For-each in Java

Introdcution

For-each is an array traversing technique in Java, which is an enhancement to for-loop. The syntax of for-each loop is as follows:

for (type var : array) 
{ 
    statements using var;
}

You can interpret the above code as:

for (int i=0; i<arr.length; i++) 
{ 
    type var = arr[i];
    statements using var;
}

But is the interpretation true? Now we are going to use JAD Java Decompiler to prove it.

Install JAD on Mac

We can use homebrew to install JAD:

brew cask jad

And brew will download and install JAD for you. Then you can use jad command to verify JAD has been installed:

Use JAD to decompile java files

Now we have a ForEachDemo.java:

public class ForEachDemo {

    public static void main(String[] args) {
        int[] array = {1,2,3,4,5};
        for (int element: array) {
            System.out.println(element);

        }
    }
}

We can use javac command to compile ForEachDemo.java, then use jad command to decompile the produced class file:

After decompilation, JAD creates a ForEachDemo.jad for us：

We can open it in a text editor：

// Decompiled by Jad v1.5.8g. Copyright 2001 Pavel Kouznetsov.
// Jad home page: http://www.kpdus.com/jad.html
// Decompiler options: packimports(3) 
// Source File Name:   ForEachDemo.java

import java.io.PrintStream;

public class ForEachDemo
{

    public ForEachDemo()
    {
    }

    public static void main(String args[])
    {
        int ai[] = {
            1, 2, 3, 4, 5
        };
        int ai1[] = ai;
        int i = ai1.length;
        for(int j = 0; j < i; j++)
        {
            int k = ai1[j];
            System.out.println(k);
        }

    }
}

From the decompiled code, we can verify our previous interpretation.

Limitation of For-each loop

From the above conclusion, we can know that for-each loops are not appropriate when you want to modify the array:

for (int num : marks) 
{
    // only changes num, not the array element
    num = num*2; 
}