java - 使用 Google Collections "No suitable method found for uniqueIndex..."将列表转换为 map (获取类型一致的问题)

Question

其中mappedRoles = Maps.uniqueIndex...可以看出Netbeans产生了如下错误:

no suitable method found for uniqueIndex(List<ReducedEntity>,<anonymous java.util.function.Function<ReducedEntity,Integer>>)
    method Maps.<K#1,V#1>uniqueIndex(Iterable<V#1>,com.google.common.base.Function<? super V#1,K#1>) is not applicable
      (cannot infer type-variable(s) K#1,V#1
        (argument mismatch; <anonymous java.util.function.Function<ReducedEntity,Integer>> cannot be converted to com.google.common.base.Function<? super V#1,K#1>...

问题演示:

package com.mycompany.testground;

import com.google.common.collect.Maps;
import java.util.ArrayList;
import java.util.List;
import java.util.Map;
import java.util.function.Function;

public class Main {
    public static void main(String[] args) {
        List<ReducedEntity> list = new ArrayList<>();
        Map<Integer, ReducedEntity> mappedRoles;
        mappedRoles = Maps.uniqueIndex(list, new Function<ReducedEntity, Integer>() {
            @Override
            public Integer apply(ReducedEntity from) {
                return from.getId();
            }
        });
    }
}

为了完整性减少了实体类(但可能没有必要)

package com.mycompany.testground;

public class ReducedEntity {
    protected Integer id;
    protected String name;

    public Integer getId() {
        return id;
    }

    public void setId(Integer id) {
        this.id = id;
    }

    public String getName() {
        return name;
    }

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

在遵循此页面上的乔斯回答后创建了以上内容 Java: how to convert a List<?> to a Map<String,?>并且我还无法理解为什么这不起作用，如图所示。

Answer 1

您正在使用 java.util.function.Function。 Maps.uniqueIndex() 方法需要一个 com.google.common.base.Function。修复您的导入。