A thorough understanding of lambda functions and interfaces

First, what is the lambda

• Functional Programming? Anonymous inner classes? concise?
• [Lambda] from academia, mainly for the actions of the simplified internal class, widely used in the functional interface.

Second, why should learn lambda?

• Do not ask me why, the director is my cousin.
• This article allows you to thoroughly learn to use lambda, wherever you can () -> {} a winded.

Three, lambda syntax

• A complete lambda expression

(Type1 param1,Type2 param2,Type3 params)->{

	statement1;
	statement2;
	....
	return data;
}
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1, () -> {}, lambda parameter, arrows, lambda body indispensable

2, parameter types may be omitted, and are basically omitted

3, lambda body braces, return and end with a semicolon if only one statement can be omitted

4, lambda expressions can be used as a reference method

No-argument lambda

• More advanced than anonymous inner classes, interfaces names, function names are omitted.
• lambda body is only one line, {} may be omitted.

There are parameters lambda

• In many cases idea can be optimized, but however. . . Submitted code base is still a pile of gray idea prompted.

Fourth, the function interface

• Lambda can be used is based on a function interface, i.e. the internal interface is only an abstract method, not only a method.
• lambda expression type inference is related with the interface function, the compiler can infer context information according to the parameter type.

Some typical interface functions

1、BiFunction

@FunctionalInterface
public interface BiFunction<T, U, R> {
    R apply(T t, U u);

    default <V> BiFunction<T, U, V> andThen(Function<? super R, ? extends V> after) {
        Objects.requireNonNull(after);
        return (T t, U u) -> after.apply(apply(t, u));
    }
}
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1, this function is widely used in the interface in java 8 stream.

2, there are two methods, only an abstract method

3, apply abstract method accepts two parameters, a type T, U, the return value is R.

4, following the get () function of the third parameter BiFunction interface, so you call the get () function can pass a line with the interface specifications lambda. For parameter 1, parameter 2 how to operate, operation, encryption. . . Lambda depends on your body.

2、Consumer

• Consumption function, the return value is void.

@FunctionalInterface
public interface Consumer<T> {
    void accept(T t);

    default Consumer<T> andThen(Consumer<? super T> after) {
        Objects.requireNonNull(after);
        return (T t) -> { accept(t); after.accept(t); };
    }
}

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• Return Value void, it is commonly used side-effects.

3、Predicate

• Assertion / test condition, define an abstract method, there must be a reference the value returned boolean.
• The most commonly used in the stream in the filter (), can be composed of predicate call chain.

//以下都可以做一个Predicate的lambda
1、(s)->s.length()>0;
2、(v)->v > 100;
3、(o)->o.startWith("a");

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4、Supplier

• It does not require input, output T. Like a container, calls get returned object.

Supplier<Integer> supplier = new Supplier<Integer>() {
            @Override
            public Integer get() {
                //返回一个随机值
                return new Random().nextInt();
            }
        };
System.out.println(supplier.get());
System.out.println(supplier.get());
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Method references

• Simply put, that is, so that you can re-use existing defined method, passing them as lambda, the use :: to represent. There are three types.
• The core point: the function signature to comply with that type of mass participation, the number of parameters, return type is void.

1, a reference method static method

• Static method acting on the object.

  public static void main(String[] args) {
    List<String> list = Lists.newArrayList("1","2","3","4");

    List<Integer> collect1 = list.stream().map(v -> Integer.parseInt(v))
        .collect(Collectors.toList());
    List<Integer> collect2 = list.stream().map(Integer::parseInt)
        .collect(Collectors.toList());
    
  }
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2, any type of pointing method reference example of a method

1, this generally refers to any type of lambda into your argument types.

2, as the following to the Senate is v, v can be written to call v.length type String :: length.

  public static void main(String[] args) {

    List<String> list = Lists
        .newArrayList("hello", "how", "are", "you", "i", "am", "fine", "thank", "you", "and",
            "you");
    Set<Integer> collect = list.stream().map(v -> v.length()).collect(Collectors.toSet());

    Set<Integer> collect1 = list.stream().map(String::length).collect(Collectors.toSet());
    System.out.println(collect);
    System.out.println(collect1);
  }
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3, point to methods of both object references

• Similarly to the first two points, not described in detail.

V. Summary

• Introduces the lambda expression and function interface
• Using the core function of the interface is to engage in the function signature, pass a qualifying signature of lambda, you can () -> {a} instigate.