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"Exception ... java.lang.ClassCastException: MyClass cannot be cast to java.lang.Comparable"
To solve this problem, we must modify the definition of the class named MyClass to make it implement the Comparable interface (assuming that we don't provide a Comparator when the TreeSet object is instantiated).
This is accomplished in the modified version of the program shown in Listing 3 .
| Listing 3 . The program named Comparable05. |
|---|
//File Comparable05.java
import java.util.*;public class Comparable05{
public static void main(String args[]){
new Worker().doIt();}//end main()
}//end class Comparable05class Worker{
public void doIt(){Iterator iter;
Collection ref;ref = new TreeSet();
Populator.fillIt(ref);iter = ref.iterator();
while(iter.hasNext()){System.out.print(iter.next());
}//end while loopSystem.out.println();
}//end doIt()}// end class Worker
class Populator{public static void fillIt(Collection ref){
ref.add(new MyClass(4));ref.add(new MyClass(4));
ref.add(new MyClass(3));ref.add(new MyClass(2));
ref.add(new MyClass(1));}//end fillIt()
}//end class Populatorclass MyClass implements Comparable{
int data;MyClass(){
data = 0;}//end noarg constructor
MyClass(int data){this.data = data;
}//end parameterized constructorpublic String toString(){
return "" + data;}//end overridden toString()
public int compareTo(Object o){if(!(o instanceof MyClass))
throw new ClassCastException();if(((MyClass)o).data<data)
return 1;if(((MyClass)o).data>data)
return -1;else return 0;
}//end compareTo()public boolean equals(Object o){
if(!(o instanceof MyClass))return false;
if(((MyClass)o).data == data)return true;
else return false;}//end overridden equals()
}//end MyClass |
The important code to note in this modified version of the program is the new definition of the class named MyClass . The other code in the program is essentially the same as in the previous version of the program.
The beginning portion of the new definition for MyClass is shown in Listing 4 .
| Listing 4 . Beginning of the class named MyClass. |
|---|
class MyClass implements Comparable{
int data;MyClass(){
data = 0;}//end noarg constructor
MyClass(int data){this.data = data;
}//end parameterized constructorpublic String toString(){
return "" + data;}//end overridden toString() |
The code shown in Listing 4 is identical to the code in the previous version with one major exception. This version of the class definition implementsthe Comparable interface. That means that this class must provide a concrete definition for the following method, which is the only method declaredin the Comparable interface:
public int compareTo(Object o)
The description of the compareTo method in the Oracle documentation begins as follows:
"Compares this object with the specified object for order. Returns a negative integer, zero, or a positive integer as this object is less than, equalto, or greater than the specified object."
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