Java——》Collectors.toMap的value为空时，报NullPointerException

╰+攻爆jí腚メ 2024-04-01 14:45 53阅读 0赞

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> [总结——》【Java】][Java]  
> [总结——》【Mysql】][Mysql]  
> [总结——》【Spring】][Spring]  
> [总结——》【SpringBoot】][SpringBoot]  
> [总结——》【MyBatis、MyBatis-Plus】][MyBatis_MyBatis-Plus]

#### Java——》Collectors.toMap的value为空时，报NullPointerException ####

*  1、操作
 *  2、现象（错误信息）
 *  3、原因
 *  4、解决

## 1、操作 ##

Collectors.toMap(key,value)时，报NullPointerException异常

## 2、现象（错误信息） ##

import java.util.Arrays;
    import java.util.List;
    import java.util.Map;
    import java.util.stream.Collectors;
    
    public class T {
        
        public static void main(String[] args) {
        
            List<Integer> list = Arrays.asList(null, 1, 2);
    
            // 当value为空时,报NullPointerException
            Map<Integer, Integer> map = list.stream().collect(Collectors.toMap(e -> e, e -> e));
            System.out.println(map);
        }
    }

Exception in thread "main" java.lang.NullPointerException
    	at java.util.HashMap.merge(HashMap.java:1225)
    	at java.util.stream.Collectors.lambda$toMap$58(Collectors.java:1320)
    	at java.util.stream.ReduceOps$3ReducingSink.accept(ReduceOps.java:169)
    	at java.util.Spliterators$ArraySpliterator.forEachRemaining(Spliterators.java:948)
    	at java.util.stream.AbstractPipeline.copyInto(AbstractPipeline.java:481)
    	at java.util.stream.AbstractPipeline.wrapAndCopyInto(AbstractPipeline.java:471)
    	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.eju.goodhouse.test.T.main(T.java:22)

## 3、原因 ##

Collectors.toMap()，key可以为null,但 value不能为null， 否则抛空指针异常nullPointerException  
![在这里插入图片描述][0840c5ed45014ccf8d02aee485a930fa.png]

## 4、解决 ##

import java.util.Arrays;
    import java.util.HashMap;
    import java.util.List;
    import java.util.Map;
    import java.util.stream.Collectors;
    
    public class T {
        
        public static void main(String[] args) {
        
            List<Integer> list = Arrays.asList(null, 1, 2);
    
            // 当value为空时,报NullPointerException
            // Map<Integer, Integer> map = list.stream().collect(Collectors.toMap(e -> e, e -> e));
            // System.out.println(map);
    
            // 方案一：
            Map<Integer, Integer> map1 = list.stream().collect(HashMap::new, (hashMap, item) -> hashMap.put(item, item), (m1, m2) -> m1.putAll(m2));
            // 方案二：
            Map<Integer, Integer> map2 = list.stream().collect(HashMap::new, (hashMap, item) -> hashMap.put(item, item), HashMap::putAll);
    
            System.out.println(map1);
            System.out.println(map2);
        }
    }

[Java]: https://blog.csdn.net/weixin_43453386/article/details/84788317
[Mysql]: https://blog.csdn.net/weixin_43453386/article/details/88667709
[Spring]: https://blog.csdn.net/weixin_43453386/article/details/124900806
[SpringBoot]: https://blog.csdn.net/weixin_43453386/article/details/84788714
[MyBatis_MyBatis-Plus]: https://blog.csdn.net/weixin_43453386/article/details/84788053
[0840c5ed45014ccf8d02aee485a930fa.png]: https://image.dandelioncloud.cn/pgy_files/images/2024/04/01/5f703186ba284282b24514265c96f914.png