Using Comparable to compare objects and sorting them in a TreeMap
- by arjacsoh
II cannot understand how should the natural ordering of class be "consistent with equals" when implementing the Comparable interface. I deteted a flaw in my program and therefore I deteced that in the documentantion of the interface Comparable. My problem is that although two Objects are considered as distinct on the base of equals method, the TreeMap structure treats them as equal and consequently does not accept the second insert. The sample code is:
public class Car implements Comparable<Car> {
int weight;
String name;
public Car(int w, String n) {
weight=w;
name=n;
}
public boolean equals(Object o){
if(o instanceof Car){
Car d = (Car)o;
return ((d.name.equals(name)) && (d.weight==weight));
}
return false;
}
public int hashCode(){
return weight/2 + 17;
}
public String toString(){
return "I am " +name+ " !!!";
}
public int compareTo(Car d){
if(this.weight>d.weight)
return 1;
else if(this.weight<d.weight)
return -1;
else
return 0;
}
/*public int compareTo(Car d){
return this.name.compareTo(d.name);
}*/
}
public static void main(String[] args) {
Car d1 = new Car(100, "a");
Car d2 = new Car(110, "b");
Car d3 = new Car(110, "c");
Car d4 = new Car(100, "a");
Map<Car, Integer> m = new HashMap<Car, Integer>();
m.put(d1, 1);
m.put(d2, 2);
m.put(d3, 3);
m.put(d4, 16);
for(Map.Entry<Car, Integer> me : m.entrySet())
System.out.println(me.getKey().toString() + " " +me.getValue());
TreeMap<Car, Integer> tm = new TreeMap<Car, Integer>(m);
System.out.println("After Sorting: ");
for(Map.Entry<Car, Integer> me : tm.entrySet())
System.out.println(me.getKey().toString() + " " +me.getValue());
}
The output is :
I am a !!! 16
I am c !!! 3
I am b !!! 2
After Sorting:
I am a !!! 16
I am c !!! 2
That is, that the object c has replaced (somewhat) object b.
If I comment the original equals method and uncomment the second equals method, which compares the objects according name, the output is the expected:
I am a !!! 16
I am c !!! 3
I am b !!! 2
After Sorting:
I am a !!! 16
I am b !!! 2
I am c !!! 3
Why does it come around in this way and what should I alter in order to insert and sort different objects with some attributes of equal value in a TreeMap?
