如何使用方法引用调用参数化方法 [英] How to invoke parameterized method with method reference

问题描述

考虑下面的代码，

interface TestInter {
    public void abc();
}


class DemoStatic {

    public static void testStatic(String abc) {
        System.out.println(abc);
    }

    public void runTest () {

        // Using lambda expression.
        String str = "demo string" ;
        TestInter demo1 = () -> DemoStatic.testStatic(str);
        demo1.abc();

        // Using method reference.
        TestInter demo2 = DemoStatic::testStatic; // This line is not compiling.
        demo2.abc();
    }
}

我们可以调用 testStatic 作为主体的方法接口的 abc（）实现if参数 testStaic（）方法将被删除，如这样回答中所述。

We can invoke the testStatic method as body of TestInter interface's abc() implementation if parameter of testStaic() method will be eliminated as described in this so answer.

但在这种情况下，我们如何为参数化方法编写方法参考 testStatic ？

But in this case how can we write method reference for parameterized method testStatic?

推荐答案

您的功能接口 TestInter 没有相应的 testStatic（String）方法。如果要使用 :: 表示法引用 testStatic（），则应添加参数：

Your functional interface TestInter does not have corresponding signature for your testStatic(String) method. If you want to refer to testStatic() using the :: notation, you should add the parameter:

interface TestInter2 {
    public void abc(String abc);
}

public void runTest () {
    TestInter2 demo2 = DemoStatic::testStatic;
    demo2.abc(str);
}

根据 Oracle Java教程，有四种方法参考：

According to Oracle Java tutorial, there are four kinds of method reference:

我准备了3个接口 - 适用于0,1和2参数。然后我们有3个静态和3个实例方法：

I prepared 3 interfaces - for 0, 1 and 2 parameters. Then we have 3 static and 3 instance methods:

interface F0 {
    void f0();
}
interface F1 {
    void f1(MetRef i1);
}
interface F2 {
    void f2(MetRef i1, MetRef i2);
}

public class MetRef {
    public static void stat0() {;}
    public static void stat1(MetRef a) {;}
    public static void stat2(MetRef a, MetRef b) {;}

    public void inst0() {;}
    public void inst1(MetRef a) {;}
    public void inst2(MetRef a, MetRef b) {;}
}

现在看看每个方法可以用作各种组合的方法参考，将它们与前一个表进行比较。请参阅不同的方法引用类型，并注意参数的来源。

Now look how each of the methods may be used as a method reference in various combinations, compare them with the previous table. See different method reference types in action and notice from where the parameters come.

public class Test {
    public static void main(String[] args) {
        final MetRef mr = new MetRef();

        final F0 mr01 = MetRef::stat0; // 1: f0() ~ MetRef.stat0() 
        final F0 mr02 = mr::inst0;     // 2: f0() ~ mr.inst0()
        final F0 mr04 = MetRef::new;   // 4: f0() ~ new MetRef()

        final F1 mr11 = MetRef::stat1; // 1: f1(i1) ~ MetRef.stat1(i1)
        final F1 mr12 = mr::inst1;     // 2: f1(i1) ~ mr.inst1(i1)
        final F1 mr13 = MetRef::inst0; // 3: f1(i1) ~ i1.inst0()       <== NOTICE!

        final F2 mr21 = MetRef::stat2; // 1: f2(i1, i2) ~ MetRef.stat2(i1, i2)
        final F2 mr22 = mr::inst2;     // 2: f2(i1, i2) ~ mr.inst2(i1, i2)
        final F2 mr23 = MetRef::inst1; // 3: f2(i1, i2) ~ i1.inst1(i2) <== NOTICE!
    }
}

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