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| Listing 4 . The class named MyClass. |
|---|
class MyClass{
int data;MyClass(){
data = 0;}//end noarg constructor
MyClass(int data){this.data = data;
}//end parameterized constructorpublic String toString(){
return "" + data;}//end overridden toString()
}//end MyClass |
That brings us to the class named TheComparator from which the Comparator object was instantiated and passed to the constructor for the TreeSet object in Listing 2 . The declaration for the class named TheComparator is shown in Listing 5 .
| Listing 5 . Beginning of the class named TheComparator. |
|---|
class TheComparator
implements Comparator,Serializable{ |
As you can see, the class named TheComparator implements both the Comparator interface and the Serializable interface.
By implementing the Comparator interface, an object instantiated from the class is eligible to be passed to the constructor for a TreeSet object, which requires an incoming parameter of type Comparator .
Here is what Oracle has to say about implementing the Serializable interface:
"Note: It is generally a good idea for comparators to implement java.io.Serializable, as they may be used as ordering methods inserializable data structures (like TreeSet, TreeMap). In order for the data structure to serialize successfully, the comparator (if provided) mustimplement Serializable."
Since the Serializable interface doesn't declare any methods, implementing the interface simply requires a declaration that the interface isbeing implemented.
The Comparator interface declares the two methods listed below:
As is always the case when implementing interfaces, a class that implements the Comparator interface must provide concrete definitions for both of these methods.
The beginning of the compare method is shown in Listing 6 .
| Listing 6 . Beginning of the compare method. |
|---|
public int compare(Object o1,Object o2){
if(!(o1 instanceof MyClass))throw new ClassCastException();
if(!(o2 instanceof MyClass))throw new ClassCastException(); |
The purpose of a Comparator is to compare the values stored in the instance variables of two objects and to return a value indicating which objectis greater .
Generally speaking, therefore, a Comparator object must be specialized to deal with a particular type of object. That type could be
The code in Listing 6 confirms that both of the objects to be compared are of the correct type, which in this case is type MyClass .
Regardless of how the type is established, the code in the compare method of the Comparator object must gain access to the instance variables of the two objects passed to the compare method as type Object . This normally requires that a downcast be performed on the incoming object references.
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