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Thread: problems with for loop

  1. #1
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    Default problems with for loop

    	case 3:
    		System.out.println("Hundens namn: ");
    	    String todelete=scan.nextLine();
     
             for (Dog h : dogs)
    	    	if (h.getname().equals(todelete))
                {
                    System.out.println(h);
    	    		dogs.remove(h);
    	    		System.out.println();
    	    		System.out.println(h+ " Är borttagen ");
                }
    	    break;


    Exception in thread "main" java.util.ConcurrentModificationException
    at java.util.AbstractList$Itr.checkForComodification( AbstractList.java:372)
    at java.util.AbstractList$Itr.next(AbstractList.java: 343)
    at Register.main(Register.java:68)


    Thats the errormessage. The program runs fine except for when I want to delete an item it shows me the errormessage above. Whats wrong?


  2. #2
    Super Moderator Norm's Avatar
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    Default Re: problems with for loop

    Did you read the API doc for the exception that is thrown?

    Try using a normal for loop to iterate through the collection.
    If you don't understand my answer, don't ignore it, ask a question.

  3. #3
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    Default Re: problems with for loop

    Norm what do you mean, using a normal for loop? I have tryed but I cant seem to make this one work. Is the last piece of my assignment...

  4. #4
    Super Moderator Norm's Avatar
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    Default Re: problems with for loop

    normal for loop?
     for(int i=0; i < collection.size(); i++) {
                 Type item = collection.get(i);
                 ...
              }

    What happened when you use this type of loop?
    If you don't understand my answer, don't ignore it, ask a question.

  5. #5
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    Default Re: problems with for loop

    I can't make it work @Norm =/

  6. #6
    Super Moderator Norm's Avatar
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    Default Re: problems with for loop

    Make a small, simple program that compiles, executes and shows the problem.

    Post the full text of the error messages you are getting.
    If you don't understand my answer, don't ignore it, ask a question.

  7. #7
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    Default Re: problems with for loop

    import java.util.ArrayList;
    import java.util.Scanner;
    import java.util.Iterator;
     
     
     
    public class Register{
    	public static void main(String []args){
     
    		double tailLength;
     
    		ArrayList<Dog> dogs = new ArrayList<Dog>();
     
    		for(;;){
    			System.out.println();
    			System.out.println("Make a choice:");
    			System.out.println("----------------------");
    			System.out.println();
    			System.out.println("1. Registrera new dog");
    			System.out.println("2. List the dogs");
    			System.out.println("3. Delete dog");
    			System.out.println("4. Exit");
     
    			Scanner keyboard= new Scanner(System.in);
     
    			int choice = Integer.parseInt (keyboard.nextLine());
     
    			switch (choice){
    			case 1:
    				System.out.println("Hundens namn: ");
    				String name=keyboard.nextLine();
     
    				System.out.println("Hundens vikt (kg): ");
    				int weight=Integer.parseInt(keyboard.nextLine());
     
    				System.out.println("Hundens Œlder: ");
    				int age=Integer.parseInt(keyboard.nextLine());
     
    				System.out.println("Ras: ");
    				String race=keyboard.nextLine();
     
    				tailLength=(double)age*weight/10;
    				if ("Tax".equalsIgnoreCase(race))
    					tailLength=3.7;
     
    				Dog dogx = new Dog(name,weight,age,race,tailLength); 
    				dogs.add(dogx);
    				System.out.println(dogx);
     
    				break;
     
    			case 2:
     
    				System.out.println("Ange svanslŠngd: ");
    				int minimumTailLength=Integer.parseInt(keyboard.nextLine());
    				for (int x=0; x<dogs.size(); x++){
    					tailLength = dogs.get(x).getTailLength();		
     
    					dogs.get(x);
    					if (tailLength >= minimumTailLength)
    						System.out.println(dogs.get(x));
    				}
    				break;
     
    			case 3:
    //See below!
     
     
    				break;
     
    			case 4: 
    			{
    				System.exit(0);
    			}
     
    			break;
     
    			default:
    			{
    				System.out.println("Fel val, try igen. ");
    			}
    			break;
    			}
    		}
     
     
    	}
    }

    public class Dog{
     
    	//Instansvariabler
    	private String name;
    	private int weight;
    	private int age;
    	private String race;
    	private double tailLength;
     
     
    	public Dog (String name, int weight, int age, String race, double tailLength){
    		//Variabler ges i konstruktorn
    		this.name=name;
    		this.weight=weight;
    		this.age=age;
    		this.race=race;
    		this.tailLength=tailLength;
     
    	}
     
    	public double getTailLength(){ 
    		return tailLength;
    	}
     
    	public String getName(){
    		return name;
    	}
     
    	public String toString(){
    		return ("Hunden "+name+" Šr en "+race+ " som vŠger: "+weight+ "Kg.\nDen Šr "+age+" "+"Œr gammal, "+"svanslŠngd: "+tailLength+" cm");
    	}
     
    }

                           case 3:
     
                                    System.out.println("Hundens namn: ");
                                String toDelete=keyboard.nextLine();
     
                             for (Dog deletedog : dogs)
                                    if (deletedog.getname().equals(toDelete))
                                {
                                            dogs.remove(deletedog);
                                            System.out.println();
                                            System.out.println(deletedog.getname() + " är borttagen");
                                }
                                            break;

    So thats the code entire program. 2 classes when pressing 3 you should be able to delete a dog from the register.
    when deleting I get this error message

    Errormessage:
    Exception in thread "main" java.util.ConcurrentModificationException
    at java.util.AbstractList$Itr.checkForComodification( AbstractList.java:372)
    at java.util.AbstractList$Itr.next(AbstractList.java: 343)
    at Register.main(Register.java:71)

  8. #8
    Super Moderator Norm's Avatar
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    Default Re: problems with for loop

    for (Dog deletedog : dogs)
    You are still using this syntax for the for loop. You need to use the syntax I suggested in post#4.


    The code you posted does NOT compile, execute and show the problem.
    If you don't understand my answer, don't ignore it, ask a question.

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