Java 8 - Convert List to Map Examples

In this tutorial we will take a look at how to convert a Java Collection List to a Map, there are multiple ways in which you can do so, we will take a look at using Streams,

Example: Student.java

Let us first create our Class called Student,

public class Student {

    int studentId;
    String studentName;
    int studentAge;

    public Student(int studentId, String studentName, int studentAge) {
        this.studentId = studentId;
        this.studentName = studentName;
        this.studentAge = studentAge;
    }

    public int getStudentId() {
        return studentId;
    }

    public void setStudentId(int studentId) {
        this.studentId = studentId;
    }

    public String getStudentName() {
        return studentName;
    }

    public void setStudentName(String studentName) {
        this.studentName = studentName;
    }

    public int getStudentAge() {
        return studentAge;
    }

    public void setStudentAge(int studentAge) {
        this.studentAge = studentAge;
    }

    @Override
    public String toString() {
        return "Student{" +
                "studentId=" + studentId +
                ", studentName='" + studentName + '\'' +
                ", studentAge=" + studentAge +
                '}';
    }
}

ListToMapExample.java

import java.util.ArrayList;
import java.util.List;
import java.util.Map;
import java.util.function.Function;
import java.util.stream.Collectors;

/**
 * Java 8 Example to Convert a 
 * Collection List Object into 
 * a Map
 * 
 * Author: Code2care.org
 * Date: 03-Apr-2022
 * Version: 1.0
 * 
 */
public class ListToMapExample {

    public static void main(String[] args) {

        //Step 1: Create some Student Objects
        Student student1 = new Student(1, "Sam", 19);
        Student student2 = new Student(2, "Mike", 18);
        Student student3 = new Student(3, "Alex", 21);

        //Step 2: Add them to a List
        List studentList = new ArrayList<>();
        studentList.add(student1);
        studentList.add(student2);
        studentList.add(student3);

        //Step 3: Converting Student List to a Map using Stream
        Map<Integer, Student> studentMap = (Map<Integer, Student>) studentList.stream()
                .collect(Collectors.toMap(Student::getStudentId, Function.identity()));

       //Step 4: Let's print the results
        for (Map.Entry<Integer, Student> entry : studentMap.entrySet()) {
            System.out.println(entry.getKey() + ":" + entry.getValue());
        }
    }

}

Note that the key that you identify from the List for the Map should be unique or else you will get java.lang.IllegalStateException: Duplicate key

Exception in thread "main" java.lang.IllegalStateException: Duplicate key 2 (attempted merging values Student{studentId=2, studentName='Mike', studentAge=18} and Student{studentId=2, studentName='Mike', studentAge=18})
	at java.base/java.util.stream.Collectors.duplicateKeyException(Collectors.java:133)
	at java.base/java.util.stream.Collectors.lambda$uniqKeysMapAccumulator$1(Collectors.java:180)
	at java.base/java.util.stream.ReduceOps$3ReducingSink.accept(ReduceOps.java:169)
	at java.base/java.util.ArrayList$ArrayListSpliterator.forEachRemaining(ArrayList.java:1655)
	at java.base/java.util.stream.AbstractPipeline.copyInto(AbstractPipeline.java:484)
	at java.base/java.util.stream.AbstractPipeline.wrapAndCopyInto(AbstractPipeline.java:474)
	at java.base/java.util.stream.ReduceOps$ReduceOp.evaluateSequential(ReduceOps.java:913)
	at java.base/java.util.stream.AbstractPipeline.evaluate(AbstractPipeline.java:234)
	at java.base/java.util.stream.ReferencePipeline.collect(ReferencePipeline.java:578)
	at ListToMapExample.main(ListToMapExample.java:35)

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