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.