Sort a typed array of object with a lambda expression

Question

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 · Accepted Answer · 2018-12-06 11:30:11Z

3

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;
        }
    }
}

edited Dec 6, 2018 at 11:30

answered Dec 6, 2018 at 11:13

Eran

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9 Comments

takesuma Over a year ago

i have done class Association<C extends Comparable<C>,V> class Table<C extends Comparable<C>,V> and now i have the error:

Exception in thread "main" java.lang.NullPointerException 	at Table.lambda$ajout$0(Table.java:15) 	at java.util.TimSort.countRunAndMakeAscending(TimSort.java:355) 	at java.util.TimSort.sort(TimSort.java:220) 	at java.util.Arrays.sort(Arrays.java:1438) 	at Table.ajout(Table.java:15) 	at main.main(main.java:546)

Eran Over a year ago

@takesuma The NullPointerException is a separate issue. You are trying to sort an array that has 1 Association instance and 9 null references. Besides, your add method always assigns to the first index of the array, which can't be right.

Eran Over a year ago

@takesuma yes, unless you change your lambda expression to handle the case where a or b are null.

takesuma Over a year ago

I tried this: Arrays.sort(asso, (@NotNull C a,@NotNull C b) -> a.cle.compareTo(b.cle)); but it don't work it's my first use of lambda so i don't know well how it works.

Eran Over a year ago

@takesuma just use Arrays.sort(asso, Comparator.nullsLast(Comparator.comparing (Association::getCle))); (and add a getter method getCle() to Association class).

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