lambda -- list转map

本是古典何须时尚 2023-02-24 13:25 11阅读 0赞

package com.example.demo.util;
    
    import com.example.demo.entity.UserPO;
    import com.google.common.collect.Maps;
    
    import java.util.ArrayList;
    import java.util.List;
    import java.util.Map;
    import java.util.stream.Collectors;
    
    /** * 过滤list中对象的某个属性非空，然后返回一个新的list */
    public class LambdaForList { 
        public static void main(String[] args) { 
            //1. 初始化数据
            UserPO user1 = new UserPO(1, "java");
            UserPO user2 = new UserPO(2, "C");
            UserPO user3 = new UserPO(3, "PHP");
            List<UserPO> users = new ArrayList<>();
            users.add(user1);
            users.add(user2);
            users.add(user3);
    
            //3. list转map
            Map<Integer, String> userMap = Maps.newHashMap();
            userMap = users.stream().collect(Collectors.toMap(UserPO::getId, UserPO::getName));
            System.out.println(userMap.toString());
        }
    }

**输出**

{ 1=java, 2=C, 3=PHP}

#### 常见错误异常 ####

`java.lang.IllegalStateException: Duplicate key java`

**对应代码块:**

package com.example.demo.util;
    
    import com.example.demo.entity.UserPO;
    import com.google.common.collect.Maps;
    
    import java.util.ArrayList;
    import java.util.List;
    import java.util.Map;
    import java.util.stream.Collectors;
    
    public class LambdaForList { 
        public static void main(String[] args) { 
            //1. 初始化数据
            UserPO user1 = new UserPO(1, "java");
            UserPO user2 = new UserPO(2, "C");
            UserPO user3 = new UserPO(1, "PHP");
            List<UserPO> users = new ArrayList<>();
            users.add(user1);
            users.add(user2);
            users.add(user3);
    
            //3. list转map
            Map<Integer, String> userMap = Maps.newHashMap();
            userMap = users.stream()
                    .collect(Collectors
                            .toMap(UserPO::getId, UserPO::getName));
            System.out.println(userMap.toString());
        }
    }

**异常信息:**

Exception in thread "main" java.lang.IllegalStateException: Duplicate key java
    	at java.util.stream.Collectors.lambda$throwingMerger$0(Collectors.java:133)
    	at java.util.HashMap.merge(HashMap.java:1254)
    	at java.util.stream.Collectors.lambda$toMap$58(Collectors.java:1320)
    	at java.util.stream.ReduceOps$3ReducingSink.accept(ReduceOps.java:169)
    	at java.util.ArrayList$ArrayListSpliterator.forEachRemaining(ArrayList.java:1382)
    	at java.util.stream.AbstractPipeline.copyInto(AbstractPipeline.java:482)
    	at java.util.stream.AbstractPipeline.wrapAndCopyInto(AbstractPipeline.java:472)
    	at java.util.stream.ReduceOps$ReduceOp.evaluateSequential(ReduceOps.java:708)
    	at java.util.stream.AbstractPipeline.evaluate(AbstractPipeline.java:234)
    	at java.util.stream.ReferencePipeline.collect(ReferencePipeline.java:499)
    	at com.example.demo.util.Test.main(Test.java:27)

**分析**：user1和user3的id都是1，导致key重复  
**解决**：[JDK8中Collectors.toMap方法报Duplicate key xxx错误信息][JDK8_Collectors.toMap_Duplicate key xxx]  
[Duplicate key][]  
**代码块:**

package com.example.demo.util;
    
    import com.example.demo.entity.UserPO;
    import com.google.common.collect.Maps;
    
    import java.util.ArrayList;
    import java.util.List;
    import java.util.Map;
    import java.util.stream.Collectors;
    
    public class LambdaForList { 
        public static void main(String[] args) { 
            //1. 初始化数据
            UserPO user1 = new UserPO(1, "java");
            UserPO user2 = new UserPO(2, "C");
            UserPO user3 = new UserPO(1, "PHP");
            List<UserPO> users = new ArrayList<>();
            users.add(user1);
            users.add(user2);
            users.add(user3);
    
            //3. list转map
            Map<Integer, String> userMap = Maps.newHashMap();
            userMap = users.stream()
                    .collect(Collectors
                            .toMap(UserPO::getId, UserPO::getName, (value1,value2) -> value2));
            System.out.println(userMap.toString());
        }
    }

**输出**

{ 1=PHP, 2=C}

[JDK8_Collectors.toMap_Duplicate key xxx]: https://blog.csdn.net/qq_19734597/article/details/86087489
[Duplicate key]: https://www.baidu.com/s?ie=UTF-8&wd=Duplicate%20key