使用 Stream.reduce() 减少对象列表
Ruducing a List of objects using Stream.reduce()
我有一个 list 的 objects，我需要将 status 等于我的 [=19] 的对象分组=]到单个定制的 count = sumOfSameObjectsCount .
我们有classMyObject
class MyObject {
Integer id;
String name;
String status;
Long count;
//constructor with attributes
//getters
//setters
}
建议实施：
List<MyObject> resultList = listOfObjects.stream()
.collect(Collectors.groupingBy(MyObject::getStatus))
.entrySet().stream()
.map(e -> e.getValue().stream()
.reduce((partialResult,nextElem) ->
{
LOGGER.info("ahaaaa! inside your reduce block ");
if(partialResult.getStatus().equals(customizedStatus)) {
LOGGER.info("equal to my customizedStatus");
return new MyObject(customizedId, customizedName, customizedStatus, partialResult.getCount()+nextElem.getCount());
} else {
LOGGER.info("not equal to my customizedStatus");
return new MyObject(partialResult.getId(), partialResult.getName(), partialResult.getStatus(), partialResult.getCount());
}
}
)
)
.map(f -> f.get())
.collect(Collectors.toList());
万一有多个 status 等于我的 customizedStatus 的对象，事情就会像魅力一样运作。
输入：
[
{
"id": XX,
"name": "nameXX",
"status": "statusXX",
"count": countXX
},
{
"id": YY,
"name": "nameYY",
"status": "statusYY",
"count": countYY
},
{
"id": ZZ,
"name": "nameZZ",
"status": "customizedStatus",
"count": countZZ
},
{
"id": ZZz,
"name": "nameZZz",
"status": "customizedStatus",
"count": countZZz
}
]
输出：
[
{
"id": XX,
"name": "nameXX",
"status": "statusXX",
"count": countXX
},
{
"id": YY,
"name": "nameYY",
"status": "statusYY",
"count": countYY
},
{
"id": customizedId,
"name": "customizedName",
"status": "customizedStatus",
"count": countZZ+countZZz
}
]
如果有一个 status 等于我的 customizedStatus 的对象，也需要对其进行自定义，不幸的是 reduce 块被跳过了！
输入：
[
{
"id": XX,
"name": "nameXX",
"status": "statusXX",
"count": countXX
},
{
"id": YY,
"name": "nameYY",
"status": "statusYY",
"count": countYY
},
{
"id": ZZ,
"name": "nameZZ",
"status": "customizedStatus",
"count": countZZ
}
]
输出：
[
{
"id": XX,
"name": "nameXX",
"status": "statusXX",
"count": countXX
},
{
"id": YY,
"name": "nameYY",
"status": "statusYY",
"count": countYY
},
{
"id": ZZ,
"name": "nameZZ",
"status": "customizedStatus",
"count": countZZ
}
]
预期输出：
[
{
"id": XX,
"name": "nameXX",
"status": "statusXX",
"count": countXX
},
{
"id": YY,
"name": "nameYY",
"status": "statusYY",
"count": countYY
},
{
"id": customizedId,
"name": "customizedName",
"status": "customizedStatus",
"count": countZZ
}
]
如果有多个相同的对象status，似乎会执行 reduce，如果没有 reduce 则根本不会执行！
有没有想过使用 groupBy 和 reduce 获得预期的输出？
更新
生成的类型不正确。因为您没有在 reduce() 中提供身份，所以它将 return 一个 Optional<Object>，而不是一个对象。
出于同样的原因（因为您正在使用不期望身份的 reduce() 风格），累加器 不会对 单个元素。引自 documentation:
Performs a reduction on the elements of this stream, using an
associative accumulation function, and returns an Optional describing
the reduced value, if any. This is equivalent to:
boolean foundAny = false;
T result = null;
for (T element : this stream) {
if (!foundAny) {
foundAny = true;
result = element;
}
else
result = accumulator.apply(result, element);
}
return foundAny ? Optional.of(result) : Optional.empty();
第一个遇到的流元素将成为部分结果并且没有更多的元素，它将被可选的按原样包装并return编辑。
一个可能的补救方法是引入 身份:
public static final Integer customizedId = 99;
public static final String customizedName = "customizedName";
public static final String customizedStatus = "customizedStatus";
public static void main(String[] args) {
List<MyObject> listOfObjects =
List.of(new MyObject(1, "nameXX", "statusXX", 1L),
new MyObject(2, "nameYY", "statusYY", 1L),
new MyObject(3, "nameZZz", "customizedStatus", 3L));
List<MyObject> result =
listOfObjects.stream()
.collect(Collectors.groupingBy(MyObject::getStatus))
.entrySet().stream()
.map(e -> e.getValue().stream()
.reduce(getIdentity(e), (partialResult, nextElem) -> accumulate(partialResult, nextElem)) )
.collect(Collectors.toList());
result.forEach(System.out::println);
}
public static MyObject getIdentity(Map.Entry<String, List<MyObject>> entry) {
return entry.getKey().equals(customizedStatus) ?
new MyObject(customizedId, customizedName, customizedStatus, 0L) :
entry.getValue().iterator().next();
}
public static MyObject accumulate(MyObject result, MyObject next) {
return result.getStatus().equals(customizedStatus) ?
new MyObject(customizedId, customizedName, customizedStatus, result.getCount() + next.getCount()) :
new MyObject(result.getId(), result.getName(), result.getStatus(), result.getCount());
}
输出：
MyObject{id=2, name='nameYY', status='statusYY', count=1}
MyObject{id=1, name='nameXX', status='statusXX', count=1}
MyObject{id=99, name='customizedName', status='customizedStatus', count=3}
你可以玩这个 Online demo
但请记住，尝试将大量条件逻辑放入流中并不是最聪明的想法，因为它变得更难阅读。
下面提供的解决方案是在问题更新之前写的，问题已经弄清楚了。虽然，他们没有针对这个特定问题，但有人可能会从中受益，因此我会保留它们。
将列表缩减为单个对象
Is there any solution to make it pass by reduce even listOfObjects entries are different by status ?
如果您想将对象列表缩减为具有预定义 id、name 和 status 的单个对象，则无需创建中间映射Collectors.groupingBy().
如果你想为此使用 reduce() 操作，你可以累加 count，然后基于它创建结果对象：
这就是它的样子（虚拟对象的类型已更改为 MyObject 以避免与 java.lang.Object 混淆）：
final Integer customizedId = // intializing the resulting id
final String customizedName = // intializing the resulting name
final String customizedStatus = // intializing the resulting status
List<MyObject> listOfObjects = // intializing the source list
MyObject resultingObject = listOfObjects.stream()
.map(MyObject::getCount)
.reduce(Long::sum)
.map(count -> new MyObject(customizedId, customizedName, customizedStatus, 0L))
.orElseThrow(); // or .orElse(() -> new MyObject(customizedId, customizedName, customizedStatus, 0L));
实现它的另一种方法是利用 MyObject 是可变的这一事实，并将其用作 collect() 操作中的容器：
MyObject resultingObject = listOfObjects.stream()
.collect(() -> new MyObject(customizedId, customizedName, customizedStatus, 0L),
(MyObject result, MyObject next) -> result.setCount(result.getCount() + next.getCount()),
(left, right) -> left.setCount(left.getCount() + right.getCount()));
我有一个 list 的 objects，我需要将 status 等于我的 [=19] 的对象分组=]到单个定制的 count = sumOfSameObjectsCount .
我们有classMyObject
class MyObject {
Integer id;
String name;
String status;
Long count;
//constructor with attributes
//getters
//setters
}
建议实施：
List<MyObject> resultList = listOfObjects.stream()
.collect(Collectors.groupingBy(MyObject::getStatus))
.entrySet().stream()
.map(e -> e.getValue().stream()
.reduce((partialResult,nextElem) ->
{
LOGGER.info("ahaaaa! inside your reduce block ");
if(partialResult.getStatus().equals(customizedStatus)) {
LOGGER.info("equal to my customizedStatus");
return new MyObject(customizedId, customizedName, customizedStatus, partialResult.getCount()+nextElem.getCount());
} else {
LOGGER.info("not equal to my customizedStatus");
return new MyObject(partialResult.getId(), partialResult.getName(), partialResult.getStatus(), partialResult.getCount());
}
}
)
)
.map(f -> f.get())
.collect(Collectors.toList());
万一有多个 status 等于我的 customizedStatus 的对象，事情就会像魅力一样运作。
输入：
[
{
"id": XX,
"name": "nameXX",
"status": "statusXX",
"count": countXX
},
{
"id": YY,
"name": "nameYY",
"status": "statusYY",
"count": countYY
},
{
"id": ZZ,
"name": "nameZZ",
"status": "customizedStatus",
"count": countZZ
},
{
"id": ZZz,
"name": "nameZZz",
"status": "customizedStatus",
"count": countZZz
}
]
输出：
[
{
"id": XX,
"name": "nameXX",
"status": "statusXX",
"count": countXX
},
{
"id": YY,
"name": "nameYY",
"status": "statusYY",
"count": countYY
},
{
"id": customizedId,
"name": "customizedName",
"status": "customizedStatus",
"count": countZZ+countZZz
}
]
如果有一个 status 等于我的 customizedStatus 的对象，也需要对其进行自定义，不幸的是 reduce 块被跳过了！
输入：
[
{
"id": XX,
"name": "nameXX",
"status": "statusXX",
"count": countXX
},
{
"id": YY,
"name": "nameYY",
"status": "statusYY",
"count": countYY
},
{
"id": ZZ,
"name": "nameZZ",
"status": "customizedStatus",
"count": countZZ
}
]
输出：
[
{
"id": XX,
"name": "nameXX",
"status": "statusXX",
"count": countXX
},
{
"id": YY,
"name": "nameYY",
"status": "statusYY",
"count": countYY
},
{
"id": ZZ,
"name": "nameZZ",
"status": "customizedStatus",
"count": countZZ
}
]
预期输出：
[
{
"id": XX,
"name": "nameXX",
"status": "statusXX",
"count": countXX
},
{
"id": YY,
"name": "nameYY",
"status": "statusYY",
"count": countYY
},
{
"id": customizedId,
"name": "customizedName",
"status": "customizedStatus",
"count": countZZ
}
]
如果有多个相同的对象status，似乎会执行 reduce，如果没有 reduce 则根本不会执行！
有没有想过使用 groupBy 和 reduce 获得预期的输出？
更新
生成的类型不正确。因为您没有在 reduce() 中提供身份，所以它将 return 一个 Optional<Object>，而不是一个对象。
出于同样的原因（因为您正在使用不期望身份的 reduce() 风格），累加器 不会对 单个元素。引自 documentation:
Performs a reduction on the elements of this stream, using an associative accumulation function, and returns an Optional describing the reduced value, if any. This is equivalent to:
boolean foundAny = false; T result = null; for (T element : this stream) { if (!foundAny) { foundAny = true; result = element; } else result = accumulator.apply(result, element); } return foundAny ? Optional.of(result) : Optional.empty();
第一个遇到的流元素将成为部分结果并且没有更多的元素，它将被可选的按原样包装并return编辑。
一个可能的补救方法是引入 身份:
public static final Integer customizedId = 99;
public static final String customizedName = "customizedName";
public static final String customizedStatus = "customizedStatus";
public static void main(String[] args) {
List<MyObject> listOfObjects =
List.of(new MyObject(1, "nameXX", "statusXX", 1L),
new MyObject(2, "nameYY", "statusYY", 1L),
new MyObject(3, "nameZZz", "customizedStatus", 3L));
List<MyObject> result =
listOfObjects.stream()
.collect(Collectors.groupingBy(MyObject::getStatus))
.entrySet().stream()
.map(e -> e.getValue().stream()
.reduce(getIdentity(e), (partialResult, nextElem) -> accumulate(partialResult, nextElem)) )
.collect(Collectors.toList());
result.forEach(System.out::println);
}
public static MyObject getIdentity(Map.Entry<String, List<MyObject>> entry) {
return entry.getKey().equals(customizedStatus) ?
new MyObject(customizedId, customizedName, customizedStatus, 0L) :
entry.getValue().iterator().next();
}
public static MyObject accumulate(MyObject result, MyObject next) {
return result.getStatus().equals(customizedStatus) ?
new MyObject(customizedId, customizedName, customizedStatus, result.getCount() + next.getCount()) :
new MyObject(result.getId(), result.getName(), result.getStatus(), result.getCount());
}
输出：
MyObject{id=2, name='nameYY', status='statusYY', count=1}
MyObject{id=1, name='nameXX', status='statusXX', count=1}
MyObject{id=99, name='customizedName', status='customizedStatus', count=3}
你可以玩这个 Online demo
但请记住，尝试将大量条件逻辑放入流中并不是最聪明的想法，因为它变得更难阅读。
下面提供的解决方案是在问题更新之前写的，问题已经弄清楚了。虽然，他们没有针对这个特定问题，但有人可能会从中受益，因此我会保留它们。
将列表缩减为单个对象
Is there any solution to make it pass by reduce even listOfObjects entries are different by status ?
如果您想将对象列表缩减为具有预定义 id、name 和 status 的单个对象，则无需创建中间映射Collectors.groupingBy().
如果你想为此使用 reduce() 操作，你可以累加 count，然后基于它创建结果对象：
这就是它的样子（虚拟对象的类型已更改为 MyObject 以避免与 java.lang.Object 混淆）：
final Integer customizedId = // intializing the resulting id
final String customizedName = // intializing the resulting name
final String customizedStatus = // intializing the resulting status
List<MyObject> listOfObjects = // intializing the source list
MyObject resultingObject = listOfObjects.stream()
.map(MyObject::getCount)
.reduce(Long::sum)
.map(count -> new MyObject(customizedId, customizedName, customizedStatus, 0L))
.orElseThrow(); // or .orElse(() -> new MyObject(customizedId, customizedName, customizedStatus, 0L));
实现它的另一种方法是利用 MyObject 是可变的这一事实，并将其用作 collect() 操作中的容器：
MyObject resultingObject = listOfObjects.stream()
.collect(() -> new MyObject(customizedId, customizedName, customizedStatus, 0L),
(MyObject result, MyObject next) -> result.setCount(result.getCount() + next.getCount()),
(left, right) -> left.setCount(left.getCount() + right.getCount()));