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As you should already know, an array is a container that can be used to store a collection of primitive values or a collection of references to objects.
The Collection interface declares a method named toArray , which can be called on a Collection object to "return an array containing all of the elements in this collection whose runtime type is that of thespecified array" .
Since you are studying this sub-series of modules to learn about the uses of the Comparator interface, this seems like an opportune time to teach you how to get an array from a collection, and how to use the Comparator interface to sort the contents of the array. (While I'm at it, I will also teach you how to sort the elements in an array of object references into naturalorder without the use of a Comparator object.)
The code in this series of modules is written with no thought given to Generics . As a result, if you copy and compile the code, you will probably get warnings about unchecked or unsafe operations .
While you will ultimately need to understand how to use Generics, that is a very complex topic. An understanding of Generics is beyond the scope of thiscourse. Therefore, for purposes of this course, you can simply ignore those warnings.
See if you can write down the output produced by the program shown in Listing 1 .
| Listing 1 . The program named Comparator 05 . |
|---|
//File Comparator05.java
//Copyright 2001, R.G.Baldwinimport java.util.*;
import java.io.Serializable;public class Comparator05{
public static void main(String args[]){
new Worker().doIt();}//end main()
}//end class Comparator05class Worker{
public void doIt(){Iterator iter;
Collection ref;Object[] array;ref = new Vector();
Populator.fillIt(ref);iter = ref.iterator();
System.out.println("Collection data");while(iter.hasNext()){
System.out.print(iter.next() + " ");}//end while loop
System.out.println();array = ref.toArray();
System.out.println("Raw array data");display(array);
//Sort the array into natural order// and display it.Arrays.sort(array);
System.out.println("Natural order sorted " +"array data");
display(array);//Sort the array into custom order
// and display it.Arrays.sort(array, new TheComparator());
System.out.println("Custom order sorted " +"array data");
display(array);iter = ref.iterator();
System.out.println("Collection data");while(iter.hasNext()){
System.out.print(iter.next() + " ");}//end while loop
System.out.println();}//end doIt()
static void display(Object[]array){
for(int i = 0; i<array.length;i++){
System.out.print(array[i]+ " ");
}//end for loopSystem.out.println();
}//end display()}// end class Worker
class Populator{public static void fillIt(Collection ref){
ref.add("Joe");ref.add("Bill");
ref.add("Tom");ref.add("JOE");
ref.add("BILL");ref.add("TOM");
}//end fillIt()}//end class Populator
class TheComparator implements Comparator,Serializable{public int compare(Object o1,Object o2){
if(!(o1 instanceof String))throw new ClassCastException();
if(!(o2 instanceof String))throw new ClassCastException();
int result = ((String)o1).compareTo(((String)o2));return result*(-1);
}//end compare()}//end class TheComparator |
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