How to use an enhanced for loop to print out the value of an ArrayList?

Question

I've been trying to figure out a way to print out values for an enhanced for loop statement for awhile now and just can't quite seem to figure out how to do it. I understand the basic concepts of an enhanced for loop but I can't seem to figure out how to use it in my scenario.

Here is the code:

 import java.util.ArrayList;

public class userGroup {

    ArrayList<User> userGroup = new ArrayList<>();
    User user0;

    public void addSampleData() {

    userGroup.add(user0);

    }

    public void printusername(){

       for (User x: userGroup)
           System.out.println(x);
    }
}

How would I implement a for loop statement in the printusername method that prints out user0 and its values? I understand that it's probably because there is an error somewhere in the creation of the ArrayList but I just can't find out where the error(s) are. Any help would be greatly appreciated by me and anybody else who may have the same issue. Thanks.

one obvious issue is.. your user0 object is a null reference, so eventually you are adding a null to your arraylist — mhasan, Nov 01 '16 at 19:44
1. You should be using generics: `ArrayList`. 2. `getUser` probably does not do what you think it does. 3. Post what you have tried with the `for` loop. — bradimus, Nov 01 '16 at 19:44
Don't name your class the same as one of its members. Also, in Java the convention is that class names have each word start with uppercase, e.g. UserGroup instead of userGroup. — azurefrog, Nov 01 '16 at 19:51
Related: http://stackoverflow.com/questions/40480/is-java-pass-by-reference-or-pass-by-value — JimmyB, Nov 01 '16 at 19:53
also idiomatic code uses camelCase like "printUserName". But then why do you have a class called *_group_ with an ArrayList which also has a single user member? Try to post clean/sensible code, so it can be useful for others as well instead of beeing confusing. — Wolf, Nov 01 '16 at 19:57
I planned on creating multiple users in that group but I read that its bets practice to use only one example when asking questions on stack overflow. I've also changed the class name to UserGroup and edited the code whilst trying a for loop. I'll post the updates. — Creative Username, Nov 01 '16 at 20:16
Do that and initialize `user0` to something other than `null` and you should be good. — bradimus, Nov 01 '16 at 20:31
You’re very close. Your creation of the ArrayList is entirely correct . Your use of the enhanced for loop is entirely correct. The issue is you’re adding a null, not a `User` object, so I assume your loop prints `null`. To have the values of the user printed, you need to create an object. If you aren’t already overriding `toString()` in the class `User`, you need to do that too (or change the `System.out.println()` to print selected values from `x`). — Ole V.V., Nov 01 '16 at 20:40

riddle_me_this · Answer 1 · 2016-11-01T20:41:19.760

I figure that you're close enough to an answer, but let's make the case a little simpler. Let's show a similar case with just one method to get some points across.

    public void printUsername() {  //note the case.  this is by convention in Java.

        //The left side is using the Collection interface, not an ArrayList.  
        //This is the weakest interface that we can use.   
        //We are also using generics to show that we have a Collection of User objects.

        //Lastly, we include the number one to show that we are 
        //creating an ArrayList with an initial capacity of 1.  We are just
        //adding one object.

        Collection<User> userGroup = new ArrayList<>(1); 

        //We create an object by using the "new" keyword
        User user0 = new User(); 

        //and now we add it to the Collection we created earlier
        userGroup.add(user0); 

        //now iterate through the loop
        for (User user: userGroup) {  //note the use of braces for clarity
            //this will call toString on the User object
            //if you don't implement toString(), 
            //you'll get a rather cryptic address (technically, a hashcode of the object)

            //try to make your variable names mean something too.
            //it's much easier to track down issues later when your variable
            //names make sense.  And it's much easier for the next person
            //trying to make sense of your code.
            System.out.println(user);  
        }
    }

If you are using Java 8 or later, you can replace your enhanced for loop with a collect call:

    userGroup.forEach(System.out::println);

How to use an enhanced for loop to print out the value of an ArrayList?

1 Answers1