I’m quite new to using Java and was trying to flatten collections within collections using map to try and get a single List. However I don’t seem to be able to get this working. In order to reproduce this I’ve created quite a simple example of what I’m trying to do. Here is what I have so far:
ClassA
import java.util.List;
public class ClassA {
private List<ClassB> listOfClassB;
public ClassA(List<ClassB> listOfClassB) {
this.listOfClassB = listOfClassB;
}
public List<ClassB> getListOfClassB() {
return this.listOfClassB;
}
}
ClassB
public class ClassB {
private String item;
public ClassB(String item) {
this.item = item;
}
public String getItem() {
return item;
}
}
Main
public class Main {
public static void main(String[] args) {
ClassB firstClass = new ClassB("A");
ClassB secondClass = new ClassB("B");
ClassB thirdClass = new ClassB("C");
ClassB fourthClass = new ClassB("D");
ArrayList<ClassB> firstClassList = new ArrayList<>();
ArrayList<ClassB> secondClassList = new ArrayList<>();
firstClassList.add(firstClass);
firstClassList.add(secondClass);
secondClassList.add(thirdClass);
secondClassList.add(fourthClass);
ArrayList<ClassA> classes = new ArrayList<>();
classes.add(new ClassA(firstClassList));
classes.add(new ClassA(secondClassList));
List<List<String>> collect = classes.stream().map(c -> c.getListOfClassB().stream().map(ClassB::getItem).collect(Collectors.toList())).collect(Collectors.toList());
}
}
As you can see on the bottom I am able to get List<List<String>> but what I’m looking to get is a List<String> of the items within ClassB. I’ve tried using a flatmap for this but I couldn’t get it working and was looking for some guidance.
Thanks in advance.
Solution:
Here is the flatmap example which works fine:
classes.stream().flatMap(aclass -> aclass.getListOfClassB().stream())
.forEach(b -> System.out.println("Class B Item Name : "+b.getItem()));
It gives the following output:
Class B Item Name : A
Class B Item Name : B
Class B Item Name : C
Class B Item Name : D
and to get the exact answer:
List<String> collect2 = classes.stream().flatMap(aclass -> aclass.getListOfClassB().stream())
.map(b -> b.getItem())
.collect(Collectors.toList());
it gives me a list as follows:
collect2 : [A, B, C, D]
A Passionate Techie 