Using contains on an ArrayList with integer arrays

Question

I have an ArrayList<int[]>, and I add an array to it.

ArrayList<int[]> j = new ArrayList<int[]>();
int[] w = {1,2};
j.add(w);

Suppose I want to know if j contains an array that has {1,2} in it without using w, since I will be calling it from another class. So, I create a new array with {1,2} in it...

int[] t = {1,2};
return j.contains(t);

...but this would return false even though w was added to the list, and w contains the exact same array as t.

Is there a way to use contains such that I can just check to see if one of the elements of the ArrayList has the array value {1,2}?

Answer 1

Arrays can only be compared with Arrays.equals().

You probably want an ArrayList of ArrayLists.

ArrayList<ArrayList<Integer>> j = new ArrayList<ArrayList<Integer>>();
ArrayList<Integer> w = new ArrayList<Integer>();
w.add(1); w.add(2);
j.add(w);
ArrayList<Integer> t = new ArrayList<Integer>();
t.add(1); t.add(2);
return j.contains(t); // should return true.

Answer 2

The problem here is that arrays don't override Object.equals(Object), So the comparison between two list entries happens with the default equals() implementation

// from Object.class
public boolean equals(Object obj) {
return (this == obj);
}

So you have to iterate over the list and check all entries using Arrays.equals(int[], int[]). Here's a Helper method that does this:

public static boolean isInList(
    final List<int[]> list, final int[] candidate){

    for(final int[] item : list){
        if(Arrays.equals(item, candidate)){
            return true;
        }
    }
    return false;
}

---

Update: Ever since Java 8, this has got a lot simpler:

public static boolean isInList(
        final List<int[]> list, final int[] candidate) {

    return list.stream().anyMatch(a -> Arrays.equals(a, candidate));
            //  ^-- or you may want to use .parallelStream() here instead
}