Dynamic array declaration in java

Question

can anyone tell me the error in this java declaration String[][] t=new String[15][15]; this works fine and if i use String[][] t=new String[][]; because i need to declare the variable t as dynamic as i am not sure how much values i am going to store in t.

Answer 1

Use ArrayList (or other array object who can handle any number of objects). A java array always has a fixed length since some memory will be reserved for the array.

ArrayList creates such array as well to store the objects. Of you add more abjects as the current reserved size of the array than ArrayList will create a new bigger array (+50% if i'm correct). There are some other implementations that act a bit different (for example they create a new array 100% of the original one when the array is full. If performance is really important for you than you can look into this.

ArrayList<ArrayList<String> t = new ArrayList<ArrayList<String>();

void fill() {
    ArrayList<String> t2 = new ArrayList<String>();
    t2.add("somestring");
    String s = "someotherstring";
    t2.add(s);
    t.add(t2);
}

Answer 2

If you don't know how big it needs to be just declare it as

String[][] t;

and once you know how big it needs to be you can do (before trying to use the array)

t = new String[15][15];

If you're never sure how big the array need to be, you'll need to use something like a List of Lists.

List<List<String>> t = new ArrayList<List<String>>;

public void add(String str, int row, int col) {
    while (row >= t.size())
        t.add(new ArrayList<String>());

    List<String> row_list = t.get(row);
    while (col >= row_list.size())
        row_list.add("");

    row_list.set(col, str);
}