此表达式的目标类型必须是Java中的功能接口

Question

lmod.forEachWithIndex( (e, i) -> lmod.set(i, e*2));

I am getting the error:

The method forEachWithIndex(Object) in the type XList is not applicable for the arguments (( e, i) -> {}) The target type of this expression must be a functional interface

What does this mean? How can I fix it?

 import java.util.*;

public class Main {

public static void main(String[] args) {

XList<Integer> lmod = XList.of(  new  Integer[]  {1,2,8, 10, 11, 30, 3, 4});  
lmod.forEachWithIndex( (e, i) -> lmod.set(i, e*2));
System.out.println(lmod);

}}

---

 import java.util.ArrayList; 
 import java.util.Collection;
 import java.util.Iterator;
 import java.util.List; 
 import java.util.Set;
 import java.util.function.Consumer;

public class XList implements Collection, Consumer {

public List<T> list = new ArrayList<T>();

public XList(T[] ints) {

    for( int i = 0; i < ints.length; i++)
        list.add(ints[i]);

}


public static XList<Integer> of(Integer[] ints) {

    return new XList(ints);
}

  public void forEachWithIndex(Object object) {

  }

   @Override
   public void accept(T t) {

   }

}

I add the function interface Consumer but I still do not know what to do next

Answer 1

You can bypass your error by below code but I will strongly recommend you to modify your forEachWithIndex parameter from Object to a BiConsumer
like

public void forEachWithIndex(BiConsumer biConsumer) {}

If you still want to strict to your implementation the here is the code may help you.

BiConsumer biConsumer = (e, i) -> lmod.set(i, e*2);
lmod.forEachWithIndex(biConsumer);

Also, I believe when you pass the BiConsumer to a method you are going to use it inside, in that case, you need to do the casting if you put the parameter as Object .