Java的Function接口与andThen组合及解读

@FunctionalInterface
public interface Function<T, R> {
    R apply(T t);
    
    default <V> Function<T, V> andThen(Function<? super R, ? extends V> after) {
        Objects.requireNonNull(after);
        return (T t) -> after.apply(apply(t));
    }
    
    default <V> Function<V, R> compose(Function<? super V, ? extends T> before) {
        Objects.requireNonNull(before);
        return (V v) -> apply(before.apply(v));
    }
    
    static <T> Function<T, T> identity() {
        return t -> t;
    }
}

// 将字符串转换为大写
Function<String, String> toUpperCase = s -> s.toUpperCase();
String result = toUpperCase.apply("hello"); // 输出：HELLO

// 将字符串转换为其长度
Function<String, Integer> lengthFunction = s -> s.length();
Integer length = lengthFunction.apply("hello"); // 输出：5

class EmailValidator implements Function<String, Boolean> {
    @Override
    public Boolean apply(String email) {
        return email != null && email.contains("@");
    }
}

// 使用自定义函数
Function<String, Boolean> validator = new EmailValidator();
boolean isValid = validator.apply("test@example.com"); // 输出：true

// 定义两个简单函数
Function<Integer, Integer> multiplyByTwo = num -> num * 2;
Function<Integer, Integer> addTen = num -> num + 10;

// 组合函数：先乘以2，再加10
Function<Integer, Integer> combined = multiplyByTwo.andThen(addTen);
int result = combined.apply(5); // 执行流程：5 * 2 + 10 = 20

// 定义三个函数
Function<String, String> removeWhitespace = s -> s.replaceAll("\\s", "");
Function<String, String> toUpperCase = s -> s.toUpperCase();
Function<String, String> addPrefix = s -> "[PREFIX] " + s;

// 组合多个函数
Function<String, String> pipeline = removeWhitespace
        .andThen(toUpperCase)
        .andThen(addPrefix);

String result = pipeline.apply("  hello world  "); 
// 执行流程："  hello world  " -> "helloworld" -> "HELLOWORLD" -> "[PREFIX] HELLOWORLD"

Function<Integer, Integer> multiplyByTwo = num -> num * 2;
Function<Integer, Integer> addTen = num -> num + 10;

// 使用 andThen：先乘2，再加10
Function<Integer, Integer> combined1 = multiplyByTwo.andThen(addTen);
int result1 = combined1.apply(5); // 计算：(5 * 2) + 10 = 20

// 使用 compose：先加10，再乘2
Function<Integer, Integer> combined2 = multiplyByTwo.compose(addTen);
int result2 = combined2.apply(5); // 计算：(5 + 10) * 2 = 30

import java.util.Arrays;
import java.util.List;
import java.util.function.Function;
import java.util.stream.Collectors;

public class StreamMapExample {
    public static void main(String[] args) {
        List<String> words = Arrays.asList("apple", "banana", "cherry");
        
        // 定义函数：转换为大写并截取前3个字符
        Function<String, String> processWord = s -> s.toUpperCase().substring(0, 3);
        
        // 在 Stream 中使用函数
        List<String> result = words.stream()
                .map(processWord)
                .collect(Collectors.toList());
        
        System.out.println(result); // 输出：[APP, BAN, CHE]
    }
}

import java.util.ArrayList;
import java.util.List;
import java.util.function.Function;

public class DynamicFunctionChain {
    public static void main(String[] args) {
        // 动态构建函数链
        List<Function<String, String>> functions = new ArrayList<>();
        functions.add(s -> s.replace(" ", "_"));
        functions.add(String::toUpperCase);
        functions.add(s -> "[" + s + "]");
        
        // 组合所有函数
        Function<String, String> pipeline = functions.stream()
                .reduce(Function.identity(), Function::andThen);
        
        String result = pipeline.apply("hello world"); 
        // 输出：[HELLO_WORLD]
    }
}

import java.util.function.Function;

public class FunctionFactory {
    // 创建一个将字符串重复指定次数的函数
    public static Function<String, String> repeatFunction(int times) {
        return s -> {
            StringBuilder sb = new StringBuilder();
            for (int i = 0; i < times; i++) {
                sb.append(s);
            }
            return sb.toString();
        };
    }
    
    public static void main(String[] args) {
        Function<String, String> triple = repeatFunction(3);
        String result = triple.apply("abc"); // 输出：abcabcabc
    }
}