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Thread: Creating Arraylist of Arraylist

  1. #1
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    Default Creating Arraylist of Arraylist

    I have an algorithm problem and i have created a class for my objects

    hard problem is how can i create a matrix that has increasing size and every rows in matrix is also increasing

    for example

    matrix is :

    0 1 2 3 4 5 6 7 ....................

    1 object

    2 object object

    3 object object object object object object object

    4 object object object object

    5

    .
    .
    .
    .
    .

    good explanation for matrix : image Untitled.jpg

    every row has increasing size and also it is not clear that howmany rows ll be created

    inside object of matrix has array, int and double value. my first idea is creation of a class name is example_name for objects

    Arraylist<example_name>[] algoritmaIci = new ArrayList[2500]; //this gave a warning
    2500 is just to assign huge size because i guess matrix size wont be bigger than 2500

    or can i create arraylist of arraylist like:


    ArrayList<Arraylist<example_name>> algoritmaIci = new ArrayList<Arraylist<example_name>>();


    what is your other advise for solution?
    Last edited by ms_ceng; December 19th, 2011 at 05:05 PM.


  2. #2
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    Default Re: Creating Arraylist of Arraylist

    Yes, you can put arraylists inside of an arraylist.

    Try your code with the compiler and see what happens. Write a small 10-20 line program and experiment with it to get the technique.

  3. #3
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    Default Re: Creating Arraylist of Arraylist

    first i tried to create type of string before use my object but i couldnt do it what is my problem in my code?

    my class is:


    import java.util.ArrayList;

    public class ArrayListDeneme {
    public static void main(String args[]) {
    ArrayList<Arraylist<String>> matrix = new ArrayList<Arraylist<String>>();

    Arraylist<String> al = new Arraylist<String>();
    al.add("row 0 col 0");

    al.add("row 2");
    al.add("zhagre");
    al.add("row4");
    al.add("row 3");
    matrix.add(al);
    Arraylist<String> al1 = new Arraylist<String>();
    al1.add("zhagre");
    al1.add("row4");
    al1.add("row 3");
    matrix.add(al1);
    int i = 0;
    int j = 0;
    System.out.print(matrix.size() + " ");

    // display contents of matrix
    for (i = 0; i < matrix.size(); i++) {
    Arraylist<String> gecici = new Arraylist<String>();
    gecici = matrix.get(i);
    for (j = 0; j < gecici.size(); j++) {
    System.out.print(gecici.get(i) + " ");
    System.out.println();
    }
    System.out.println();
    }
    }
    }


    after i wrote this class eclipse advised to add class for inner arraylist and advised to create add() size() and get() method in this inner arraylist class i carried on implement this advise and then my code didnt run

    my inner arraylist class is :


    package yuzswing.denem;

    public class Arraylist<T> {
    Arraylist<String> ekle;
    public Arraylist() {


    }
    public void add(String string) {
    this.ekle.add(string);

    }
    public int size() {
    return this.ekle.size();

    }
    public String get(int i) {
    return this.ekle.get(i);

    }


    }

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    Default Re: Creating Arraylist of Arraylist

    what is my problem in my code?
    Please explain what problems you are having.
    Does it compile? If not, post the errors.
    Does it execute without errors? If not post the errors.
    Does it give the wrong results? Show the results and explain what is wrong with them.

  5. #5
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    Default Re: Creating Arraylist of Arraylist

    Exception in thread "main" java.lang.NullPointerException
    at yuzswing.denem.Arraylist.add(Arraylist.java:10)
    at yuzswing.denem.ArrayListDeneme.main(ArrayListDenem e.java:10)


    when i click Arraylist.java:10 ==> this.ekle.add(string);

    ArrayListDeneme.java:10 ==> al.add("row 0 col 0");


    i just clicked run (Ctrl+F11)

  6. #6
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    Default Re: Creating Arraylist of Arraylist

    Exception in thread "main" java.lang.NullPointerException
    at yuzswing.denem.Arraylist.add(Arraylist.java:10)
    at yuzswing.denem.ArrayListDeneme.main(ArrayListDenem e.java:10)
    There is a null variable at line 10 in your program. Look at the code on that line and see what variable is null and then see why it does not have a valid value.
    If you can not tell which variable is null, add a println call just before line 10 and print out the values of all the variables on line 10 so you can see which one is null.

  7. #7
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    Default Re: Creating Arraylist of Arraylist

    it is ok so sorry problem is that i wrote Arraylist not ArrayList

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    Default Re: Creating Arraylist of Arraylist

    thanks for greet answer

  9. #9
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    Default Re: Creating Arraylist of Arraylist

    Does the program work now?

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    Default Re: Creating Arraylist of Arraylist

    yes it is working after i changed Arraylist to ArrayList and deleted my Arraylist class

    Now i finished my artificial intelligence lecture homework project but i m getting this exceptions.


    Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException
    at matrix.calisiyor.AstarPanel.Diziyiolustur(AstarPan el.java:357)
    at matrix.calisiyor.AstarPanel.coz(AstarPanel.java:28 4)
    at matrix.calisiyor.AstarPanel$5.actionPerformed(Asta rPanel.java:148)
    at javax.swing.AbstractButton.fireActionPerformed(Unk nown Source)
    at javax.swing.AbstractButton$Handler.actionPerformed (Unknown Source)
    at javax.swing.DefaultButtonModel.fireActionPerformed (Unknown Source)
    at javax.swing.DefaultButtonModel.setPressed(Unknown Source)
    at javax.swing.plaf.basic.BasicButtonListener.mouseRe leased(Unknown Source)
    at java.awt.AWTEventMulticaster.mouseReleased(Unknown Source)
    at java.awt.Component.processMouseEvent(Unknown Source)
    at javax.swing.JComponent.processMouseEvent(Unknown Source)
    at java.awt.Component.processEvent(Unknown Source)
    at java.awt.Container.processEvent(Unknown Source)
    at java.awt.Component.dispatchEventImpl(Unknown Source)
    at java.awt.Container.dispatchEventImpl(Unknown Source)
    at java.awt.Component.dispatchEvent(Unknown Source)
    at java.awt.LightweightDispatcher.retargetMouseEvent( Unknown Source)
    at java.awt.LightweightDispatcher.processMouseEvent(U nknown Source)
    at java.awt.LightweightDispatcher.dispatchEvent(Unkno wn Source)
    at java.awt.Container.dispatchEventImpl(Unknown Source)
    at java.awt.Window.dispatchEventImpl(Unknown Source)
    at java.awt.Component.dispatchEvent(Unknown Source)
    at java.awt.EventQueue.dispatchEventImpl(Unknown Source)
    at java.awt.EventQueue.access$000(Unknown Source)
    at java.awt.EventQueue$1.run(Unknown Source)
    at java.awt.EventQueue$1.run(Unknown Source)
    at java.security.AccessController.doPrivileged(Native Method)
    at java.security.AccessControlContext$1.doIntersectio nPrivilege(Unknown Source)
    at java.security.AccessControlContext$1.doIntersectio nPrivilege(Unknown Source)
    at java.awt.EventQueue$2.run(Unknown Source)
    at java.awt.EventQueue$2.run(Unknown Source)
    at java.security.AccessController.doPrivileged(Native Method)
    at java.security.AccessControlContext$1.doIntersectio nPrivilege(Unknown Source)
    at java.awt.EventQueue.dispatchEvent(Unknown Source)
    at java.awt.EventDispatchThread.pumpOneEventForFilter s(Unknown Source)
    at java.awt.EventDispatchThread.pumpEventsForFilter(U nknown Source)
    at java.awt.EventDispatchThread.pumpEventsForHierarch y(Unknown Source)
    at java.awt.EventDispatchThread.pumpEvents(Unknown Source)
    at java.awt.EventDispatchThread.pumpEvents(Unknown Source)
    at java.awt.EventDispatchThread.run(Unknown Source)


    what is problem in here why i m getting null position error.

  11. #11
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    Default Re: Creating Arraylist of Arraylist

    sorry for huge exceptions

  12. #12
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    Default Re: Creating Arraylist of Arraylist

    Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException
    at matrix.calisiyor.AstarPanel.Diziyiolustur(AstarPan el.java:357)
    at matrix.calisiyor.AstarPanel.coz(AstarPanel.java:28 4)
    There is a variable on line 357 that has a null value. See post#6

  13. #13
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    Default Re: Creating Arraylist of Arraylist

    356 gidilen=0;
    357 ornek.set_g(gidilen);


    these are snippet of my code yes it has zero value not null i need to set zero value for my zero step in algorithm. why does java suppose that it is null

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    Default Re: Creating Arraylist of Arraylist

    why does java suppose that it is null
    The computer KNOWS there is a variable with a null value. If you can not see what variable is null, you need to add a println statement that prints out the values of all the variables on the line where the exception occurs.
    357 ornek.set_g(gidilen);
    What is the value of ornek?

  15. #15
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    Default Re: Creating Arraylist of Arraylist

    ornek is an instance of a class, ornek is my object that i defined to use and
    that line one of its values have been set


    i think i found error point there is a part of code below i need to get i and j value of buttons matrix. But i could not access i and j value insede the actionperformed. This create my null value



    private void BlockButtonSec() {
    secim = 2;
    for (final int i = 0; i < 50; i++) {
    for (int j = 0; j < 50; j++) {
    butonlar[i][j].addActionListener(new ActionListener() {
    public void actionPerformed(ActionEvent event) {
    System.out.print(i); // icant reach i
    System.out.print(j);
    // while (secim == 2) {
    //
    // if (event.getSource() == butonlar[i][j]) {
    // butonlar[i][j].setBackground(Color.GRAY);
    //
    // koordinatlarengel[index].setCoordinate_x(i);
    // koordinatlarengel[index].setCoordinate_y(j);
    // index++;
    // }
    // }

    }
    });
    }
    }
    }



    System.out.print(i); i cant reach "i" and eclipse advised that make "i value" is a final but after i changed it as a final "for statement" loop create error that final value can not be increased and eclipse advised to delete final it is just like infinite advise cycle

    thank you very much for your attention and concern

  16. #16
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    Default Re: Creating Arraylist of Arraylist

    What variable is/was null?

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