Java Programming Forums - The Java Community - Loops & Control Statements

Help with nested loops. So confusing.

abstractmind — Tue, 18 Jun 2013 20:01:03 GMT

So for some reason, the idea of nested for loops COMPLETELY blows my mind. Consider this following code I wrote(I wrote this through experimentation and trial and error, I have no idea how it works.):

Code :

public class nestedloops {
 
	/**
	 * @param args
	 */
	public static void main(String[] args) {
		for (int r = 1; r <= 10; r++) {
			System.out.println();
			for (int c = 1; c <= 10; c++) {
				int product = r * c;
				if (product < 10) {
					System.out.print(" " + product + " ");
				} else {
					System.out.print(product + " ");
				}
 
			}
		}
	}
}

Now I was wondering if someone could thoroughly explain to me how this works, what is does is print a multiplication table for the numbers 1 - 10, on a grid. I don't understand how it prints each line 1 - 9, and then the way it prints all the way down 10 times confuses my mind. I know this is a really big newbie question, but arrays have never been understood in my mind. Whoever can give me a nice explanation to this probably very simple problem, HUGE kudos to you :)

Best way avoiding ConcurrentModifications

Jergetson — Thu, 13 Jun 2013 18:09:44 GMT

Hey.

I have a HashMap like this:

Code Java:

HashMap<Player, Integer> players = new HashMap<Player, Integer>();

The int is for the amount of seconds the player has left. Each second one second is removed from the person: (this is inside a thread)

Code Java:

for (final Iterator<Player> it = players.keySet().iterator(); it.hasNext(); ) {
    final Player player = it.next();
 
    if (player.isOnline()) {
        int seconds = getTimeLeft(player);
 
        if (seconds <= 0) {
            if (!(player.isDead())) {
                player.setHealth(0); // This kills the player
            }
        }
 
        setTimeLeft(player, --seconds);
    } else {
        it.remove();
    }
}

If the player has 0 seconds or less, the player is killed and later runs a method to update everything correspondingly. The player can also die naturally, either by another player or from eg. starvation, so thats why I can't necessarily add the check directly to the thread / loop. This is the code which is run when the player dies:

Code Java:

public void playerKilled(Player player) {
        // Some validation before this
        players.remove(player);
        spectators.add(player);
}

The problem with this is that it always throws a ConcurrentModificationException. I changed the HashMap to ConcurrentHashMap and that fixed it, but that forces me add a ton of null checks everywhere and it ends up extremely ugly and occasionally fails for some reason.

So the question is: What would be the best solution to avoid both of these errors while retaining the functionality?

Random Boolean in an if statement (tricky, can't seem to figure out)

Xhalite — Wed, 29 May 2013 01:37:18 GMT

Okay, so I have a class and in this class I have a method that runs a random boolean generator and I want to make it so that, depending on the value, it does one thing or the other. It's part of the battle class of my console game, Dungeoneer.(Which I will make a thread for soon, I may need help there lol).

Anyhow, here is the current code, and it has a massive error that I can't seem to work around. Basically, I want
to make the method blockCheck be a part of the battle system and essentially it works like this: "if player block true then take no damage, else take random damage(which I'll make shortly).

Code Java:

import java.util.Random;
 
//This class is the battle system
 
public class Battle {
 
    private static Random rnd = new Random();
 
    public static boolean blockCheck() {
        return rnd.nextBoolean();
        if (blockCheck() == false) {
            //TODO block code
        }
        else {
           //TODO damage code
 
       }
   }
 
}

So, what can I do to make this if statement work without error(aside from actually putting code in it)?

Intersection of two loops

sengreen — Sun, 26 May 2013 16:48:20 GMT

Hi everybody!!!
I want to get intersection of two loops. For example

Code :

class uniqueKey{
	public static void main(String args[]){
		for(int i=1;i<10;i++){
			if(i==5 || i==6 || i==7)
				continue;
			System.out.println(i);
		}
	}
}

Out put of this code is
1
2
3
4
8
9
Now scenario is something like this:
Suppose 5,6 and 7 are coming from database in a while loop and we want to print only those values that are not coming from database say from 1 to 10. I tried a lot but no success. Plz help me in this regard.....

[SOLVED] A while-loop is SUPPOSED to replace a substring

SOG — Fri, 24 May 2013 22:42:15 GMT

I notice that my method doesn't work even though I tried to get it's while-loop set-up to do the following things:

Have each token of a string get put into another one
Replace each given-substring (string2) with it's assigned-replacement (string3) as the string's (string1) tokens pass by
Have it's final answer returned as a variable to the main-method

Here is my code that has each of it's parts with a comment...

Code Java:

     // For the replacement of a substring with another one.
     public static String replaceSubstring(String string1, String string2, String string3)
     {
          Scanner keys = new Scanner(System.in);
 
          // Use a StringTokenizer-object for breaking a string
          // down to the point of finding that matching-substring
          // in general.
          StringTokenizer numOne = new StringTokenizer(string1, " ", true);
          StringTokenizer num1 = new StringTokenizer(string1, " ", true);
          StringTokenizer numWon = new StringTokenizer(string1, " ", true);
 
          String attacher = new String();
 
          // Find the substring with the while-loop.
          while (numOne.hasMoreTokens())
          {
               if (num1.nextToken() == string2)
               {
                    attacher += string3;
               }
               if (numWon.nextToken() != string2)
               {
                    attacher += numWon.nextToken();
               }
          }
 
          // Finally, return the String-object.
          return attacher;
     }

...and here is the output-part I want to avoid at runtime.



Exception in thread "main" java.util.NoSuchElementException

        at java.util.StringTokenizer.nextToken(StringTokenize  r.java:349)

        at MiscellaneousStr.replaceSubstring(MiscellaneousStr  .java:93)

        at MiscellaneousSDemo.main(MiscellaneousSDemo.java:84  )

So as you can see, I have to avoid getting a message that refers to having no more elements in an enumeration.

Any suggestions?