1
String playerlist;

for(Player p : allplayers){
    playerlist += p.getDisplayName() + ", ";
}

sendMessage(playerlist);

How can I accomplish this? I need to have a variable inside my for-loop, and then access it outside of the loop.

Thanks, and sorry if this is a very nooby question. I just can't figure it out.

DannyF247
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3 Answers3

7

You're close. You need to change the first line to

String playerlist = "";

In Java it's illegal to use a variable before it's initialized, and the line

playerlist += p.getDisplayName() + ", ";

desugars(*) to

String temp = playerlist;
playerlist = temp + (p.getDisplayName() + ", ");

Once you initialize playerlist, you are able to read off its contents into the temporary variable, and then update playerlist.

There are two higher-level suggestions, though.

  1. If you have a join method available, as some libraries provide (as do the the .NET and Python platforms), you can just use a join, which is easier to read.
  2. However, if you don't have a join method available (AFAIK, the Java standard library provides no such thing), you should use a StringBuilder, consistent with @anubhava's suggestion. You'll get better performance on large numbers of strings, and the code is easier to understand, which is a win-win.

EDIT: As @edalorzo rightly mentions in the comments, you will only get an error for not initializing a local variable, and both static and instance fields are automatically initialized. Specifically, numeric types (int, long, double, etc.) are initialized to 0, booleans are initialized to false, and reference types are initialized to null.

The spec explicitly describes the automatic initialization of fields, so you can rely on it, but it's rarely what you want. Normally, you will want a non-default value of the field when you use it later, since null isn't terribly useful (and many languages, particularly functional languages like Standard ML, go without null altogether). So I would recommend initializing all the fields of your objects in the constructor.

(*): The "desugaring" I show above is almost accurate: in truth, playerlist is evaluated only once, which doesn't affect the behavior of this particular program.

Adam Mihalcin
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  • WOW that was easy, thanks so much :D I thought I tried every combination, I guess I forgot one :) I will accept as best answer once it lets me (12 mins). – DannyF247 May 02 '12 at 04:00
  • Perhaps it is worth mentioning that only local variables require explicit initialization as stated in [JLS](http://docs.oracle.com/javase/specs/jls/se7/html/jls-4.html#jls-4.12.5). Since class and instance variables are explicitly initialized and parameters receive a value through their corresponding argument. – Edwin Dalorzo May 02 '12 at 04:13
6

If you initialize your variable outside loop you can get that:

String playerlist = "";

But since you are manipulating Strings it will be much better to use StringBuilder instead of String. Reason is the String is immutable and you will be creating a new String in each iteration inside for loop. However StringBuilder is mutable and has various overloaded append methods to support your operation. Using StringBuilder your code will become like this:

StringBuilder playerlist = new StringBuilder();

for (Player p : allplayers) {
    playerlist.append(p.getDisplayName()).append(", ");
}
anubhava
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0

The local variable playerlist may not have been initialized. In java for loop statement,local variables must first initialize and then use in loop block. You have to initialize playerlist. You can use following code:

String playerlist = null;

for(Player p : allplayers)
{
    playerlist += p.getDisplayName() + ", ";
}

sendMessage(playerlist);

But it is better use following code:

StringBuilder playerlist = new StringBuilder();

for(Player p : allplayers)
{
    playerlist.append(p.getDisplayName()).append(", ");
}
sendMessage(playerlist.toString());
Sam
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  • -1 Initializing `playerlist` to `null` will cause incorrect behavior (the literal string "null" at the beginning of the resulting string). See http://stackoverflow.com/a/693617/960195. – Adam Mihalcin May 02 '12 at 04:08
  • `playList` local variable may be empty so a String variable instantiated un-useful. In this sample `sendMessage` method has to check `null` state of incoming value. – Sam May 02 '12 at 04:32