[英]Creating a map of String and Object from a List in Java 8
I have the following classes: 我有以下课程:
class Data {
String systemId;
String fileName;
int x;
int y;
Data(String systemId, String fileName, int x, int y) {
this.systemId = systemId;
this.fileName = fileName;
this.x = x;
this.y = y;
}
public String getSystemId() {
return systemId;
}
public void setSystemId(String systemId) {
this.systemId = systemId;
}
public String getFileName() {
return fileName;
}
public void setFileName(String fileName) {
this.fileName = fileName;
}
public int getX() {
return x;
}
public void setX(int x) {
this.x = x;
}
public int getY() {
return y;
}
public void setY(int y) {
this.y = y;
}
}
class Result {
int x;
int y;
Result(int x, int y) {
this.x = x;
this.y = y;
}
public int getX() {
return x;
}
public void setX(int x) {
this.x = x;
}
public int getY() {
return y;
}
public void setY(int y) {
this.y = y;
}
}
List<Data> dataList = new ArrayList<>();
Data x1 = new Data("n1", "f1", 1, 2);
Data x2 = new Data("n1", "f1", 3, 4);
Data x3 = new Data("n1", "f1", 5, 6);
Data x4 = new Data("n1", "f2", 7, 8);
Data x5 = new Data("n2", "f1", 9, 10);
Data x6 = new Data("n2", "f2", 11, 12);
Data x7 = new Data("n3", "f1", 13, 14);
Data x8 = new Data("n4", "f1", 15, 16);
dataList.add(x1);dataList.add(x2);dataList.add(x3);dataList.add(x4);dataList.add(x5);dataList.add(x6);dataList.add(x7);dataList.add(x8);
I want to use Java streams to create a Map<String, List<Result>>
out of the given input list. 我想使用Java流从给定的输入列表中创建
Map<String, List<Result>>
。 Also, the list values needs to be sorted in ascending order according to fields (x and y) 另外,列表值需要根据字段(x和y)以升序排序
I need the output map to be as follows: 我需要输出映射如下:
{"n1:f1" : [(1, 2), (3, 4), (5, 6)]
"n1:f2" : [(7, 8)]
"n2:f1" : [(9, 10)]
"n2:f2" : [(11, 12)]
"n3:f1" : [(13, 14)]
"n4:f1" : [(15, 16)]
}
The key for the map is the combination of systemid and filename concatenated by colon. 映射的键是由冒号连接的systemid和文件名的组合。 I tried grouping by the combination of systemid and filename but was not able to proceed with that approach.
我尝试通过systemid和filename的组合进行分组,但是无法继续使用该方法。
You can use groupingBy
collector with mapping
downstream as : 您可以将
groupingBy
收集器与下游mapping
一起使用:
Map<String, List<Result>> map = data.stream()
.collect(Collectors.groupingBy(a -> a.getSystemId() + ":" + a.getFileName()
, Collectors.mapping(a -> new Result(a.getX(), a.getY()),
Collectors.toList())));
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