Sort a typed array of object with a lambda expression

takesuma :

I run into a problem that I do not understand. In the method of adding in the Table class I would like to use:

Arrays.binarySearch(asso, (a,b) -> a.cle.compareTo(b.cle));

Where asso is my array of Association objects containing the key and the value.

By executing this code with the type String, I get an error "cannot find symbol" on the method compareTo(Object).

There my code:

test:

Table<String,String> table0 = new Table<String,String>(10);
table0.add("1","int");

method:

import java.util.Arrays;

public class Table<C,V>{

  Association[] asso;

  public Table(int n){
    asso = new Association[n];
  }

  public void add(C cle, V valeur){
    asso[0] = new Association<C,V>(cle,valeur);
    Arrays.sort(asso, (a,b) -> a.cle.compareTo(b.cle));
  }

  public class Association<C,V>{

    public C cle;
    public V valeur;

    public Association(C cle,V valeur){
      this.cle = cle;
      this.valeur = valeur;
    }
  }
}

And the error:

../src/Table.java:15: error: cannot find symbol
Arrays.sort(asso, (a,b) -> a.cle.compareTo(b.cle));
                                ^
symbol:   method compareTo(Object)
location: variable cle of type C
where C is a type-variable:
C extends Object declared in class Table.Association

Thank you for your help.

Eran :

Add a type bound to the generic type parameter C:

class Table<C extends Comparable<C>,V>

Otherwise the compiler doesn't know that C implements Comparable (and therefore must have the compareTo method).

Also don't use raw type (Association[]) for the array. Use:

Association<C,V>[] asso;

On second thought, I also suggest you make the Association class static:

public class Table<C extends Comparable<C>,V> {

    Association<C,V>[] asso;

    public Table(int n) {
        asso = new Association[n];
    }

    public void add(C cle, V valeur) {
        asso[0] = new Association<C,V>(cle,valeur);
        Arrays.sort(asso, (a,b) -> a.cle.compareTo(b.cle));
    }

    public static class Association<C,V> {

        public C cle;
        public V valeur;

        public Association(C cle,V valeur){
            this.cle = cle;
            this.valeur = valeur;
        }
    }
}