Java Iterator: OutOfMemory (OOM) Scenario

This simple example explains how to iterate through Java Iterator and avoid possible OutOfMemory (OOM) in Java. This post is filed under Java Web Tutorial.

Possible reason: forget to use Iterator.next() inside loop.

Good code: Below example just iterator through ArrayList (Collection) and returns the same value with the help of  java.util.Iterator.

package com.crunchify;

import java.util.ArrayList;
import java.util.Iterator;

/**
 * @author Crunchify.com
 */

public class CrunchifyIteratorOOM {

        public static void main(String[] args) {
                ArrayList<String> company = new ArrayList();
                company.add("eBay");
                company.add("Paypal");
                company.add("Google");
                Iterator comapnyIterator = company.iterator();

                int i= 1;
                while(comapnyIterator.hasNext()) {
                        String companyName = (String)comapnyIterator.next();
                        System.out.print(i + " ");
                        System.out.println(companyName);
                        i++;
                }
        }
}

Output:

1 eBay
2 Paypal
3 Google

Bad code: Now just comment out below two lines from above code.

// String companyName = (String)comapnyIterator.next();
// System.out.println(companyName);

Output: You should see your Java program going into infinite loop which eventually cause OOM.

What is the best practice? Always use enhanced Java 8 for loop.

package com.crunchify;

import java.util.ArrayList;
import java.util.Iterator;

/**
 * @author Crunchify.com
 */

public class CrunchifyIteratorOOM {

        public static void main(String[] args) {
                ArrayList<String> company = new ArrayList();
                company.add("eBay");
                company.add("Paypal");
                company.add("Google");

// Same example as above but enhanced for loop.
                for (String s : company) {
                    System.out.println(s);
                }
        }
}

How can I check if memory usage is increasing over time?

• Run this program
• Go to macOS Terminal (in Mac) or Command prompt (in Windows)
• Type: jConsole and Enter
• Select process associated with your java program as in below diagram

• Monitor / Analyse Memory/ CPU usage and you should see it increase over time and finally OOM.

Sometimes this small thing could leads to Memory Leak/ OOM in major production projects.

The post Java Iterator: OutOfMemory (OOM) Scenario appeared first on Crunchify.

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