Vector.size() using in for loop

Question

hi,
i would like to know whether performance can be improved by using Vector.size() outside a loop than inside a loop
int size=VectorObj.size();
for(int i=0;i<size;i++)
is the above a better code than below
for(int i=0;i<VectorObj.size();i++)
please let me have your comment on this

Answer 1

It will be faster in the first case, but as others have already mentioned, you will run into problems if your vector is changing during the loop. If your vector IS changing, then case 1 could: a) miss the new additions to the vector, or b) throw an ArrayIndexOutOfBoundsException because the vector is smaller than it originally was (ie an item was removed during the loop).
Case 2 may also give you trouble: it may miss some elements at the end of the vector if some elements are removed during the loop. If you have "one" "two" and "three" in the vector, then remove "two" when you read it, the loop will never get to "three".
A solution to these is to clone the vector for the loop and use the original vector for additions and removals.
import java.util.Vector;
public class VectorLoop{
     public static void main(String[] args){
          bad1a();
          try{
               bad1b();
          }catch(Exception ex){
               System.out.println("Exception thrown");
          bad2();
          good();
     public static void bad1a(){//new element not reached
          System.out.println("\nBad 1a:");
          Vector vec = new Vector();
          vec.addElement("one");
          vec.addElement("two");
          vec.addElement("three");
          int size = vec.size();
          for(int i=0; i<size; i++){
               System.out.println(vec.elementAt(i).toString());
               if(vec.elementAt(i).equals("two")){
                    vec.addElement("four");
     public static void bad1b(){//exception is thrown
          System.out.println("\nBad 1b:");
          Vector vec = new Vector();
          vec.addElement("one");
          vec.addElement("two");
          vec.addElement("three");
          int size = vec.size();
          for(int i=0; i<size; i++){
               System.out.println(vec.elementAt(i).toString());
               if(vec.elementAt(i).equals("two")){
                    vec.removeElementAt(i);
     public static void bad2(){//"three" not reached
          System.out.println("\nBad 2:");
          Vector vec = new Vector();
          vec.addElement("one");
          vec.addElement("two");
          vec.addElement("three");
          for(int i=0; i<vec.size(); i++){
               System.out.println(vec.elementAt(i).toString());
               if(vec.elementAt(i).equals("two")){
                    vec.removeElementAt(i);
     public static void good(){
          System.out.println("\nGood:");
          Vector vec = new Vector();
          vec.addElement("one");
          vec.addElement("two");
          vec.addElement("three");
          Vector vec2 = (Vector)vec.clone();
          int size = vec2.size();
          for(int i=0; i<size; i++){
               System.out.println(vec2.elementAt(i).toString());
               if(vec2.elementAt(i).equals("two")){
                    vec.removeElementAt(i); //remove from ORIGINAL vector.
}Of course the over-head of cloning will slow you down...another solution may be to have an add vector and a delete vector wich will keep track of items to be added/removed AFTER the loop...there are other complexities that will crop up with this, however.
I believe that the clone() solution is the only thread-safe one.
Hope this helps.