Collectors.toMap中的NullPointerException

错误日志如下： 

java.lang.NullPointerException: null
	at 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.Collections$2.tryAdvance(Collections.java:4719)
	at java.util.Collections$2.forEachRemaining(Collections.java:4727)
	at java.util.stream.AbstractPipeline.copyInto(AbstractPipeline.java:482)
	at java.util.stream.AbstractPipeline.wrapAndCopyInto(AbstractPipeline.java:472)
	at java.util.stream.ReduceOps$ReduceOp.evaluateSequential(ReduceOps.java:708)
	at java.util.stream.AbstractPipeline.evaluate(AbstractPipeline.java:234)
	at java.util.stream.ReferencePipeline.collect(ReferencePipeline.java:566)
	at com.xxx.xxx.service.impl.DynamicFormColumnServiceImpl.lambda$getCodePropertyValue$24(DynamicFormColumnServiceImpl.java:465)
	at java.util.stream.Collectors.lambda$toMap$58(Collectors.java:1321)
	at java.util.stream.ReduceOps$3ReducingSink.accept(ReduceOps.java:169)
	at java.util.HashMap$EntrySpliterator.forEachRemaining(HashMap.java:1723)
	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:566)
	at com.xxx.xxx.service.impl.DynamicFormColumnServiceImpl.getCodePropertyValue(DynamicFormColumnServiceImpl.java:463)

代码： 

Map> listMap=list.stream()
.collect(Collectors.toMap(PriceOverseasPostConfig::getCountryCode,PriceOverseasPostConfig::getCountryManagers));

查看Collectors.toMap方法源码发现，在Collectors.toMap的调用过程中并不是我们平常常用的put方法，而是merge。如下：

default V merge(K key, V value,
            BiFunction 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;
    }

Objects.requireNonNull(value); value 为null则报空指针错误。

解决方案：

使用filter过滤所有NULL值

Map> listMap=list.stream().filter(f->CollectionUtil.isNotEmpty(f.getCountryManagers()))
                .collect(Collectors.toMap(PriceOverseasPostConfig::getCountryCode,PriceOverseasPostConfig::getCountryManagers));

或

Map> collect = list.stream()
                .collect(HashMap::new, (m, v)->m.put(v.getCountryCode(), v.getCountryManagers()), HashMap::putAll);