Introduction

Recently, I was blessed with the opportunity to use Java because of my work. I updated my ancient Java (1.3) knowledge.

What is a lambda expression?

This is a function to easily implement a functional interface.

This alone is refreshing, so I would like to explain it with an example.

Compare lambda expressions with code

Code that doesn't use lambda expressions

public static void main(String[] args) {

  class Sample implements Runnable {
    public void run() {
      System.out.println("I Love Pengin!");
    }
  }

  Runnable runner = new Sample();
  times( 10, runner );
}

public static void times( int count, Runnable runner ) {
  for( int i = 0; i < count; i++ ) {
    runner.run();
  }
}

Code using lambda expressions

public static void main(String[] args) {

  funcCall( 10, () -> System.out.println("I Love Pengin!") );
}
public static void times( int count, Runnable runner ) {
  for( int i = 0; i < count; i++ ) {
    runner.run();
  }
}

What is good to use?

The example is the processing to the times function that repeats the specified processing multiple times. If you compare them, you can see that the lambda expression is quite complete.

The method of substituting a part of the processing with another object is called delegation. By using a lambda expression, I think the advantage is that this delegation is much easier to write.

How to write a lambda expression

Format

Let's look at the format when using a lambda expression

() -> System.out.println("I Love Pengin!")

The first () represents the method argument. We don't use arguments this time, but you can use them.
-> describes the actual processing content. If the processing content spans multiple processes, write -> {Processing} here.

When using arguments and return values, the implementation of the called side (times function in this case) is slightly different.

Lambda expression with arguments

When using

By specifying the argument of the lambda expression, the argument can be used during processing.

(i) -> System.out.println("I Love Pengin!" + i)

When making

It is necessary to use the function interface properly according to the type of argument and return value to be used. If there is one argument and no return value, it will be as follows.

public static void times( int count, Consumer<Integer> runner ) {
  for( int i = 0; i < count; i++ ) {
    runner.accept(new Integer(i));
  }
}

The interface to use properly is as follows.

type	Function interface	Method
0 arguments that do not return a value	Runnable	run
1 argument that returns no value	Consumer	accept
2 arguments that do not return a value	BiConsumer	accept
0 arguments that return a value	Supplier	get
1 argument that returns a value	Function	apply
2 arguments that return a value	BiFunction	apply
1 argument that returns a boolean value	Predicate	test
Two arguments that return a boolean value	BiPredicate	test
1 argument that returns the operation result	UnaryOperator	test
2 arguments that return the operation result	BinaryOperator	test

A Java user over a dozen years ago tried to study the functions of Java8 (Lambda expression).