Question

I have this method:

private SortedMap<String, SpaceObjectImpl> catalog = new TreeMap<String, SpaceObjectImpl>();

public Collection<SpaceObject> getSpaceObjects(){
    SortedSet<SpaceObject> temp = new TreeSet<SpaceObject>(catalog.values());
    return temp;
}

while compareTo is defined here:

public int compareTo(SpaceObjectImpl s){
    ....
}

class SpaceObjectImpl implements SpaceObject

When i run program, ClassCastException is thrown. Any idea why?

error:

SpaceObjectImpl cannot be cast to java.lang.Comparable(in TreeMap)

Answer 1

Your SpaceObjectImpl must implement Comparable interface.

The following is an example by mkyong that illustrates this concept:

public class Fruit implements Comparable<Fruit>{

    private String fruitName;
    private String fruitDesc;
    private int quantity;

    public Fruit(String fruitName, String fruitDesc, int quantity) {
        super();
        this.fruitName = fruitName;
        this.fruitDesc = fruitDesc;
        this.quantity = quantity;
    }

    public String getFruitName() {
        return fruitName;
    }
    public void setFruitName(String fruitName) {
        this.fruitName = fruitName;
    }
    public String getFruitDesc() {
        return fruitDesc;
    }
    public void setFruitDesc(String fruitDesc) {
        this.fruitDesc = fruitDesc;
    }
    public int getQuantity() {
        return quantity;
    }
    public void setQuantity(int quantity) {
        this.quantity = quantity;
    }

    public int compareTo(Fruit compareFruit) {

        int compareQuantity = ((Fruit) compareFruit).getQuantity(); 

        //ascending order
        return this.quantity - compareQuantity;

        //descending order
        //return compareQuantity - this.quantity;

    }   
}

Here, the Fruit class has implemented Comparable interface with type as Fruit and has also overriden compareTo method.