Convert a List<SomeObject> toList<AnotherObject> in java 8

Question

I am trying to convert a List of Object to another list fo Object. But i am trying to chose the best way using with or without streams by java8 way with more readability.

Lets say i have a list like below:

List<SomeObject> list = new ArrayList<SomeObject>;

SomeObject:

public class SomeObject{
  private String var1;
  private String var2;
  private String var3;
  private String var4;
  //getters and setters
}

AnotherObject:

public class AnotherObject{
  private String var1;
  private List<String> var2;
  //getters and setters
}

But in the another object if the var1 is same for multiple Objects i want to save the var2 value in a list of String .Example below:

SomeObject so = new SomeObject();
so.setVar1("key1");
so.setVar2("value1");
SomeObject so1 = new SomeObject();
so1.setVar1("key2");
so1.setVar2("value2"); 
SomeObject so2 = new SomeObject();
so2.setVar1("key2");
so2.setVar2("value3");    
list.add(so);
list.add(so1);
list.add(so2);

Now from the above List i want to create List of AnotherObject which has just var1 and var2 and if var1 is same in two SoemObject then i want the var2 values in the list and save it for var1.

Any suggestions are appreciated and helpful.

Answer 1

You seem to be looking for groupingBy based on var1 and then mapping to var2 as a list of values. This could be done as:

List<AnotherObject> anotherList = list.stream()
        .collect(Collectors.groupingBy(
                SomeObject::getVar1, Collectors.mapping(
                        SomeObject::getVar2, Collectors.toList())))
        .entrySet().stream()
        .map(e -> new AnotherObject(e.getKey(), e.getValue()))
        .collect(Collectors.toList());