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I am learning Java through self-reading. Right now I am doing a exercise. I am trying to create variable size of 2D-array then assign random number from 10 to 100 and put it into each array.

the problem I am encountering is don't know how to get each 2D-array out and put it into a string then show it through dialog after finish creating variable object.

here is my code.

import java.security.SecureRandom;
import javax.swing.JOptionPane;
import java.util.ArrayList;

public class Random {
    public int randomNum;
    public String ID;

    public Random(String ID, int initialValue) {
        SecureRandom randomNumbers = new SecureRandom();
        this.ID = ID;
        this.randomNum = initialValue;
        int randomValue = randomNumbers.nextInt(99) + 1;
        randomNum = randomValue;
    }

    public int getRandomNum() {
        return randomNum;
    }

    public String getID() {
        return ID;
    }
}

class RandomText {
    public static void main(String[] args) {
        int ans = Integer.parseInt(JOptionPane.showInputDialog("How many random number you want to show?"));

        ArrayList < Random > randomNum = new ArrayList < Random > ();
        for (int i = 0; i < ans; i++) {
            randomNum.add(new Random("ID " + Integer.toString(i), 0));
        }
        String result;
        for (int i = 0; i < ans; i++) {
            result = result + ?????? +"\n";
        }
        JOptionPane.showMessageDialog(null, result ")
    }
}
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9
  • 2
    "create variable size of 2D-array" & "how to get each 2D-array" - There are no arrays in your code. Commented Jan 23, 2018 at 15:31
  • 1
    You do not have any 2 dimensional arrays? Commented Jan 23, 2018 at 15:31
  • 1
    "Solve the ans" is an expression I am not familiar with. Are you asking how to print a List? Or how to loop over the contents of a List? Commented Jan 23, 2018 at 15:34
  • 1
    Well, you didn't create a 2D-ArrayList. You have a basic 1 dimensional arraylist of your custom "Random" class. A 2D-ArrayList would be one where every element of the outer ArrayList would be an ArrayList itself: ArrayList<ArrayList<?>> Commented Jan 23, 2018 at 15:39
  • 1
    Something similar to this requirement is How to create a Multidimensional ArrayList in Java? which contains examples. Commented Jan 23, 2018 at 15:42

2 Answers 2

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2

You're missing a couple of things to make this work.

  1. Add a toString() method inside your Random class. The toString() method in an object is used to change an object to a string representation of the local variables (In "Random" case, you want to return a string with the ID and the randomNum, see below code).

  2. String result; needs to be assigned an initial value in order to use '+='. Change it to String result = "";

  3. Now that we have a "toString()" method, you can append it to result with result = result + randomNum.get(i).toString();

    import java.security.SecureRandom;
import javax.swing.JOptionPane;
import java.util.ArrayList;

public class Random {
    public int randomNum;
    public String ID;

    public Random(String ID,int initialValue){
        SecureRandom randomNumbers = new SecureRandom();
        this.ID = ID;
        this.randomNum = initialValue;
        int randomValue = randomNumbers.nextInt(99)+1;
        randomNum = randomValue;
    }

    public int getRandomNum(){
        return randomNum;
    }

    public String getID(){
        return ID;
    }

    public String toString(){
        return ID + ": " + randomNum;
    }
}

class RandomText{
    public static void main(String[] args) {
        int ans = Integer.parseInt(JOptionPane.showInputDialog
                ("How many random number you want to show?"));

        ArrayList<Random> randomNum = new ArrayList<Random>();
        for (int i = 0; i < ans; i++) {
            randomNum.add(new Random("ID " + Integer.toString(i),0));
        }
        String result = "";
        for (int i = 0; i < ans; i++) {
            result = result + randomNum.get(i).toString() + "\n";
        }
        JOptionPane.showMessageDialog(null, result);

    }
}
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1 Comment

Thanks for your time to solve this question first! I will try best to understand every code you wrote here!
1

I don't see why you need a 2D array when you can just use the Random#getID method you created:

String result;
for (Random random : randomNum) {
    result += random.getID() " : " + random.getRandomNum() + "\n";
}

But here is a method that uses a Map to create a 2D array list:

    Map<String, List<Integer>> idNums = new HashMap<String, List<Integer>>();
    randomNum.stream().forEach(r -> {
        if (idNums.get(r.getID()) != null) {
            idNums.get(r.getID()).add(r.getRandomNum());
        } else {
            idNums.put(r.getID(), Arrays.asList(r.getRandomNum()));
        }
    });

    ArrayList<List<Integer>> ids = new ArrayList<List<Integer>>();
    idNums.entrySet().forEach(e -> ids.add(e.getValue()));
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3 Comments

Thanks for your help to write down how to make 2D-arraylist!
@Let'sYo answer updated.
Thanks for your help again!

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