Comparing different type of Objects with comparable

Question

A.java

public class A implements Comparable {
    private String id;
    private String name;

    public A(String a, String b) {
        id = a;
        name = b;
    }

    public String getId() {
        return id;
    }

    public void setId(String id) {
        this.id = id;
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public int compareTo(Object o) {
        A a = (A) o;
        return id.compareTo(a.getId());
    }
}

B.java

public class B implements Comparable {
    private String b_id;
    private String other;

    public B(String a, String b) {
        b_id = a;
        other = b;
    }

    public String getBId() {
        return b_id;
    }

    public void setBId(String id) {
        this.b_id = id;
    }

    public String getOther() {
        return other;
    }

    public void setOther(String other) {
        this.other = other;
    }

    public int compareTo(Object o) {
        B b = (B) o;
        return b_id.compareTo(b.getId());
    }
}

Learn.java

public class Learn {

    public static void main(String[] args) {

        List<A> listA = new ArrayList<A>();
        List<B> listB = new ArrayList<B>();
        List<Object> listAll = new ArrayList<Object>();
        listA.add(new A("aa", "bb"));
        listA.add(new A("ae", "bbn"));
        listA.add(new A("dfr", "GSDS"));
        listB.add(new B("nm", "re"));
        listB.add(new B("asd", "asfa"));

        listAll.addAll(listA);
        listAll.addAll(listB);
        Collections.sort(listAll);
        for (Object o : listAll) {
            if (o instanceof A)
                System.out.println(o.getId);
            else if (o instanceof B)
                Syatem.out.println(o.getBId);
        }

    }

}

The error i get is at the line Collections.sort(listAll); It says.

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

What to do? Also is the rest of the logic all right?

What i am trying to do is have a list of A and list of B with one attribute same as id; though the variable name is not the same. i.e id in A and bid in B. Now i put both the lists in ListAll and do sort on them on the same variable id/bid. I have A and B implementing Comparable.

and my listAll is of type Object?

how do I do it? thanks.

Answer 1

You could add a common base class and implement comparison there, as in:

abstract class AandBComparable implements Comparable {

  public int compareTo(Object o) {
    AandBComparable ab = (AandBComparable) o;
    return getId().compareTo(ab.getId());
  }

  public abstract String getId();
}