Your error message indicates that you should implement Comparable
instead of Comparator in MyOwnClass. However, doing that will not
solve the problem. You will then get error messages about comparing
Integer and String. The problem is that you are adding instances of 3
different classes to TreeSet and they need to be compared to each
other. This can be done by implementing a Comparator class that
compares everything as strings and supply an instance of that class to
the TreeSet constructor. Here is one way to do it:
----------------- MyTreeSet.java
package mytreeset;
import java.util.TreeSet;
import java.util.Iterator;
import java.util.Comparator;
public class Main {
public static void main(String[] args) {
TreeSet ts = new TreeSet(new MyComparator());
ts.add("Tree Set Item 1");
ts.add("Tree Set Item 2");
MyOwnClass O1 = new MyOwnClass();
O1.setShape("Circle");
ts.add(O1);
MyOwnClass O2 = new MyOwnClass();
O2.setShape("Triangle");
ts.add(O2);
ts.add(4);
ts.add(7);
ts.add(9);
Iterator it = ts.iterator();
while(it.hasNext())
{
System.out.println(it.next());
}
}
static class MyComparator implements Comparator
{
public int compare(Object o1, Object o2)
{
return o1.toString().compareTo(o2.toString());
}
}
}
----------------- MyOwnClass.java
package mytreeset;
public class MyOwnClass
{
private String Shape;
public String getShape() {
return Shape;
}
public void setShape(String Shape) {
this.Shape = Shape;
}
public MyOwnClass() {
}
@Override
public String toString()
{
return this.Shape;
}
}
-----------------
Hope this helps.
Dag
On Oct 4, 7:58 pm, yongchun gao <gaoyongc...@gmail.com> wrote:
> Hello, All,
>
> I can not figure out how to slove the problem. Could you help me?
>
> Here is the classes.
>
> ------------------------------------------------------------------------------------------------------------
> package mytreeset;
>
> import java.util.TreeSet;
> import java.util.Iterator;
>
> public class Main {
>
> public static void main(String[] args) {
>
> TreeSet ts = new TreeSet();
>
> ts.add("Tree Set Item 1");
> ts.add("Tree Set Item 2");
>
> MyOwnClass O1 = new MyOwnClass();
> O1.setShape("Circle");
> ts.add(O1);
>
> MyOwnClass O2 = new MyOwnClass();
> O2.setShape("Triangle");
> ts.add(O2);
>
> ts.add(4);
> ts.add(7);
> ts.add(9);
>
> Iterator it = ts.iterator();
> while(it.hasNext())
> {
> System.out.println(it.next());
> }
> }
>
> }
>
> ------------------------------------------------------------------------------------------------------------
>
> package mytreeset;
>
> import java.util.Comparator;
>
> public class MyOwnClass implements Comparator
> {
> private String Shape;
>
> public String getShape() {
> return Shape;
> }
>
> public void setShape(String Shape) {
> this.Shape = Shape;
> }
>
> public MyOwnClass() {
> }
>
> public boolean equals(Object obj)
> {
> MyOwnClass M1=(MyOwnClass) obj;
> if (M1.Shape.length()==this.Shape.length())
> return true;
> else
> return false;
> }
>
> public int compare(Object o1, Object o2) {
> MyOwnClass M1=(MyOwnClass) o1;
> MyOwnClass M2=(MyOwnClass) o2;
> if (M1.Shape.length()>M2.Shape.length())
> return 1;
> else if (M1.Shape.length()==M2.Shape.length())
> return 0;
> else
> return -1;
> }
>
> }
>
> ------------------------------------------------------------------------------------------------------------
> When I run the main class, I got
>
> run:
> Exception in thread "main" java.lang.ClassCastException:
> mytreeset.MyOwnClass cannot be cast to java.lang.Comparable
> at java.util.TreeMap.put(TreeMap.java:542)
> at java.util.TreeSet.add(TreeSet.java:238)
> at mytreeset.Main.main(Main.java:18)
> Java Result: 1
> BUILD SUCCESSFUL (total time: 0 seconds)
>
> Thanks
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