1. Introduction
When converting an object List into a Map form, you fill the Map by looping as shown below. Let's look at how to convert a List -> Map using the streams introduced in Java 8.
@Test
public void convert_students_to_map_of_nameVsAge_beforeJava8() {
int max = 3;
List<Student> students = TestUtil.getStudentSample(max);
Map<String, Integer> nameVsAgeMap = new HashMap<>();
Student student;
for (int i = 0; i < students.size(); i++) {
student = students.get(i);
nameVsAgeMap.put(student.getName(), student.getAge());
}
assertThat(nameVsAgeMap.size()).isEqualTo(max);
}
2. Converting List -> Map
2.1 Converting from List to Map Using Streams in Java 8
collect() converts the elements of a stream into the data type we want. A library called Collectors provides basic methods, and you can use toMap(), which converts into a Map form, to convert List -> Map.
In the example, name and age of Student, which become the key and value of the Map, are passed as arguments to toMap() to actually create a Map data type.
@Test
public void convert_students_to_map_of_nameVsAge() {
int max = 3;
List<Student> students = TestUtil.getStudentSample(max);
Map<String, Integer> nameVsAgeMap = students
.stream()
.collect(Collectors.toMap(
i1 -> i1.getName(),
i2 -> i2.getAge())
);
assertThat(nameVsAgeMap.size()).isEqualTo(max);
log.info("nameVsAgeMap : {}", nameVsAgeMap);
}
You can write the lambda expression more concisely through method references.
@Test
public void convert_students_to_map_of_nameVsAge_method_reference() {
int max = 3;
List<Student> students = TestUtil.getStudentSample(max);
//method reference
Map<String, Integer> nameVsAgeMap = students
.stream()
.collect(Collectors.toMap(
Student::getName,
Student::getAge)
);
assertThat(nameVsAgeMap.size()).isEqualTo(max);
}
This is an example of converting into a Map data type in the form of id : Student.
@Test
public void convert_students_to_map_of_idVsStudent() {
int max = 3;
List<Student> students = TestUtil.getStudentSample(max);
Map<Integer, Student> idVsStudentMap = IntStream.range(0, max).boxed()
.collect(Collectors.toMap(
i1 -> i1 + 1,
i2 -> students.get(i2)
));
idVsStudentMap.forEach((it, it2) -> log.info("{}", it));
assertThat(idVsStudentMap.size()).isEqualTo(max);
}
2.2 When There Is Duplicate Data in the List - An Exception Occurs
In the process of converting List -> Map, if there is a duplicate key in the Map, a java.lang.IllegalStateException: Duplicate key exception occurs.
@Test
public void 중복키가_존재하는_경우_IllegalStateException_발생() {
int max = 3;
List<Student> students = TestUtil.getStudentSample(max);
students.add(new Student("name1", 30)); //add a duplicate name
//throw occurs - java.lang.IllegalStateException: Duplicate key
assertThatThrownBy(() -> students.stream()
.collect(Collectors.toMap(
Student::getName,
Student::getAge)
)).hasMessage("Duplicate key name1 (attempted merging values 11 and 30)");
}
To prepare for cases where duplicates occur as above, you can provide a mergeFunction method as the third argument of toMap(). The MergeFunciton argument can contain the logic for how to handle duplicates when storing into the Map.
(oldValue, newValue) -> oldValuechooses the oldValue value when there is a duplicate
@Test
public void 중복키가_존재하는_경우_3rd_인자에_merge함수로_해결() {
int max = 3;
List<Student> students = TestUtil.getStudentSample(max);
students.add(new Student("name1", 30)); //add a duplicate name
Map<String, Integer> nameVsAgeMap = students
.stream()
.collect(Collectors.toMap(
Student::getName,
Student::getAge,
(oldValue, newValue) -> {
log.info("oldValue : {} newValue : {}", oldValue, newValue);
return oldValue;
})
);
assertThat(nameVsAgeMap.size()).isEqualTo(max);
}
3. Wrapping Up
We looked at how to convert from List -> Map form using Java 8 streams. Compared to the code style before Java 8, code written in the stream form seems to have the advantage of letting you understand the code much faster.
For the full source, please refer to github.