for loop optimization

Sorry to say, but @Jigar's answer is incorrect. This is the correct answer. (tldr; don't use for : each).

import java.util.ArrayList;
import java.util.List;

public class LoopTest {

    public static void main(String s[]) {

        long start, end;

        List<Integer> a =  new ArrayList<Integer>();

        for (int i = 0; i < 2500000; i++) {
            a.add(i);
        }

        ///// TESTING FOR : EACH LOOP

        start = System.currentTimeMillis();

        for (Integer j : a) {
            int x = j + 3;
        }

        end = System.currentTimeMillis();

        System.out.println(end - start
                + " milli seconds for [ Integer j : a ] ");

        ////// TESTING DEFAULT LOOP

        start = System.currentTimeMillis();
        for (int i = 0; i < a.size(); i++) {
            int x = a.get(i) + 3;
        }

        end = System.currentTimeMillis();

        System.out.println(end - start
                + " milli seconds for [ int i = 0; i < a.length; i++ ] ");


        ////// TESTING SLIGHTLY OPTIMIZED LOOP

        start = System.currentTimeMillis();
        int size = a.size();
        for (int i = 0; i < size; i++) {
            int x = a.get(i) + 3;
        }

        end = System.currentTimeMillis();

        System.out.println(end - start
                + " milli seconds for [ int i = 0; i < size; i++ ] ");        

        //// TESTING MORE OPTIMIZED LOOP

        start = System.currentTimeMillis();
        for (int i = size; --i >= 0;) {
            int x = a.get(i) + 3;
        }

        end = System.currentTimeMillis();

        System.out.println(end - start
                + " milli seconds for [ int i = size; --i >= 0; ] ");       

    }

}

The results:

96 milli seconds for [ Integer j : a ] 
57 milli seconds for [ int i = 0; i < a.length; i++ ] 
31 milli seconds for [ int i = 0; i < size; i++ ] 
31 milli seconds for [ int i = size; --i >= 0; ]

You can make up your own mind, but too much attribution is given to the JVM optimizer. You still have to be smart with your own code, and using for : each notation is NOT a good idea (almost ever). As you can see, you have a good idea by putting size in its own variable.

Even though some of these optimization may be JVM-dependent (and some may kick in with the JIT), it's important to know what Java does and what Java doesn't do.

It is better to use for-each loop [more readable]

for (Flower flower :flowers){
    //...
}

I have dumped instructions using javap for the following code:

public void forLoop1() {
    List<String> lst = new ArrayList<String>();
    for (int i = 0; i < lst.size(); i++) {
        System.out.println("hi");
    }
}

public void forLoop2() {
    List<String> lst = new ArrayList<String>();
    int size = lst.size();
    for (int i = 0; i < size; i++) {
        System.out.println("hi");
    }
}

public void forLoop1();
  Code:
   0:   new     #2; //class java/util/ArrayList
   3:   dup
   4:   invokespecial   #3; //Method java/util/ArrayList."<init>":()V
   7:   astore_1
   8:   iconst_0
   9:   istore_2
   10:  iload_2
   11:  aload_1
   12:  invokeinterface #4,  1; //InterfaceMethod java/util/List.size:()I
   17:  if_icmpge       34
   20:  getstatic       #5; //Field java/lang/System.out:Ljava/io/PrintStream;
   23:  ldc     #6; //String hi
   25:  invokevirtual   #7; //Method java/io/PrintStream.println:(Ljava/lang/Str
ing;)V
   28:  iinc    2, 1
   31:  goto    10
   34:  return

public void forLoop2();
  Code:
   0:   new     #2; //class java/util/ArrayList
   3:   dup
   4:   invokespecial   #3; //Method java/util/ArrayList."<init>":()V
   7:   astore_1
   8:   aload_1
   9:   invokeinterface #4,  1; //InterfaceMethod java/util/List.size:()I
   14:  istore_2
   15:  iconst_0
   16:  istore_3
   17:  iload_3
   18:  iload_2
   19:  if_icmpge       36
   22:  getstatic       #5; //Field java/lang/System.out:Ljava/io/PrintStream;
   25:  ldc     #6; //String hi
   27:  invokevirtual   #7; //Method java/io/PrintStream.println:(Ljava/lang/Str
ing;)V
   30:  iinc    3, 1
   33:  goto    17
   36:  return

It doesn't optimize for me.

java version "1.6.0_22" Java(TM) SE Runtime Environment (build 1.6.0_22-b04) Java HotSpot(TM) Client VM (build 17.1-b03, mixed mode, sharing)

So if you need to choose from mentioned two, go for second, but I personally would go for for-each.

for-each Performance

From Item 46 in Effective Java by Joshua Bloch :

The for-each loop, introduced in release 1.5, gets rid of the clutter and the opportunity for error by hiding the iterator or index variable completely. The resulting idiom applies equally to collections and arrays:
// The preferred idiom for iterating over collections and arrays
for (Element e : elements) {
    doSomething(e);
}
When you see the colon (:), read it as “in.” Thus, the loop above reads as “for each element e in elements.” Note that there is no performance penalty for using the for-each loop, even for arrays. In fact, it may offer a slight performance advantage over an ordinary for loop in some circumstances, as it computes the limit of the array index only once. While you can do this by hand (Item 45), programmers don’t always do so.

for loop optimization

for-each Performance

See Also

Tags:

Java

Optimization

Coding Style

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