线程“主”java.lang.ClassCastException 中的异常：setcollection.Enseignant 无法转换为 java.lang.Comparable

Question

Exception in thread "main" java.lang.ClassCastException: setcollection.Enseignant cannot be cast to java.lang.Comparable

public class Test {

    /**
    * @param args the command line arguments
    */
    public static void main(String[] args) {
        Enseignant e=new Enseignant();
        Enseignant e1=new Enseignant(1,"bargaoui","haythem");
        Enseignant e2=new Enseignant(12,"islem","ben ahmed");
        System.out.println(e1.toString());
        System.out.println(e2.toString());
        System.out.println(e1.equals(e2));
        TreeSet<Enseignant> treeset = new TreeSet<Enseignant>(); 
        Enseignant E1=new Enseignant(1,"mohamed","ben brahim");
        Enseignant E2=new Enseignant(2,"imed","rouabi");
        Enseignant E3=new Enseignant(3,"salim","ali");
        Enseignant E4=new Enseignant(4,"zayneb","sassi");
        Enseignant E5=new Enseignant(5,"khalid","louati");
        Enseignant E6=new Enseignant(6,"hatem"," himadi");


        treeset.add(E1);
        treeset.add(E2);
        treeset.add(E3);
        treeset.add(E4);
        treeset.add(E5);
        treeset.add(E6);
        System.out.println(treeset.contains(E4));
        System.out.println(treeset.contains(E1.getId()));
        System.out.println(treeset.remove(E6));
        Iterator iterator = treeset.iterator();

        // afficher tous les éléments de TreeSet
        System.out.println("éléments de TreeSet");     
        while (iterator.hasNext()){
        System.out.println(iterator.next());
        }
    }
}

Exception in thread "main" java.lang.ClassCastException:

Answer 1

Since you are using a TreeSet, some comparisons will be done and a Comparator<> is needed.

There are two things that you can do.

1. Implement Comparable<> in Enseignant

public class Enseignant implements Comparable<Enseignant> {
...
  public int compareTo(Enseignant other) {
    // Compare here
    return 0;
  }
...
}

2. Add a Comparator on creation of TreeSet

TreeSet<Enseignant> treeset = new TreeSet<>(new Comparator<Enseignant>(){
  public int compare(Enseignant e1, Enseignant e2) {
    // Compare here.
    return 0;
  }
});