java里的右面两列没有了_java – Collectors.groupingBy由两列不起作用
我想使用Collectors.groupingBy将两列分组.
我写了以下内容：
public class TestGroupKey
{
public static void main(String... args) {
List list = Arrays.asList(
new Item(1, 1, 1),
new Item(1, 1, 2),
new Item(1, 1, 3),
new Item(1, 2, 1),
new Item(1, 2, 2),
new Item(2, 1, 1),
new Item(2, 2, 1),
new Item(2, 3, 1),
new Item(2, 4, 1),
new Item(2, 4, 2),
new Item(2, 4, 3),
new Item(2, 4, 4),
new Item(3, 1, 1),
new Item(3, 1, 2),
new Item(3, 1, 3),
new Item(3, 1, 4),
new Item(3, 2, 1),
new Item(3, 2, 2)
);
Map> tupleGrouping =
list.stream().collect(Collectors.groupingBy(item -> item.customKey));
tupleGrouping.entrySet().stream()
.forEach(entry -> {
System.out.println(entry.getKey());
});
}
public static class Item {
public int topLevelId;
public int secondLevelId;
public int itemId;
public CustomKey customKey;
public Item(int topLevelId, int secondLevelId, int itemId) {
this.topLevelId = topLevelId;
this.secondLevelId = secondLevelId;
this.itemId = itemId;
this.customKey = new CustomKey(topLevelId, secondLevelId);
}
@Override
public String toString() {
return String.format("%d%d%d", this.topLevelId, this.secondLevelId, this.itemId);
}
}
public static class CustomKey {
public int topLevelId;
public int secondLevelId;
public CustomKey(int topLevelId, int secondLevelId) {
this.topLevelId = topLevelId;
this.secondLevelId = secondLevelId;
}
@Override
public String toString() {
return String.format("%d%d", this.topLevelId, this.secondLevelId);
}
}
}
预期的结果是
11
12
21
22
23
24
31
32
但实际结果是
24
31
23
22
12
21
24
31
31
24
12
32
32
11
11
11
31
我认为groupingBy不起作用.
使用CustomKey类有什么问题？
此外,嵌套的映射键正在工作：
Map>> entryGrouping =
list.stream()
.collect(Collectors.groupingBy(atta1 -> atta1.topLevelId,
Collectors.groupingBy(atta2 -> atta2.secondLevelId)));
解决方法:
您的CustomKey类不会覆盖Object的equals方法,因此groupingBy认为两个CustomKeys只有在它们是同一个对象时才相等(在您的示例中永远不会这样,因为您为每个Item创建了一个新的CustomKey实例).
public static class CustomKey {
...
@Override
public int hashCode ()
{
// if you override equals, you should also override hashCode
}
@Override
public boolean equals (Object other)
{
if (other == this)
return true;
if (!(other instanceof CustomKey))
return false;
CustomKey okey = (CustomKey) other;
return this.topLevelId == okey.topLevelId && this.secondLevelId == okey.secondLevelId;
}
...
}
标签：java,java-8,java-stream,grouping
来源： https://codeday.me/bug/20190828/1754445.html