In this tutorial you’ll see how to convert a Stream to list using collector method and utility methods like toList()
and toCollection() of Collectors class in Java Stream API.
1. A simple example to collect Stream elements into an ArrayList.
import java.util.List;
import java.util.stream.Collectors;
import java.util.stream.Stream;
public class StreamToList {
public static void main(String[] args) {
Stream<String> streamElem = Stream.of("A", "B", "C", "D");
List<String> listFromStream = streamElem.collect(Collectors.toList());
System.out.println(listFromStream.getClass().getName());
System.out.println("Elements in the list- " + listFromStream);
}
}
Output
java.util.ArrayList Elements in the list- [A, B, C, D]
As you can see type of the List returned is ArrayList.
2. If you want to convert the Stream into a LinkedList then you can use Collectors.toCollection() method.
public class StreamToList {
public static void main(String[] args) {
Stream<String> streamElem = Stream.of("A", "B", "C", "D");
List<String> listFromStream = streamElem.collect(Collectors.toCollection(LinkedList :: new));
System.out.println(listFromStream.getClass().getName());
System.out.println("Elements in the list- " + listFromStream);
}
}
Output
java.util.LinkedList Elements in the list- [A, B, C, D]
As you can see now the type of the List is LinkedList.
3. Converting Stream to List while filtering some of the elements by providing a condition. For example if you want to store only those elements in the List which are greater than 10 then you can filter other elements.
import java.util.Arrays;
import java.util.List;
import java.util.stream.Collectors;
public class StreamToList {
public static void main(String[] args) {
List<Integer> numList = Arrays.asList(6, 10, 5, 6, 7, 8, 12, 22);
List<Integer> listFromStream = numList.stream().filter(e -> e > 10).collect(Collectors.toList());
System.out.println("Elements in the list- " + listFromStream);
}
}
Output
Elements in the list- [12, 22]
4. An example with custom object to store only those Students in the List who have scored more than 90.
Student class used is as follows-public class Student {
private int rollNo;
private String name;
private String stream;
private int marks;
Student(int rollNo, String name, String stream, int marks){
this.rollNo = rollNo;
this.name = name;
this.stream = stream;
this.marks = marks;
}
public int getRollNo() {
return rollNo;
}
public String getName() {
return name;
}
public String getStream() {
return stream;
}
public int getMarks() {
return marks;
}
@Override
public String toString() {
return "Roll Number: " + getRollNo()
+ " Name: " + getName() + " Marks: " + getMarks();
}
}
public class StreamToList {
public static void main(String[] args) {
List<Student> studentList = Arrays.asList(new Student(1, "Mercy", "Science", 75),
new Student(2, "Ram", "Science", 99),
new Student(3, "Priscilla", "Art", 68),
new Student(4, "Jacques", "Maths", 97),
new Student(5, "Peter", "Science", 75));
List<Student> listFromStream = studentList.stream().filter(s -> s.getMarks() > 90).collect(Collectors.toList());
System.out.println("Elements in the list- " + listFromStream);
}
}
Output
Elements in the list- [Roll Number: 2 Name: Ram Marks: 99, Roll Number: 4 Name: Jacques Marks: 97]
5. If you want to get only the student names in the list then you can use map method along with collect.
public class StreamToList {
public static void main(String[] args) {
List<Student> studentList = Arrays.asList(new Student(1, "Mercy", "Science", 75),
new Student(2, "Ram", "Science", 99),
new Student(3, "Priscilla", "Art", 68),
new Student(4, "Jacques", "Maths", 97),
new Student(5, "Peter", "Science", 75));
List<String> listFromStream = studentList.stream().map(s -> s.getName()).collect(Collectors.toList());
System.out.println("Elements in the list- " + listFromStream);
}
}
Output
Elements in the list- [Mercy, Ram, Priscilla, Jacques, Peter]
That's all for the topic Java Stream - Convert a Stream to List. If something is missing or you have something to share about the topic please write a comment.
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