NullPointerException
v 为null，报空指针错误
k为null， 运行正常。
以往的认知：HashMap中k,v都是可以存null值的。在上面的测试用例中可以看到，v为null其实会报错。
查看API可以看到
publicstatic<T,K,U,MextendsMap<K,U>>Collector<T,?,M>toMap(Function<?superT,?extendsK>keyMapper,Function<?superT,?extendsU>valueMapper,BinaryOperator<U>mergeFunction,Supplier<M>mapSupplier){BiConsumer<M,T>accumulator=(map,element)->map.merge(keyMapper.apply(element),valueMapper.apply(element),mergeFunction);returnnewCollectorImpl<>(mapSupplier,accumulator,mapMerger(mergeFunction),CH_ID);}
@OverridepublicVmerge(Kkey,Vvalue,BiFunction<?super V,?super V,?extends V>remappingFunction){if(value==null)thrownewNullPointerException();if(remappingFunction==null)thrownewNullPointerException();inthash=hash(key);Node<K,V>[]tab;Node<K,V>first;intn,i;intbinCount=0;TreeNode<K,V>t=null;Node<K,V>old=null;if(size>threshold||(tab=table)==null||(n=tab.length)==0)n=(tab=resize()).length;if((first=tab[i=(n-1)&hash])!=null){if(first instanceof TreeNode)old=(t=(TreeNode<K,V>)first).getTreeNode(hash,key);else{Node<K,V>e=first;Kk;do{if(e.hash==hash&&((k=e.key)==key||(key!=null&&key.equals(k)))){old=e;break;}++binCount;}while((e=e.next)!=null);}}if(old!=null){Vv;if(old.value!=null)v=remappingFunction.apply(old.value,value);elsev=value;if(v!=null){old.value=v;afterNodeAccess(old);}elseremoveNode(hash,key,null,false,true);returnv;}if(value!=null){if(t!=null)t.putTreeVal(this,tab,hash,key,value);else{tab[i]=newNode(hash,key,value,first);if(binCount>=TREEIFY_THRESHOLD-1)treeifyBin(tab,hash);}++modCount;++size;afterNodeInsertion(true);}returnvalue;}
从代码中可以看到，HashMap.merge 会对v进行判空。
目前我的解决办法是在流中加上判空过滤
System.out.println(list.stream().filter(t->t.getName()!=null).collect(Collectors.toMap(TestDO::getId,TestDO::getName,(k1,k2)->k1)));
Duplicate key
查看toMap的API及相关注释
/*
* <p>If the mapped keys contains duplicates (according to
* {@link Object#equals(Object)}), an {@code IllegalStateException} is
* thrown when the collection operation is performed. If the mapped keys
* may have duplicates, use {@link #toMap(Function, Function, BinaryOperator)}
* instead.
*/publicstatic<T,K,U>Collector<T,?,Map<K,U>>toMap(Function<?superT,?extendsK>keyMapper,Function<?superT,?extendsU>valueMapper){returntoMap(keyMapper,valueMapper,throwingMerger(),HashMap::new);}
如果有重复的key，使用toMap(Function, Function）会抛异常，可以使用toMap(Function, Function, BinaryOperator)来解决。
优化后的代码是
publicclassMailTest{publicstaticvoidmain(String[]args){List<TestDO>list=Lists.newArrayList();TestDO t1=newTestDO(1,"a");TestDO t2=newTestDO(1,"b");list.add(t1);list.add(t2);System.out.println(list.stream().collect(Collectors.toMap(TestDO::getId,TestDO::getName,(k1,k2)->k1)));System.out.println(list.stream().collect(Collectors.toMap(TestDO::getId,TestDO::getName,(k1,k2)->k2)));System.out.println(list.stream().collect(Collectors.toMap(TestDO::getId,TestDO::getName,(k1,k2)->k1+k2)));}}
从执行结果来看，对于重复的key,对他的value可以有三种取值情况：
(k1, k2) -> k1，取第一个值
(k1, k2) -> k2)，取第二个值
(k1, k2) -> k1 + k2)，对两个值做某些操作后取结果。（例如基本类型可以进行加减乘除）。
作者：snoweek
链接：https://www.jianshu.com/p/6a37e245ff46
来源：简书
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