda expression is used to implement functional interfaces.

Why lambda expression?

It’s used to pass a piece of executable code to a method as an argument rather than implementing them in a separate method/class and call inside the function.

When lambda expression?

To distribute the processing of collections over multiple threads in parallel by effectively utilizing the multicore CPUs

In the beginning, we were told that lambda expressions implement functional interfaces. The functional interface is an interface that contains only one abstract method. But it may contain more than one default and static method.

@FunctionalInterface annotation is used to represent a functional interface. This annotation is not mandatory, but it helps the compiler to throw errors if the interface is not a functional interface at compile time.

Inside the java.util.function package, functional interfaces are defined.

The java.util.function package contains more than 40 functional interfaces and they are organized into 4 categories. They are

  • Supplier<T> {...}
  • Consumer<T> {...}
  • Predicate<T> {...}
  • Function<T, R> {...}

Explaining the functional interfaces is not in the scope of this article. But I’ll give a brief overview of functional interfaces to recap.

  • Supplier<T> is used to get a result of type T. It contains an abstract method T get() .
  • Consumer<T> processes the argument type T and doesn’t return anything. It contains an abstract method void accept(T t).
  • Predicate<T> negates the argument type T and returns the type boolean. It contains an abstract method boolean test(T t).
  • Function<T, R> computes the argument type T and returns the type R. It contains an abstract method R apply(T t) .

Now we have the basic knowledge of functional interfaces. Let’s implement these functional interfaces using a lambda expression.

Implementing Supplier Interface

We know that the Supplier interface contains only one abstract method get() which doesn’t take any arguments but returns a type of T.

So the lambda expression will look like this

Supplier<Double> randomValue = () -> Math.random();System.out.println(randomValue.get());

Implementing Consumer Interface

As we know Consumer interface contains only one abstract method accept() which takes a parameter of type T and doesn’t return anything.

So the lambda expression will look like this

Consumer<String> printString = s -> System.out::println;printString.accept("Hello World");

Implementing Predicate Interface

The Predicate interface contains only one abstract method test() which takes an argument of type T and returns boolean.

So the lambda expression will look like this

Predicate<Integer> isGreaterThanZero = i -> (i >0) ;System.out.println(isGreaterThanZero.test(20));

Implementing Function Interface

The Function interface contains only one abstract method apply() which takes a parameter of type T and returns a type R.

So the lambda expression will look like this

Function<String,Integer> length = s -> s.length();System.out.println(length.apply(“Hello World”));

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