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
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);
}
@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();
int hash = hash(key);
Node<K,V>[] tab; Node<K,V> first; int n, i;
int binCount = 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; K k;
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) {
V v;
if (old.value != null)
v = remappingFunction.apply(old.value, value);
else
v = value;
if (v != null) {
old.value = v;
afterNodeAccess(old);
}
else
removeNode(hash, key, null, false, true);
return v;
}
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);
}
return value;
}
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.
*/
public static <T, K, U>
Collector<T, ?, Map<K,U>> toMap(Function<? super T, ? extends K> keyMapper,
Function<? super T, ? extends U> valueMapper) {
return toMap(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
public class MailTest {
public static void main(String[] args) {
List<TestDO> list = Lists.newArrayList();
TestDO t1 = new TestDO(1, "a");
TestDO t2 = new TestDO(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）。
