[英]Java 8 stream groupingBy String value
I have a JSON file containing data in the form: 我有一个JSON文件,其中包含以下形式的数据:
{
"type":"type1",
"value":"value1",
"param": "param1"
}
{
"type":"type2",
"value":"value2",
"param": "param2"
}
I also have an object like this: 我也有这样的对象:
public class TestObject {
private final String value;
private final String param;
public TestObject(String value, String param) {
this.value = value;
this.param = param;
}
}
What I want is to create a Map<String, List<TestObject>>
that contains a list of TestObject
s for each type. 我要创建一个
Map<String, List<TestObject>>
,其中包含每种类型的TestObject
列表。
This is what I coded: 这是我编写的代码:
Map<String, List<TestObject>> result = jsonFileStream
.map(this::buildTestObject)
.collect(Collectors.groupingBy(line -> JsonPath.read(line, "$.type")));
Where the method buildTestObject
is: 方法
buildTestObject
在哪里:
private TestObject buildTestObject(String line) {
return new TestObject(
JsonPath.read(line, "$.value"),
JsonPath.read(line, "$.param"));
}
This does not work because the map()
function returns a TestObject
, so that the collect
function does not work on the JSON String line anymore. 这不起作用,因为
map()
函数返回TestObject
,因此collect
函数不再适用于JSON String行。
In real life, I cannot add the "type" variable to the TestObject
file, as it is a file from an external library. 在现实生活中,我无法将“ type”变量添加到
TestObject
文件,因为它是来自外部库的文件。
How can I group my TestObject
s by the type in the JSON file? 如何按JSON文件中的类型对
TestObject
分组?
You can move the mapping operation to a down stream collector of groupingBy: 您可以将映射操作移至groupingBy的下游收集器:
Map<String, List<TestObject>> result = jsonFileStream
.collect(Collectors.groupingBy(line -> JsonPath.read(line, "$.type"),
Collectors.mapping(this::buildTestObject, Collectors.toList())));
This will preserve the string so you can extract the type as a classifier, and applies the mapping to the elements of the resulting groups. 这将保留字符串,以便您可以提取类型作为分类器,并将映射应用于结果组的元素。
You can also use the toMap
collector to accomplish the task at hand. 您也可以使用
toMap
收集器完成手头的任务。
Map<String, List<TestObject>> resultSet = jsonFileStream
.collect(Collectors.toMap(line -> JsonPath.read(line, "$.type"),
line -> new ArrayList<>(Collections.singletonList(buildTestObject(line))),
(left, right) -> {
left.addAll(right);
return left;
}
));
In addition to the Stream solution, it's worth pointing out that Java 8 also significantly improved the Map
interface, making this kind of thing much less painful to achieve with a for
loop than had previously been the case. 除了Stream解决方案外,值得指出的是Java 8还显着改进了
Map
接口,这使得使用for
循环实现这种事情比以前for
轻松得多。 I am not familiar with the library you are using, but something like this will work (you can always convert a Stream
to an Iterable
). 我不熟悉您正在使用的库,但是类似的东西会起作用(您可以始终将
Stream
转换为Iterable
)。
Map<String, List<TestObject>> map = new HashMap<>();
for (String line : lines) {
map.computeIfAbsent(JsonPath.read(line, "$.type"), k -> new ArrayList<>())
.add(buildTestObject(line));
}
声明:本站的技术帖子网页,遵循CC BY-SA 4.0协议,如果您需要转载,请注明本站网址或者原文地址。任何问题请咨询:yoyou2525@163.com.