interface Strategy {
    String approach(String msg);
}

class Soft implements Strategy {
    public String approach(String msg) {
        return msg.toLowerCase() + "?";
    }
}

class Unrelated {
    static String twice(String msg) {
        return msg + " " + msg;
    }
}

public class Strategize {

    Strategy strategy;
    String msg;
    Strategize(String msg) {
        strategy = new Soft(); // [1] 構(gòu)建默認(rèn)的 Soft
        this.msg = msg;
    }

    void communicate() {
        System.out.println(strategy.approach(msg));
    }

    void changeStrategy(Strategy strategy) {
        this.strategy = strategy;
    }

    public static void main(String[] args) {
        Strategy[] strategies = {
                new Strategy() { // [2] Java 8 以前的匿名內(nèi)部類
                    public String approach(String msg) {
                        return msg.toUpperCase() + "!";
                    }
                },
                msg -> msg.substring(0, 5), // [3] 基于 Ldmbda 表達(dá)式，實(shí)例化 interface
                Unrelated::twice // [4] 基于 方法引用，實(shí)例化 interface
        };
        Strategize s = new Strategize("Hello there");
        s.communicate();
        for(Strategy newStrategy : strategies) {
            s.changeStrategy(newStrategy); // [5] 使用默認(rèn)的 Soft 策略
            s.communicate(); // [6] 每次調(diào)用 communicate() 都會(huì)產(chǎn)生不同的行為
        }
    }
}

interface Description {
    String brief();
}

interface Body {
    String detailed(String head);
}

interface Multi {
    String twoArg(String head, Double d);
}

public class LambdaExpressions {

    static Body bod = h -> h + " No Parens!"; // [1] 一個(gè)參數(shù)時(shí)，可以不需要擴(kuò)展 ()， 但這是一個(gè)特例
    static Body bod2 = (h) -> h + " More details"; // [2] 正常情況下的使用方式
    static Description desc = () -> "Short info"; // [3] 沒(méi)有參數(shù)的情況下的使用方式
    static Multi mult = (h, n) -> h + n; // [4] 多參數(shù)情況下的使用方式

    static Description moreLines = () -> { 
        // [5] 多行代碼情況下使用 `{}` + `return` 關(guān)鍵字
        // （在單行的 Lambda 表達(dá)式中 `return` 是非法的）
        System.out.println("moreLines()");
        return "from moreLines()";
    };

    public static void main(String[] args) {
        System.out.println(bod.detailed("Oh!"));
        System.out.println(bod2.detailed("Hi!"));
        System.out.println(desc.brief());
        System.out.println(mult.twoArg("Pi! ", 3.14159));
        System.out.println(moreLines.brief());
    }
}

interface Callable { // [1] 單一方法的接口（重要）
    void call(String s);
}

class Describe {
    void show(String msg) { // [2] 符合 Callable 接口的 call() 方法實(shí)現(xiàn)
        System.out.println(msg);
    }
}

public class MethodReferences {
    static void hello(String name) { // [3] 也符合 call() 方法實(shí)現(xiàn)
        System.out.println("Hello, " + name);
    }

    static class Description {
        String about;

        Description(String desc) {
            about = desc;
        }

        void help(String msg) { // [4] 靜態(tài)類的非靜態(tài)方法
            System.out.println(about + " " + msg);
        }
    }

    static class Helper {
        static void assist(String msg) { // [5] 靜態(tài)類的靜態(tài)方法，符合 call() 方法
            System.out.println(msg);
        }
    }

    public static void main(String[] args) {
        Describe d = new Describe();
        Callable c = d::show; // [6] 通過(guò)方法引用創(chuàng)建 Callable 的接口實(shí)現(xiàn)
        c.call("call()"); // [7] 通過(guò)該實(shí)例 call() 方法調(diào)用 show() 方法

        c = MethodReferences::hello; // [8] 靜態(tài)方法的方法引用
        c.call("Bob");

        c = new Description("valuable")::help; // [9] 實(shí)例化對(duì)象的方法引用
        c.call("information");

        c = Helper::assist; // [10] 靜態(tài)方法的方法引用
        c.call("Help!");
    }
}

// 方法引用與 Runnable 接口的結(jié)合使用

class Go {
    static void go() {
        System.out.println("Go::go()");
    }
}

public class RunnableMethodReference {

    public static void main(String[] args) {

        new Thread(new Runnable() {
            public void run() {
                System.out.println("Anonymous");
            }
        }).start();

        new Thread(
                () -> System.out.println("lambda")
        ).start();

        new Thread(Go::go).start();		// 通過(guò) 方法引用創(chuàng)建 Runnable 實(shí)現(xiàn)的引用
    }
}

// 未綁定的方法引用是指沒(méi)有關(guān)聯(lián)對(duì)象的普通方法
class X {
    String f() {
        return "X::f()";
    }
}

interface MakeString {
    String make();
}

interface TransformX {
    String transform(X x);
}

public class UnboundMethodReference {

    public static void main(String[] args) {
        // MakeString sp = X::f;       // [1] 你不能在沒(méi)有 X 對(duì)象參數(shù)的前提下調(diào)用 f()，因?yàn)樗?X 的方法
        TransformX sp = X::f;       // [2] 你可以首個(gè)參數(shù)是 X 對(duì)象參數(shù)的前提下調(diào)用 f()，使用未綁定的引用，函數(shù)式的方法不再與方法引用的簽名完全相同
        X x = new X();
        System.out.println(sp.transform(x));      // [3] 傳入 x 對(duì)象，調(diào)用 x.f() 方法
        System.out.println(x.f());      // 同等效果
    }
}

package com.github.xiao2shiqi.lambda;

// 未綁定的方法與多參數(shù)的結(jié)合運(yùn)用
class This {
    void two(int i, double d) {}
    void three(int i, double d, String s) {}
    void four(int i, double d, String s, char c) {}
}
interface TwoArgs {
    void call2(This athis, int i, double d);
}
interface ThreeArgs {
    void call3(This athis, int i, double d, String s);
}
interface FourArgs {
    void call4(
            This athis, int i, double d, String s, char c);
}

public class MultiUnbound {

    public static void main(String[] args) {
        TwoArgs twoargs = This::two;
        ThreeArgs threeargs = This::three;
        FourArgs fourargs = This::four;
        This athis = new This();
        twoargs.call2(athis, 11, 3.14);
        threeargs.call3(athis, 11, 3.14, "Three");
        fourargs.call4(athis, 11, 3.14, "Four", 'Z');
    }
}

class Dog {
    String name;
    int age = -1; // For "unknown"
    Dog() { name = "stray"; }
    Dog(String nm) { name = nm; }
    Dog(String nm, int yrs) {
        name = nm;
        age = yrs;
    }
}

interface MakeNoArgs {
    Dog make();
}

interface Make1Arg {
    Dog make(String nm);
}

interface Make2Args {
    Dog make(String nm, int age);
}

public class CtorReference {
    public static void main(String[] args) {
        // 通過(guò) ::new 關(guān)鍵字賦值給不同的接口，然后通過(guò) make() 構(gòu)建不同的實(shí)例
        MakeNoArgs mna = Dog::new; // [1] 將構(gòu)造函數(shù)的引用交給 MakeNoArgs 接口
        Make1Arg m1a = Dog::new; // [2] …………
        Make2Args m2a = Dog::new; // [3] …………
        Dog dn = mna.make();
        Dog d1 = m1a.make("Comet");
        Dog d2 = m2a.make("Ralph", 4);
    }
}

// 使用 @FunctionalInterface 注解強(qiáng)制執(zhí)行此 “函數(shù)式方法” 模式
@FunctionalInterface
interface Functional {
    String goodbye(String arg);
}

interface FunctionalNoAnn {
    String goodbye(String arg);
}

public class FunctionalAnnotation {
    // goodbye
    public String goodbye(String arg) {
        return "Goodbye, " + arg + "!";
    }

    public static void main(String[] args) {
        FunctionalAnnotation fa = new FunctionalAnnotation();

        // FunctionalAnnotation 沒(méi)有實(shí)現(xiàn) Functional 接口，所以不能直接賦值
//        Functional fac = fa;      // Incompatible ?

        // 但可以通過(guò) Lambda 將函數(shù)賦值給接口 （類型需要匹配）
        Functional f = fa::goodbye;
        FunctionalNoAnn fna = fa::goodbye;
        Functional fl = a -> "Goodbye, " + a;
        FunctionalNoAnn fnal = a -> "Goodbye, " + a;
    }
}

class Foo {}

class Bar {
    Foo f;
    Bar(Foo f) { this.f = f; }
}

class IBaz {
    int i;
    IBaz(int i) { this.i = i; }
}

class LBaz {
    long l;
    LBaz(long l) { this.l = l; }
}

class DBaz {
    double d;
    DBaz(double d) { this.d = d; }
}

public class FunctionVariants {
    // 根據(jù)不同參數(shù)獲得對(duì)象的函數(shù)表達(dá)式
    static Function<Foo, Bar> f1 = f -> new Bar(f);
    static IntFunction<IBaz> f2 = i -> new IBaz(i);
    static LongFunction<LBaz> f3 = l -> new LBaz(l);
    static DoubleFunction<DBaz> f4 = d -> new DBaz(d);
    // 根據(jù)對(duì)象類型參數(shù)，獲得基本數(shù)據(jù)類型返回值的函數(shù)表達(dá)式
    static ToIntFunction<IBaz> f5 = ib -> ib.i;
    static ToLongFunction<LBaz> f6 = lb -> lb.l;
    static ToDoubleFunction<DBaz> f7 = db -> db.d;
    static IntToLongFunction f8 = i -> i;
    static IntToDoubleFunction f9 = i -> i;
    static LongToIntFunction f10 = l -> (int)l;
    static LongToDoubleFunction f11 = l -> l;
    static DoubleToIntFunction f12 = d -> (int)d;
    static DoubleToLongFunction f13 = d -> (long)d;

    public static void main(String[] args) {
        // apply usage examples
        Bar b = f1.apply(new Foo());
        IBaz ib = f2.apply(11);
        LBaz lb = f3.apply(11);
        DBaz db = f4.apply(11);

        // applyAs* usage examples
        int i = f5.applyAsInt(ib);
        long l = f6.applyAsLong(lb);
        double d = f7.applyAsDouble(db);

        // 基本類型的相互轉(zhuǎn)換
        long applyAsLong = f8.applyAsLong(12);
        double applyAsDouble = f9.applyAsDouble(12);
        int applyAsInt = f10.applyAsInt(12);
        double applyAsDouble1 = f11.applyAsDouble(12);
        int applyAsInt1 = f12.applyAsInt(13.0);
        long applyAsLong1 = f13.applyAsLong(13.0);
    }
}

import java.util.function.BiConsumer;

class In1 {}
class In2 {}

public class MethodConversion {

    static void accept(In1 in1, In2 in2) {
        System.out.println("accept()");
    }

    static void someOtherName(In1 in1, In2 in2) {
        System.out.println("someOtherName()");
    }

    public static void main(String[] args) {
        BiConsumer<In1, In2> bic;

        bic = MethodConversion::accept;
        bic.accept(new In1(), new In2());

        // 在使用函數(shù)接口時(shí)，名稱無(wú)關(guān)緊要——只要參數(shù)類型和返回類型相同。Java 會(huì)將你的方法映射到接口方法。
        bic = MethodConversion::someOtherName;
        bic.accept(new In1(), new In2());
    }
}

import java.util.Comparator;
import java.util.function.*;

class AA {}
class BB {}
class CC {}

public class ClassFunctionals {

    static AA f1() { return new AA(); }
    static int f2(AA aa1, AA aa2) { return 1; }
    static void f3 (AA aa) {}
    static void f4 (AA aa, BB bb) {}
    static CC f5 (AA aa) { return new CC(); }
    static CC f6 (AA aa, BB bb) { return new CC(); }
    static boolean f7 (AA aa) { return true; }
    static boolean f8 (AA aa, BB bb) { return true; }
    static AA f9 (AA aa) { return new AA(); }
    static AA f10 (AA aa, AA bb) { return new AA(); }

    public static void main(String[] args) {
        // 無(wú)參數(shù)，返回一個(gè)結(jié)果
        Supplier<AA> s = ClassFunctionals::f1;
        s.get();
        // 比較兩個(gè)對(duì)象，用于排序和比較操作
        Comparator<AA> c = ClassFunctionals::f2;
        c.compare(new AA(), new AA());
        // 執(zhí)行操作，通常是副作用操作，不需要返回結(jié)果
        Consumer<AA> cons = ClassFunctionals::f3;
        cons.accept(new AA());
        // 執(zhí)行操作，通常是副作用操作，不需要返回結(jié)果，接受兩個(gè)參數(shù)
        BiConsumer<AA, BB> bicons = ClassFunctionals::f4;
        bicons.accept(new AA(), new BB());
        // 將輸入?yún)?shù)轉(zhuǎn)換成輸出結(jié)果，如數(shù)據(jù)轉(zhuǎn)換或映射操作
        Function<AA, CC> f = ClassFunctionals::f5;
        CC cc = f.apply(new AA());
        // 將兩個(gè)輸入?yún)?shù)轉(zhuǎn)換成輸出結(jié)果，如數(shù)據(jù)轉(zhuǎn)換或映射操作
        BiFunction<AA, BB, CC> bif = ClassFunctionals::f6;
        cc = bif.apply(new AA(), new BB());
        // 接受一個(gè)參數(shù)，返回 boolean 值： 測(cè)試參數(shù)是否滿足特定條件
        Predicate<AA> p = ClassFunctionals::f7;
        boolean result = p.test(new AA());
        // 接受兩個(gè)參數(shù)，返回 boolean 值，測(cè)試兩個(gè)參數(shù)是否滿足特定條件
        BiPredicate<AA, BB> bip = ClassFunctionals::f8;
        result = bip.test(new AA(), new BB());
        // 接受一個(gè)參數(shù)，返回一個(gè)相同類型的結(jié)果，對(duì)輸入執(zhí)行單一操作并返回相同類型的結(jié)果，是 Function 的特殊情況
        UnaryOperator<AA> uo = ClassFunctionals::f9;
        AA aa = uo.apply(new AA());
        // 接受兩個(gè)相同類型的參數(shù)，返回一個(gè)相同類型的結(jié)果，將兩個(gè)相同類型的值組合成一個(gè)新值，是 BiFunction 的特殊情況
        BinaryOperator<AA> bo = ClassFunctionals::f10;
        aa = bo.apply(new AA(), new AA());
    }
}

// 創(chuàng)建處理 3 個(gè)參數(shù)的函數(shù)式接口
@FunctionalInterface
public interface TriFunction<T, U, V, R> {
    
    R apply(T t, U u, V v);
}

public class TriFunctionTest {
    static int f(int i, long l, double d) { return 99; }

    public static void main(String[] args) {
        // 方法引用
        TriFunction<Integer, Long, Double, Integer> tf1 = TriFunctionTest::f;
        // Lamdba 表達(dá)式
        TriFunction<Integer, Long, Double, Integer> tf2 = (i, l, d) -> 12;
    }
}

import java.util.function.Function;

interface FuncSS extends Function<String, String> {}        // [1] 使用繼承，輕松創(chuàng)建屬于自己的函數(shù)式接口

public class ProduceFunction {
    // produce() 是一個(gè)高階函數(shù)：既函數(shù)的消費(fèi)者，產(chǎn)生函數(shù)的函數(shù)
    static FuncSS produce() {
        return s -> s.toLowerCase();    // [2] 使用 Lambda 表達(dá)式，可以輕松地在方法中創(chuàng)建和返回一個(gè)函數(shù)
    }

    public static void main(String[] args) {
        FuncSS funcSS = produce();
        System.out.println(funcSS.apply("YELLING"));
    }
}

class One {}
class Two {}

public class ConsumeFunction {
    static Two consume(Function<One, Two> onetwo) {
        return onetwo.apply(new One());
    }

    public static void main(String[] args) {
        Two two = consume(one -> new Two());
    }
}

public class ClosureExample {
    public static void main(String[] args) {
        int a = 10;
        int b = 20;

        // 這是一個(gè)閉包，因?yàn)樗东@了外部作用域中的變量 a 和 b
        IntBinaryOperator closure = (x, y) -> x * a + y * b;

        int result = closure.applyAsInt(3, 4);
        System.out.println("Result: " + result); // 輸出 "Result: 110"
    }
}

import java.util.function.Function;

public class FunctionComposition {
    static Function<String, String> f1 = s -> {
        System.out.println(s);
        return s.replace('A', '_');
    },
    f2 = s -> s.substring(3),
    f3 = s -> s.toLowerCase(),
    // 重點(diǎn)：使用函數(shù)組合將多個(gè)函數(shù)組合在一起
    // compose 是先執(zhí)行參數(shù)中的函數(shù)，再執(zhí)行調(diào)用者
    // andThen 是先執(zhí)行調(diào)用者，再執(zhí)行參數(shù)中的函數(shù)
    f4 = f1.compose(f2).andThen(f3);        

    public static void main(String[] args) {
        String s = f4.apply("GO AFTER ALL AMBULANCES");
        System.out.println(s);
    }
}

public class PredicateComposition {
    static Predicate<String>
            p1 = s -> s.contains("bar"),
            p2 = s -> s.length() < 5,
            p3 = s -> s.contains("foo"),
            p4 = p1.negate().and(p2).or(p3);    // 使用謂詞組合將多個(gè)謂詞組合在一起，negate 是取反，and 是與，or 是或

    public static void main(String[] args) {
        Stream.of("bar", "foobar", "foobaz", "fongopuckey")
                .filter(p4)
                .forEach(System.out::println);
    }
}

public class CurryingAndPartials {
    static String uncurried(String a, String b) {
        return a + b;
    }

    public static void main(String[] args) {
        // 柯里化的函數(shù)，它是一個(gè)接受多參數(shù)的函數(shù)
        Function<String, Function<String, String>> sum = a -> b -> a + b;
        System.out.println(uncurried("Hi ", "Ho"));

        // 通過(guò)鏈?zhǔn)秸{(diào)用逐個(gè)傳遞參數(shù)
        Function<String, String> hi = sum.apply("Hi ");
        System.out.println(hi.apply("Ho"));

        Function<String, String> sumHi = sum.apply("Hup ");
        System.out.println(sumHi.apply("Ho"));
        System.out.println(sumHi.apply("Hey"));
    }
}

import java.util.function.Function;

public class Curry3Args {
    public static void main(String[] args) {
        // 柯里化函數(shù)
        Function<String,
                Function<String,
                        Function<String, String>>> sum = a -> b -> c -> a + b + c;

        // 逐個(gè)傳遞參數(shù)
        Function<String, Function<String, String>> hi = sum.apply("Hi ");
        Function<String, String> ho = hi.apply("Ho ");
        System.out.println(ho.apply("Hup"));
    }
}

import java.util.function.IntFunction;
import java.util.function.IntUnaryOperator;

public class CurriedIntAdd {
    public static void main(String[] args) {
        IntFunction<IntUnaryOperator> curriedIntAdd = a -> b -> a + b;
        IntUnaryOperator add4 = curriedIntAdd.apply(4);
        System.out.println(add4.applyAsInt(5));
    }
}