sublist of arraylist???

[Suzi Kapoor]

Hi,

I was wondering if there is an efficient way of creating a sublist from an arraylist( not List)

[Suzi Kapoor]

I searched and realised that Generics could be used.
Could someone explain casting a arraylsit into list with some example.

Thanks in advance

[Garrett Rowe]

An ArrayList is a List (it implements the List interface).

import java.util.List;
import java.util.ArrayList;
public class SLTest
{
   public static void main(String[] args)
   {
      List<String> list = new ArrayList<String>();
      list.add("a");
      list.add("b");
      list.add("c");
      list.add("d");
      list.add("e");
 
     printList(list);
 
     List<String> sub = list.subList(1, 4);
 
     printList(sub);
   }
 
   public static void printList(List<String> aList)
   {
      for (int i = 0; i < aList.size() - 1; i++)
      {
         System.out.print(aList.get(i) + ", ");
      }
      System.out.print(aList.get(aList.size() - 1) + "\n");
   }
}

[ May 04, 2006: Message edited by: Garrett Rowe ]

[Jeff Albertson]

Originally posted by Suzi Kapoor:
I was wondering if there is an efficient way of creating a sublist from an arraylist( not List)

java.util.ArrayList implements interface java.util.List, so your "not List" restriction doesn't make sense. As far as creating a sublist, check out List method subList. Its API states:

Returns a view of the portion of this list between the specified fromIndex, inclusive, and toIndex, exclusive. (If fromIndex and toIndex are equal, the returned list is empty.) The returned list is backed by this list, so non-structural changes in the returned list are reflected in this list, and vice-versa. The returned list supports all of the optional list operations supported by this list.

The most important thing here is the fact that the sublist is *backed* by the original list, so alterations to the sublist affect the backing list. If you don't want this, copy the sublist (copying deeply if you need to).

Demo:

import java.util.*;
 
public class Test {
    private int value;
    public Test(int value) {this.value = value;}
    public String toString() {return String.valueOf(value);}
    public void triple() {value *= 3;}
 
    public static void main(String[] args) {
        exampleNotUsingGenerics();
        exampleUsingGenerics();
    }
 
    static void exampleNotUsingGenerics() {
        System.out.println("exampleNotUsingGenerics:");
        List a = new ArrayList();
        for (int i=0; i<10; ++i)
            a.add(new Test(i));
        List b = a.subList(3,6);
        System.out.println("a="+a + ", b="+b);
        for(Iterator i=b.iterator(); i.hasNext(); ) {
            Test x = (Test) i.next();
            x.triple();
        }
        System.out.println("now a="+a + ", b="+b);
    }
 
    static void exampleUsingGenerics() {
        System.out.println("exampleUsingGenerics:");
        List<Test> a = new ArrayList<Test>();
        for (int i=0; i<10; ++i)
            a.add(new Test(i));
        List<Test> b = a.subList(3,6);
        System.out.println("a="+a + ", b="+b);
        for(Test x : b)
            x.triple();
        System.out.println("now a="+a + ", b="+b);
    }
}

Originally posted by Suzi Kapoor:
I searched and realised that Generics could be used.
Could someone explain casting a arraylsit into list with some example.

Generics are orthogonal to your questions. Here you are asking something much more basic: how to "upcast" an arraylist reference to List. This is implicit in the syntax, for example:

//example 1:
List a = new ArrayList(); 
 
//example 2:
ArrayList a = ...
List b = ...
b = a;
 
//example 3:
void f(List a) {...}
 
ArrayList b = ...
f(b);
 
//example 4:
List g() {
    ArrayList a = ...
    return a;
}

[Suzi Kapoor]

Thanks for ur help.....