在Java中，如何创建每个都带有一个参数的函数指针列表？

Question

I've already looked here , that works for an argument less method list containing Runnables. I need to have Consumer in my case. This is what I have so far:

import com.google.common.collect.ImmutableList;
import java.util.List;
import java.util.function.Consumer;

public class experiment {

 private static List<Consumer<String>> activities;

 public experiment (){
 activities = ImmutableList.of(
     (userId) -> this::bar,        // throws error
     (userId) -> this::foo);
 }

 public void bar(String x) {
   System.out.println(x);
 }

 public void foo(String x) {
   System.out.println(x);
 }

 public static void main(String []args) {

  for (int i=0; i<2; i++) {
    activities.get(i).accept("activity #" + i);
  }
 }
}

The error I see is void is not a functional interface .

I don't understand why I'm getting that, both bar and foo implement the accept method of the Consumer interface. Is there something I'm overlooking here?

Answer 1

I believe you want to write ImmutableList.of(this::bar, this::foo) instead of ImmutableList.of((userId) -> this::bar, (userId) -> this::foo) .

If you do want to use lambdas instead of method references, you should instead write ImmutableList.of((userId) -> this.bar(userId), (userId) -> this.foo(userId)) .

Answer 2

Your lambda's are wrong. You either supply a method reference, or you supply a lambda expression. You're trying an incorrect combo.

Also, your two methods foo() and bar() should be static , and your instance variable should not be. To show how instance methods could be called, I've added baz() and qux() .

Here's showing full example:

public class Experiment {

    private List<Consumer<String>> activities;

    public Experiment() {
        this.activities = ImmutableList.of(
            (userId) -> Experiment.bar(userId),
            Experiment::foo,
            (userId) -> this.baz(userId),
            this::qux
        );
    }

    public static void bar(String x) {
        System.out.println(x);
    }

    public static void foo(String x) {
        System.out.println(x);
    }

    public void baz(String x) {
        System.out.println(this.hashCode() + ": " + x);
    }

    public void qux(String x) {
        System.out.println(this.hashCode() + ": " + x);
    }

    public static void main(String[] args) {
        Experiment ex = new Experiment();
        for (int i = 0; i < ex.activities.size(); i++) {
            ex.activities.get(i).accept("activity #" + i);
        }
    }
}