Error when using Collections.sort()

Question

I'm wondering why i'm getting an error after trying to sort the List. The error happens when i try to sort the list containing "Student" objects.

import java.lang.reflect.Array;
import java.util.*;
import java.util.ArrayList;

public class mainProgram {
    public static void main(String[] args) {
        List<Student> classStudents = new ArrayList<Student>();

    //Add 4 students to List
        classStudents.add(new Student("Charles", 22));
        classStudents.add(new Student("Chris", 25));
        classStudents.add(new Student("Robert", 23));
        classStudents.add(new Student("Adam", 21));

    //sort and print
        Collections.sort(classStudents);    //Why does this not work?
        System.out.println("classStudent(Sorted) ---> "
        + Arrays.toString(classStudents.toArray()));
    }
}

class Student implements Comparable<Student>{
    // fields
    private String name;
    private int age;

    //Constructor
    public Student(String name, int age){ 
        name = name;
        age = age;
        }
    //Override compare
    public int CompareTo(Student otherStudent){
        if(this.age == otherStudent.age){
            return 0;
        } else if(this.age < otherStudent.temp){
            return -1;
        } else{
            return 1;
        }
    }

    //Override toString
    public String toString(){
        return this.name + " " + this.age;
    }
}

interface Comparable<Student>{
    int CompareTo(Student otherStudent); 
    String toString();
}

The Error is:

Bound mismatch: The generic method sort(List<T>) of type Collections is not applicable for the arguments (List<Student>). The inferred type Student is not a valid substitute for the bounded parameter <T extends Comparable<? super T>>

I do not understand what the error means and how to fix it. Your input will be very much appreciated.

Pshemo · Accepted Answer · 2015-07-01T22:17:24.617

It may not be so obvious based on error message, but if you take a look at documentation of Collection.sort you fill find that this method (like rest of standard API methods) expects list of instances implementing already defined in API interface which in this case is java.lang.Comparable.

So don't try to create your own Comparable interface. Remember that if some method expects some type, this type must be already defined somewhere (otherwise class with such method wouldn't compile because method can't use some unknown/undefined type).

Also comparing method defined in java.util.Comparable is compareTo not CompareTo (Java is case sensitive so these methods are not treated as equal).

Other problems are

Student doesn't have temp field so
```
} else if (this.age < otherStudent.temp) {
```
should probably be
```
} else if (this.age < otherStudent.age) {
```
or even better, instead of this age comparing conditions if(..<..){-1} else if(..==..){0} else {1} simply use
```
return Integer.compare(age, otherStudent.age);
```
in constructor you need this.name to determine which name you are refering to. this.name means field of this instance, name is reference to variable passed to constructor. So you need
```
public Student(String name, int age) {
    this.name = name;
    this.age = age;
}
```

score 2 · Answer 2 · answered Jul 01 '15 at 21:53

2

Java defines its own Comparable interface. delete yours and import theres, the collections method uses the java one.

answered Jul 01 '15 at 21:53

Neil Locketz

4,228
1
23
34

score 2 · Answer 3 · answered Jul 02 '15 at 07:28

Made small changes, have a look:

MainProgram.java

    import java.util.ArrayList;
    import java.util.Arrays;
    import java.util.Collections;
    import java.util.List;

    public class MainProgram {

        public static void main(String[] args) {
            System.out.println("HI !!!");
            List<Student> classStudents = new ArrayList<Student>();

        //Add 4 students to List
            classStudents.add(new Student("Charles", 22));
            classStudents.add(new Student("Chris", 25));
            classStudents.add(new Student("Robert", 23));
            classStudents.add(new Student("Adam", 21));

        //sort and print
            Collections.sort(classStudents);    //Why does this not work?
            System.out.println("classStudent(Sorted) ---> "
            + Arrays.toString(classStudents.toArray()));
        }
    }

Student.java

class Student implements java.lang.Comparable<Student>{
    // fields
    private String name;
    private int age;

    //Constructor
    public Student(String name, int age){ 
        this.name = name;
        this.age = age;
        }

    //Override compare
    public int compareTo(Student otherStudent){
        if(this.age == otherStudent.age){
            return 0;
        } else if(this.age < otherStudent.age){
            return -1;
        } else{
            return 1;
        }
    }

    //Override toString
    public String toString(){
        return this.name + " " + this.age;
    }
}

Output : classStudent(Sorted) ---> [Adam 21, Charles 22, Robert 23, Chris 25]

score 0 · Answer 4 · edited May 23 '17 at 11:51

0

Your Student class needs to implement the java.lang.Comparable interface if you are going to sort on it. Otherwise, how could the method possibly know on what criteria to sort?

This should be helpful: Why should a Java class implement comparable?

edited May 23 '17 at 11:51

Community

1
1

answered Jul 01 '15 at 21:52

Kon

10,702
6
41
58

score 0 · Answer 5 · answered Jul 01 '15 at 22:10

First, there's a typo where you override the Comparable interface method, "compareTo()" --- it should be a lower-case "c", not upper-case.

Second, I don't know what you're trying to do with the last bit that starts, "interface Comparable". Drop that part --- you don't need it.

The Comparable interface is part of the API.

Cheers.

Error when using Collections.sort()

5 Answers5