我們之前寫Lambda表示式，如果Lambda表示式有對應方法，我們可以直接引用方法作為Lambda表示式。這種做法叫做方法引用。方法引用用”::”分隔。

例子

@Test
public void methodRefTest() {
    MethodReference.Class1 class1 = new MethodReference.Class1();
    double result = doubleUnatuOperator(4.5, class1::return1);
    System.out.println("result:" + result);
}

public double doubleUnatuOperator(double num, DoubleUnaryOperator doubleUnaryOperator) {
    return doubleUnaryOperator.applyAsDouble(num);
}
static class Class1 {

    public double return1(double i) {
        return 1 + i;
    }

}
 

這裡doubleSupplier的表示式呼叫Class1.return1方法，就可以使用方法引用。

方法引用有四種

引用靜態方法

ContainingClass::staticMethodName

例子: String::valueOf，對應的Lambda：(s) -> String.valueOf(s)

比較容易理解，和靜態方法呼叫相比，只是把.換為::

引用特定物件的例項方法

containingObject::instanceMethodName

例子: x::toString，對應的Lambda：() -> this.toString()

與引用靜態方法相比，都換為例項的而已

引用建構函式

ClassName::new

例子: String::new，對應的Lambda：() -> new String()

建構函式本質上是靜態方法，只是方法名字比較特殊。

引用特定型別的任意物件的例項方法

這個不清楚

例子

public class MethodReference {

    @Test
    public void staticMethodRefTest() {
        double result = doubleUnatuOperator(4.5, Class1::staticReturn1);
        System.out.println("result:" + result);
    }

    @Test
    public void methodRefTest() {
        MethodReference.Class1 class1 = new MethodReference.Class1();
        double result = doubleUnatuOperator(4.5, class1::return1);
        System.out.println("result:" + result);
    }

    @Test
    public void constructorRefTest() {
        Class1 class1 = supplier(Class1::new);
        System.out.println(class1);
    }

    @Test
    public void ContainingTypeRefTest() {
        String[] stringArray = {"Barbara", "James", "Mary", "John",
                "Patricia", "Robert", "Michael", "Linda"};
        Arrays.sort(stringArray, (a, b) -> {
            return a.compareToIgnoreCase(b);
        });
        System.out.println(Arrays.toString(stringArray));
        Arrays.sort(stringArray, String::compareToIgnoreCase);
        System.out.println(Arrays.toString(stringArray));
        
    }

    public static double doubleUnatuOperator(double num, DoubleUnaryOperator doubleUnaryOperator) {
        return doubleUnaryOperator.applyAsDouble(num);
    }

    public Object unaryOperator(Object t,UnaryOperator<Object> operator) {
        return operator.apply(t);
    }

    public Class1 supplier(Supplier<Class1> supplier) {
        return supplier.get();
    }


    static class Class1 {

        private String name;

        public String getName() {
            return name;
        }

        public void setName(String name) {
            this.name = name;
        }

        public Class1() {

        }

        public Class1(String name) {
            System.out.println(name + "構造成功");
        }

        public double return1(double i) {
            return 1 + i;
        }

        public static double staticReturn1(double i) {
            return i + 1;
        }

        public double add(Class1 a) {
            return 12.0;
        }

        @Override
        public String toString() {
            return "Class1{" +
                    "name='" + name + '\'' +
                    '}';
        }
    }
}