Why does this code sometimes throw a NullPointerException?

Posted by Tiberiu Hajas on Stack Overflow See other posts from Stack Overflow or by Tiberiu Hajas
Published on 2010-03-16T21:16:47Z Indexed on 2010/04/13 17:33 UTC
Read the original article Hit count: 181

Filed under:

java

|

hashmap

Consider the following Java source:

if( agents != null ) {
  for( Iterator iter = agents.keySet().iterator(); iter.hasNext(); ) {
    // Code that uses iter.next() ...
    //
  }
}

The agents is a HashMap.

Why does the for statement sometimes throw a NullPointerException?

Thank you.

© Stack Overflow or respective owner

Related posts about java

Tomcat 6: Access Control Exception?

as seen on Server Fault - Search for 'Server Fault'
I'm trying to setup a tomcat6 server, and I'm trying to match another setup someone else established. However, my deployment (default Ubuntu install) uses a policy.d/ directory structure, and the established server just uses a catalina.policy file. I've tried setting every entry in policy.d to match… >>> More
Problem in creation MDB Queue connection at Jboss StartUp

as seen on Stack Overflow - Search for 'Stack Overflow'
I am not able to create a Queue connection in JBOSS4.2.3GA Version & Java1.5, as I am using MDB as per the below details. I am putting this MDB in a jar file(named utsJar.jar) and copied it in deploy folder of JBOSS, In the test env. this MDB works well but in another env. [ env settings and… >>> More
failing to establish connection between Postgres db and gwt

as seen on Stack Overflow - Search for 'Stack Overflow'
Hi, I am using Postgres and gwt 2.0 for one of my applications. I am facing problem connecting to the database. When I try to connect it gives "ClassNotFoundException". Here is what I get when I try to connect to database: java.lang.ClassNotFoundException: org.postgresql.Driver at java.net… >>> More
failing to establish connection between postgre db and gwt

as seen on Stack Overflow - Search for 'Stack Overflow'
Hi, For i am using postgre and gwt 2.0 for one of my applications. I am facing problem connecting to the database. When i try to connect it gives "ClassNotFoundException". Here is what i get when i try to connect to database: java.lang.ClassNotFoundException: org.postgresql.Driver at java.net… >>> More
Migration and deployement problems JBoss 4.2.2.GA to JBoss 6.0.0.M2

as seen on Stack Overflow - Search for 'Stack Overflow'
Hi, I'm trying to migrate an application running on JBoss 4.2.2.GA to JBoss 6.0.0.M2 I give you some log to explain my problem : boot.log : 2010-03-16 09:59:29,406 ERROR [org.jboss.system.server.profileservice.ProfileServiceBootstrap] (Thread-2) Failed to load profile: Summary of incomplete deployments… >>> More

Related posts about hashmap

creating Object equality "HashMap" in ActionScript3 as java HashMap

as seen on Stack Overflow - Search for 'Stack Overflow'
const jonny1 : Person = new Person("jonny", 26); const jonny2 : Person = new Person("jonny", 26); const table : Dictionary = new Dictionary(); table[jonny1] = "That's me"; trace(table[jonny1]) // traces: "That's me" trace(table[jonny2]) // traces: undefined. But I want use Dictionary like this… >>> More
How to sort hashmap?

as seen on Stack Overflow - Search for 'Stack Overflow'
Hi All ! I have hashmap and its keys are like "folder/1.txt,folder/2.txt,folder/3.txt" and value has these text files data. Now i am stucked. I want to sort this list. But it does not let me do it :( Here is my hashmap data type: HashMap<String, ArrayList<String>> following function… >>> More
Efficient way to get the most used keys in a HashMap - Java

as seen on Stack Overflow - Search for 'Stack Overflow'
Hi! I have a HashMap where the key is a word and the value is a number of occurrences of that string in a text. Now I'd like to reduce this HashMap to only 15 most used words (with greatest numbers of occurrences). Do you have any idea to do this efficiently? >>> More
Java Generics Type Safety warning with recursive Hashmap

as seen on Stack Overflow - Search for 'Stack Overflow'
Hi, I'm using a recursive tree of hashmaps, specifically Hashmap map where Object is a reference to another Hashmap and so on. This will be passed around a recursive algorithm: foo(String filename, Hashmap<String, Object> map) { //some stuff here for (Entry<String, Object> entry… >>> More
Java: Last access of 2D HashMap

as seen on Stack Overflow - Search for 'Stack Overflow'
I have the following structure: HashMap< String, HashMap< String, String Now i want to know the last accessed element in the 2nd dimension. I know there is TreeMap which makes sense in the 1rst dimension but after that it doesn't make any sense. How can I keep track of a 2D HashMap ordering… >>> More