How to use Function interface in Java8

Introduction to the Function interface in Java 8

Java 8 provides a functional interface Function. This interface represents doing some operations on a parameter and then returning the value after the operation. This interface has an abstract method apply, which indicates the operation on the parameters.

// Java Function 接口的定义
@FunctionalInterface
public interface Function<T, R> {
R apply(T t);

default <V> Function<V, R> compose(Function<? super V, ? extends T> before) {
    Objects.requireNonNull(before);
    return (V v) -> apply(before.apply(v));
}

default <V> Function<T, V> andThen(Function<? super R, ? extends V> after) {
    Objects.requireNonNull(after);
    return (T t) -> after.apply(apply(t));
}

static <T> Function<T, T> identity() {
    return t -> t;
}

Usage of Function interface

apply The method is used as follows:

Function<Integer, Integer> add = p -> p + 10;
Integer result = add.apply(10);
// 这里会输出 20，因为这个函数定义的操作时把参数加上 10 后返回
System.out.println(result);

compose The parameter of the method is also a Function object. When calling the compose method on the Function of A, pass in the Function object of B, and then get a new Function object C. The implementation logic of C's Function object is to first use B's apply method to operate the parameters, and then use the result as a parameter Pass to the apply method of this Function object of A, and then return the result after execution. The

addThen method has a similar principle, except that the order of internal execution methods is different.

Function<Integer, Integer> multiplyTen = a -> a * 10;
Function<Integer, Integer> addTen = a -> a + 10;
// 先增加 10，然后再乘 10，输出结果 110
Function<Integer, Integer> addTenThenMultiplyTen = multiplyTen.compose(addTen);
System.out.println(addTenThenMultiplyTen.apply(1));

// 先乘 10，然后再加 10，输出结果 20
Function<Integer, Integer> multiplyTenAddTenThen = multiplyTen.andThen(addTen);
System.out.println(multiplyTenAddTenThen.apply(1));

Function interface instance

public class CheckUtils {
    private static final Function<String, String> lengthCheck = params -> {
        if (params.length() > 100) {
            throw new RuntimeException("Length exceed max limit.");
        }
        return params;
    };

    private static final Function<String, String> invalidCharacterCheck = str -> {
        if (!str.matches("^[a-f,A-F]$")) {
            throw new RuntimeException("Contains invalid character.");
        }
        return str;
    };
    /**
     * 这里的公共方法组合了该类中的基本校验逻辑构成一个复合的逻辑
     */
    public static void checkStringLengthAndPhoneNumber(String string) {
        invalidCharacterCheck.compose(lengthCheck).apply(string);
    }
}

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