Function และ BiFunction

สิ่งที่ฟังก์ชันแทน

import java.util.function.Function;

public class Main {
    public static void main(String[] args) {
        Function<String, Integer> length = s -> s.length();
        System.out.println(length.apply("hello"));
    }
}

การเรียกใช้ apply

import java.util.function.Function;

public class Main {
    public static void main(String[] args) {
        Function<String, String> upper = String::toUpperCase;
        System.out.println(upper.apply("java"));
    }
}

การประกอบด้วย andThen

import java.util.function.Function;

public class Main {
    public static void main(String[] args) {
        Function<Integer, Integer> times2 = n -> n * 2;
        Function<Integer, Integer> plus3 = n -> n + 3;
        System.out.println(times2.andThen(plus3).apply(5));
    }
}

การประกอบด้วย compose

import java.util.function.Function;

public class Main {
    public static void main(String[] args) {
        Function<Integer, Integer> times2 = n -> n * 2;
        Function<Integer, Integer> plus3 = n -> n + 3;
        System.out.println(times2.compose(plus3).apply(5));
    }
}

Function.identity

import java.util.function.Function;

public class Main {
    public static void main(String[] args) {
        Function<String, String> id = Function.identity();
        System.out.println(id.apply("unchanged"));
    }
}

ทำความรู้จัก BiFunction

import java.util.function.BiFunction;

public class Main {
    public static void main(String[] args) {
        BiFunction<Integer, Integer, Integer> add = (a, b) -> a + b;
        System.out.println(add.apply(4, 6));
    }
}

BiFunction กับชนิดที่ผสมกัน

import java.util.function.BiFunction;

public class Main {
    public static void main(String[] args) {
        BiFunction<String, Integer, String> repeat = (s, n) -> s.repeat(n);
        System.out.println(repeat.apply("ab", 3));
    }
}

BiFunction กับ andThen

import java.util.function.BiFunction;
import java.util.function.Function;

public class Main {
    public static void main(String[] args) {
        BiFunction<Integer, Integer, Integer> add = (a, b) -> a + b;
        Function<Integer, String> label = n -> "sum=" + n;
        System.out.println(add.andThen(label).apply(7, 8));
    }
}

ฟังก์ชันในคอลเลกชัน

import java.util.HashMap;
import java.util.Map;

public class Main {
    public static void main(String[] args) {
        Map<String, Integer> lengths = new HashMap<>();
        lengths.computeIfAbsent("hello", String::length);
        System.out.println(lengths);
    }
}

การเก็บฟังก์ชันไว้ในตัวแปร

import java.util.function.Function;

public class Main {
    public static void main(String[] args) {
        Function<Integer, Integer> square = n -> n * n;
        Function<Integer, Integer> cube = n -> n * n * n;
        Function<Integer, Integer> chosen = square;
        System.out.println(chosen.apply(5));
    }
}

การสร้างสายการแปลงข้อมูล

import java.util.function.Function;

public class Main {
    public static void main(String[] args) {
        Function<String, String> trim = String::trim;
        Function<String, String> upper = String::toUpperCase;
        Function<String, Integer> len = String::length;
        System.out.println(trim.andThen(upper).andThen(len).apply("  abc "));
    }
}

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