Java collection grouping with custom key-CodePudding

I have a list of students studentList and i want to seperate them based on their class and section into a Map<CustomKey, List<Student>>.

class Student {
public String Name;
public String Class;
public String Section;
}

I have a custom Key Class

class CustomKey {
public String Class;
public String Section;
}

I'm trying to group them using this, but we can clearly see what I'm doing wrong.

studentList.stream().collect(Collectors.groupingBy(x -> new CustomKey(x.Class, x.Section)));

The expression inside groupingBy is incorrect. Extending the Student Class is not allowed :(

I want to create this output

{
    {"ClassA", "SectionA"}: [{name, class, section}, {name, class, section}],
    {"ClassA", "SectionB"}: [{name, class, section}],
    {"ClassB", "SectionA"}: [{name, class, section}, {name, class, section}],
}

My knowledge is very limited on JAVA. And any help/pointers are welcomed. Also, Apologies for the messed-up cases.

CodePudding user response：

i used your code exactly like below and it works for me

 public void testCustomGroupBy(){
    List<Student> studentList = new ArrayList<>();
    studentList.add(new Student("name_1","class_1","section_1"));
    studentList.add(new Student("name_2","class_1","section_1"));
    studentList.add(new Student("name_3","class_2","section_1"));
    studentList.add(new Student("name_4","class_2","section_2"));
    studentList.add(new Student("name_5","class_2","section_2"));
    Map<CustomKey, List<Student>> studentsBySection = studentList.stream().collect(Collectors.groupingBy(x -> new CustomKey(x.getClassName(), x.getSection())));
    studentsBySection.entrySet().forEach(System.out::print);
}

create your pojo like below

class Student {
public String name;
public String className;
public String section;
// getters and setters

public Student(String name, String className, String section) {
    this.name = name;
    this.className = className;
    this.section = section;
}

}

class CustomKey {
public String className;
public String section;
// getters and setters

public CustomKey(String className, String section) {
    this.className = className;
    this.section = section;
}

}

output

CustomKey(className=class_2, section=section_2)=[Student(name=name_4, className=class_2, section=section_2), Student(name=name_5, className=class_2, section=section_2)]
CustomKey(className=class_2, section=section_1)=[Student(name=name_3, className=class_2, section=section_1)]
CustomKey(className=class_1, section=section_1)=[Student(name=name_1, className=class_1, section=section_1), Student(name=name_2, className=class_1, section=section_1)]

CodePudding user response：

Well, CustomKey is matched by its equals method, which you don't have overridden. If you implement equals (and also hashCode) properly, then it'll work.

What's even better, is to create a record for CustomKey:

public record CustomKey(String className, String section) { }

This'll let the compiler auto-generate its equals and hashCode implementations for free, as well as getters, the canonical constructor and the toString method. The above record definition is equivalent to this:

public final class CustomKey {

    private final String className;
    private final String section;

    public CustomKey(String className, String section) {
        this.className = className;
        this.section = section;
    }

    public String className() {
        return className;
    }

    public String section() {
        return section;
    }

    @Override
    public boolean equals(Object o) {
        return getClass() == o.getClass()
            && Objects.equals(className, o.className)
            && Objects.equals(section, o.section);
    }

    @Override
    public int hashCode() {
        return Objects.hash(className, section);
    }

    @Override
    public String toString() {
        return "CustomKey[className="   className   ", section="   section   "]";
    }