grouping objects java 8

Question

I have something like the below :

public class MyClass {
private Long stackId
private Long questionId
}

A collection of say 100, where the

Answer 1

You can use the java8 groupingBy collector. Like this:

import org.junit.Test;

import java.util.ArrayList;
import java.util.List;
import java.util.Set;
import java.util.stream.Collectors;

public class RandomTest {

    class MyClass {
        private Long stackId;
        private Long questionId;

        public MyClass(Long stackId, Long questionId) {
            this.stackId = stackId;
            this.questionId = questionId;
        }

        public Long getStackId() {
            return stackId;
        }

        public Long getQuestionId() {
            return questionId;
        }
    }

    public class MyOtherClass {
        private Long stackId;
        private Set questionIds;

        public MyOtherClass(Long stackId, Set questionIds) {
            this.stackId = stackId;
            this.questionIds = questionIds;
        }

        public Long getStackId() {
            return stackId;
        }

        public Set getQuestionIds() {
            return questionIds;
        }
    }

    @Test
    public void test() {
        List classes = new ArrayList<>();
        List otherClasses = new ArrayList<>();

        //populate the classes list
        for (int j = 1; j <= 25; j++) {
            for (int i = 0; i < 4; i++) {
                classes.add(new MyClass(0L + j, (100L*j) + i));
            }
        }

        //populate the otherClasses List
        classes.stream().collect(Collectors
                .groupingBy(MyClass::getStackId, Collectors.mapping(MyClass::getQuestionId, Collectors.toSet())))
                .entrySet().stream().forEach(
                longSetEntry -> otherClasses.add(new MyOtherClass(longSetEntry.getKey(), longSetEntry.getValue())));

        //print the otherClasses list
        otherClasses.forEach(myOtherClass -> {
            System.out.print(myOtherClass.getStackId() + ": [");
            myOtherClass.getQuestionIds().forEach(questionId-> System.out.print(questionId + ","));
            System.out.println("]");
        });
    }
}