java multidimensional arraylist

Question

I just wrote this function to add values into Multidimensional arraylist, but dont get it why all added values overwrites into recent added value. I need to figure out why this happening. As javadoc says it has to act like this " Appends the specified element to the end of this list". And i dont see any place at his code, where previously added value could be overwriten . One more strange thing is that size of array increases

Here is my output:

[0.4, 0.42, 0.4, 1.2200000000000002, 0.0010000000000000009] after add 0 [0.4, 0.42, 0.4, 1.2200000000000002, 0.0010000000000000009] after add 1 [0.4, 0.42, 0.4, 1.2200000000000002, 0.0010000000000000009] after add 2 [0.4, 0.42, 0.4, 1.2200000000000002, 0.0010000000000000009] after add 3 [0.4, 0.42, 0.4, 1.2200000000000002, 0.0010000000000000009] after add 4 [0.4, 0.42, 0.42, 1.24, 0.01200000000000001] after add 0 [0.4, 0.42, 0.42, 1.24, 0.01200000000000001] after add 1 [0.4, 0.42, 0.42, 1.24, 0.01200000000000001] after add 2 [0.4, 0.42, 0.42, 1.24, 0.01200000000000001] after add 3 [0.4, 0.42, 0.42, 1.24, 0.01200000000000001] after add 4 [0.4, 0.42, 0.42, 1.24, 0.01200000000000001] after add 5 [0.4, 0.44, 0.4, 1.2400000000000002, 0.011999999999999983] after add 0 [0.4, 0.44, 0.4, 1.2400000000000002, 0.011999999999999983] after add 1 [0.4, 0.44, 0.4, 1.2400000000000002, 0.011999999999999983] after add 2 [0.4, 0.44, 0.4, 1.2400000000000002, 0.011999999999999983] after add 3 [0.4, 0.44, 0.4, 1.2400000000000002, 0.011999999999999983] after add 4 [0.4, 0.44, 0.4, 1.2400000000000002, 0.011999999999999983] after add 5 [0.4, 0.44, 0.4, 1.2400000000000002, 0.011999999999999983] after add 6

And here is my function:

public void createArrayList(ArrayList<Double> list,ArrayList<Double>
listLaid,int size,double sum, double absValue){   
ArrayList<Double> workArray = new ArrayList<>();  
ArrayList<ArrayList<Double>>
workMatrix= new ArrayList<>();

double abs=0;
for(int a = 0; a < size ;a++){
     for(int b = 0; b < size ; b++){
           for (int c = 0 ; c < size; c++){

             abs =Math.abs(sum - list.get(a) - list.get(b) - list.get(c));
                 if( abs <= absValue){
                       double totalSum = listLaid.get(a) + listLaid.get(b) +                                            
                                         listLaid.get(c);
                       workArray.clear();
                       workArray.add(listLaid.get(a));
                       workArray.add(listLaid.get(b));
                       workArray.add(listLaid.get(c));
                       workArray.add(totalSum);
                       workArray.add(abs);

                       workMatrix.add(workArray);

                  }
              }
         } 
    }                   
} 
for (int i = 0; i < workMatrix.size(); i++)   
System.out.println(darbineMat.get(i)+"  after add  "+i);

Marco13 · Accepted Answer · 2014-05-21 16:29:30Z

2

You are always adding the same instance of a list to the list. That means that all rows of the final workMatrix are identical.

You have to create a new ArrayList for each new row. Replacing

workArray.clear();

with

workArray = new ArrayList<Double>();

should be sufficient here.

EDIT: The concept of "references" may be confusing at the first glance (and there are programming languages where this concept does not exist in the same way). But when you are creating an instance of a list like this

ArrayList<Double> workArray = new ArrayList<Double>();

then workArray is only a reference to the instance that you are creating. You could imagine this like a "handle" (and in fact, it's more like a "pointer" in C/C++, but this might become too confusing now). The key point is that when you write

ArrayList<ArrayList<Double>> workMatrix = new ArrayList<>();
ArrayList<Double> workArray = new ArrayList<>();  
workMatrix.add(workArray);

then you only add the reference to the workMatrix list. This reference still refers to the one and only instance that you created:

// This prints "true": The first element of the matrix
// and the workArray are IDENTICAL. (Not only equal, 
// but really IDENTICAL)
System.out.println(workMatrix.get(0) == workArray);

edited May 21, 2014 at 16:29

answered May 21, 2014 at 16:11

Marco13

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2 Comments

user3661552 Over a year ago

Thank you very much! It worked! But still I dont get it why. It seems that I am clearing the old arraylist and adding new values from 0 index. I had tried removeAll() didnt work either

Marco13 Over a year ago

@user3661552 Added an EDIT. But these are rather basic concepts, and you can find many resources explaining this in more detail (and with some nice images) on the web.

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