How to filter a list variable of an object in java?

Question

I have an object Student

public class Student {
   String name;
   String[] classes;

    public Student(String name, String[] classes){
        this.name = name;
        this.classes = classes;
    }

   //getter & setter
}

I want to filter the classes. For example, I have

Student sa = new Student("John", "math, physics")
Student sb = new Student("Jack", "math, english")
Student sc = new Student("Maria", "math, chemistry")

So how do I get the Student with math vairable? Do I need to write a new Predicate or is there an existing method? Thank you.

You can't just create an array from "math, physics", you need to do something like new String[]{"math", "physics"} — Jeppz
– Jeppz, Commented Apr 30, 2018 at 13:49

Link64 · Accepted Answer · 2018-04-30 13:54:17Z

There is a lot wrong with your code.

First: If your constructor requires a String[] parameter, you have to provide a String[] not a String. Call it like this:

Student sa = new Student("John", new String[]{"math", "physics"});
Student sb = new Student("Jack", new String[]{"math", "english"});
Student sc = new Student("Maria", new String[]{"math", "chemistry"});

You should make your fields private and use getters, instead of making them package private. You can read about visibility here

Now, you have three Student objecs. You cant put them into a Stream the easiest way like this:

Arrays.stream(new Student[]{sa, sb, sc})

You can now filter the stream with something like this:

Arrays.stream(new Student[]{sa, sb, sc})
    .filter(s -> {
        for(String className : s.classes) {
            if(className.equals("math")) return true;
        }
        return false;
    })

Which will give you a Stream of all Students that have the class math within their classes field. You can now collect your elements or do more work with it.

ernest_k · Accepted Answer · 2018-04-30 14:02:52Z

1

You can use a seaparate find method to locate a class in the array:

List<Student> mathStudents = studentList.stream()
                  .filter(student -> findClass(student.getClasses(), "math"))
                  .collect(Collectors.toList());

Sorting is needed for binary search, but it's better done before the search, preferably in constructor of Student, as suggested by AlexH

private static boolean findClass(String[] classes, String search) {

    Arrays.sort(classes); 
    return 0 <= Arrays.binarySearch(classes, search);
}

edited Apr 30, 2018 at 14:02

answered Apr 30, 2018 at 13:47

ernest_k

45.5k5 gold badges58 silver badges107 bronze badges

1 Comment

AxelH Over a year ago

I would suggest to sort the array in Student constructor. This would be simpler to look at chemistry later on ;)

Ousmane D. · Accepted Answer · 2018-04-30 20:25:07Z

1

Assuming you have a List<Student> populated with the correct data, you can accomplish the task at hand like so:

List<Student> result = 
     students.stream()
             .filter(student -> Arrays.stream(student.getClasses())
                                      .anyMatch("math"::equals))
             .collect(Collectors.toList());

This creates a stream from the list of students and filters them to retain only students who have a "math" class.

answered Apr 30, 2018 at 20:25

Ousmane D.

56.6k8 gold badges101 silver badges134 bronze badges

4 Comments

dexhunter Over a year ago

Thank you for answering. Could I ask how to filter two classes such as math and physics? Thanks!

Ousmane D. Over a year ago

@DexD.Hunter no probs and sure, you can do it like this -->

List<Student> result =                 students.stream()                         .filter(s -> {                             Set<String> classes =Set.of(s.getClasses());                             return  classes.contains("math") && classes.contains("physics");                         })                         .collect(Collectors.toList());

Ousmane D. Over a year ago

@DexD.Hunter if you're not using java-9 then change Set<String> classes =Set.of(s.getClasses()); to List<String> classes = Arrays.asList(s.getClasses());

dexhunter Over a year ago

Works like a charm. Thank you very much!

Collectives™ on Stack Overflow

How to filter a list variable of an object in java?

3 Answers 3

Comments

1 Comment

4 Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

3 Answers 3

Comments

1 Comment

4 Comments

Your Answer

Sign up or log in

Post as a guest

Related