CODEWITHSUNDEEP
javaarrays
user2192774
user2192774

Reputation: 3907

How to free memory in 2-D array

I have a long code. But the logic is as follows: I have a 2-D array in Java. So, I have values inside locations: [0][0], [0][1], [0][2] and values in:[1][0], [1][1], [1][2]. At this point, I made some comparisons, and I am done from the first row. I want to free the memory of: [0][0], [0][1], [0][2] and move to the next locations [2][0], [2][1], [2][2].

How can I do something like this. I can not overwrite [0][0], [0][1], [0][2],my code already programmed to move to the next row which is [2][0], [2][1], [2][2] but I faced memory constraints and I want to free memory from the first row [0][0], [0][1], [0][2] as I do not need it any more. I just need the current row and the previous to make a comparison. So, I want to delete the first row whenever I finish the comparison.

UPDATE: I tried to assign NULL to the unused array locations as the following:

for (int f = 0; f <= capacity; f++)
                      {  
                         table[f][i-2] = (Integer) null;  
                      }

My array is of type int and I only need the last two columns. The first one is not needed once I move one more position forward. When I applied the above code to assign NULL, I got:

java.lang.NullPointerException

Upvotes: 0

Views: 266

Answers (2)

Ilya
Ilya

Reputation: 29731

set null to each reference in row 

for (int i = 0; i < arr.length; i++)
{  
   arr[0][i] = null;  
}

arr should be array of references, not primal types

EDIT
for primal types (e.g. int) you can use wrapper-class (e.g. java.lang.Integer instead of int : Integer[][] instead of int[][])

EDIT2
Equivalent for previous loop is: 

arr[0] = new Integer[arr.length];

array new Integer[arr.length] contains null values

Upvotes: 6

Joni
Joni

Reputation: 111399

Java doesn't have 2D arrays. They are mimicked by arrays whose elements are arrays.

You can set the first array of the array of arrays to null; this allows the GC to collect it and everything it contains.

arr[0]=null;

Note that this will work for arrays of both of arrays of primitive types and of objects. You don't need a loop.

An alternative to using null is using an empty array:

// Assuming arr is int[][]
private static final int[] EMPTY_ROW = {};
...
arr[0] = EMPTY_ROW;

Using an empty array you may be able to avoid adding null-checks and use simpler code in general, while obtaining the same effects as with null: the reference to the first row is removed, so it becomes eligible for garbage collection.

If you are interested in design patterns, this can be considered an application of the Null Object Pattern.

Upvotes: 4

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