How to initialise an array without knowing the size it should be

Question

I would like to write a method which returns an array containing only the numbers from input that are greater than or equal to n.

So far I have the following:

public static int[] filter(int[] input, int n) {
 int counter = 0;
 int[] res = new int[input.length];
 for(int i = 0; i < input.length; i++) {
 if(input[i] >= n) {
 res[counter++] = input[i];
 }
 } 
 return res;
}

I am aware that int[] res = new int[input.length] is incorrect because it initialises the array with the size of the input array, but I cannot get it to work otherwise.

As a result, for int[] input = {4,3,2,5,1,6} and int n = 3 I get 4 3 5 6 0 0.

I presume it has something to do with the counter and I tried int[] res = new int[counter] but since counter = 0 it has length 0.

My question: how can I initialise the array res when I don't know how large it will be?

• azro: use a List when you need not-fixed size container :) look about ArrayList. Without that you need at the end to create a new array (3rd one) with the size you need, then copy the too-long array into the 3rd one

Answer 1

1. Shorten: use java 8 Stream API

public static int[] filter(int[] input, int n) {
 return IntStream.of(input).filter(i -> i >= n).toArray();
}

2. Easiest: use a non-fixed size wrapper, like a List

public static Integer[] filter(int[] input, int n) {
 List<Integer> res = new ArrayList<>();
 for (int val : input) {
 if (val >= n) {
 res.add(val);
 }
 }
 return res.toArray(new Integer[0]);
}

3. Arrays only: copy the needed value into an array with the appropriate size

public static int[] filter(int[] input, int n) {
 int counter = 0;
 int[] res = new int[input.length];
 for (int val : input) {
 if (val >= n) {
 res[counter++] = val;
 }
 }
 int[] result = new int[counter];
 System.arraycopy(res, 0, result, 0, counter);
 return result;
}

The System.arraycopy line can be replaced by

for (int i = 0; i < counter; i++) {
 result[i] = res[i];
}

• h4ckerlinguist: How would you do this without List? We are not allowed to use such classes in my programming course and so we have to get around things like that with loops.
• azro: @h4ckerlinguist Read my answer fully, the last proposition use arrays only
• h4ckerlinguist: Is there a way to not use System.arraycopy?
• azro: @h4ckerlinguist yes, it can be replaced by another loop, look my edit

Answer 2

Don’t create the array; let the JDK do it for you:

public static int[] filter(int[] input, int n) {
 return Arrays.stream(input)
 .filter(i -> i >= n)
 .toArray();
}

It’s a lot less code too.

• azro: That isn't a valid filter ;)

Answer 3

Try ArrarList as follows:

import java.util.ArrayList; //
ArrayList<String> res = new ArrayList<String>();