使用Collectors.toMap()报NullPointerException

记录下最近工作中在使用 stream().collect(Collectors.toMap())出现的空指针问题

问题代码

@Test
public void test2() {List<Student> studentList =Stream.of(new Student("1", "张三"), new Student("2", null)).collect(Collectors.toList());Map<String, String> map =studentList.stream().map(student -> Pair.of(student.getUserId(), student.getUserName())).collect(Collectors.toMap(Pair::getKey, Pair::getValue, (v1, v2) -> v1));System.out.println(map);
}@Data
@AllArgsConstructor
public static class Student implements Serializable {private static final long serialVersionUID = -3058531222330506251L;private String userId;private String userName;
}

java.lang.NullPointerExceptionat java.util.HashMap.merge(HashMap.java:1226)at java.util.stream.Collectors.lambda$toMap$58(Collectors.java:1320)at java.util.stream.ReduceOps$3ReducingSink.accept(ReduceOps.java:169)at java.util.stream.ReferencePipeline$3$1.accept(ReferencePipeline.java:193)at java.util.ArrayList$ArrayListSpliterator.forEachRemaining(ArrayList.java:1384)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.yxxmg.collection.CollectionTest.test2(CollectionTest.java:53)

按照我们正常思维逻辑这不应该会出现这个问题，想当然理解成put(key,value)方式

问题分析

1. 首先分析出错代码

   Map<String, String> map =studentList.stream().map(student -> Pair.of(student.getUserId(), student.getUserName())).collect(Collectors.toMap(Pair::getKey, Pair::getValue, (v1, v2) -> v1));
   

2. Pair.of

   /*** <p>Creates an immutable pair of two objects inferring the generic types.</p>** <p>This factory allows the pair to be created using inference to* obtain the generic types.</p>** @param <L> the left element type* @param <R> the right element type* @param left  the left element, may be null* @param right  the right element, may be null* @return a pair formed from the two parameters, not null*/
   public static <L, R> Pair<L, R> of(final L left, final R right) {return ImmutablePair.of(left, right);
   }
   

   这边可以看出key，value都可以为空

3. 分析Collectors.toMap(Pair::getKey, Pair::getValue, (v1, v2) -> v1)

   public static <T, K, U>Collector<T, ?, Map<K,U>> toMap(Function<? super T, ? extends K> keyMapper,Function<? super T, ? extends U> valueMapper,BinaryOperator<U> mergeFunction) {return toMap(keyMapper, valueMapper, mergeFunction, HashMap::new);
   }
   

   public static <T, K, U, M extends Map<K, U>>Collector<T, ?, M> toMap(Function<? super T, ? extends K> keyMapper,Function<? super T, ? extends U> valueMapper,BinaryOperator<U> mergeFunction,Supplier<M> mapSupplier) {BiConsumer<M, T> accumulator= (map, element) -> map.merge(keyMapper.apply(element),valueMapper.apply(element), mergeFunction);return new CollectorImpl<>(mapSupplier, accumulator, mapMerger(mergeFunction), CH_ID);
   }
   

   看下Map接口的默认merge方法

   default V merge(K key, V value,BiFunction<? super V, ? super V, ? extends V> remappingFunction) {Objects.requireNonNull(remappingFunction);Objects.requireNonNull(value);V oldValue = get(key);V newValue = (oldValue == null) ? value :remappingFunction.apply(oldValue, value);if(newValue == null) {remove(key);} else {put(key, newValue);}return newValue;
   }
   

   该接口可以看出value不能为空，再看下我们使用的HashMap的重写的merge方法

   @Override
   public V merge(K key, V value,BiFunction<? super V, ? super V, ? extends V> remappingFunction) {if (value == null)throw new NullPointerException();if (remappingFunction == null)throw new NullPointerException();...return value;
   }
   

   从源码可以看出value==null直接报空指针异常了