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}?

Clement P · Accepted Answer · 2011-01-31 09:02:22Z

11

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.

answered Jan 31, 2011 at 9:02

Clement P

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2 Comments

Mark Peters Over a year ago

That won't solve the real problem unfortunately. What if w had 1,2,3?

Clement P Over a year ago

I thought the user wanted to compare exact arrays, he didn't mention a slice of the array.

Sean Patrick Floyd · Accepted Answer · 2017-06-08 15:57:05Z

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
}

Mark Peters · Accepted Answer · 2011-01-31 09:10:31Z

0

You need to iterate through the list and manually check whether an array matches your criteria.

public static boolean containsSubArray(List<int[]> j, int[] sub) {
   for ( int[] arr : j ) {
      if (arr contains elements of sub) {
         return true;
      }
   }
   return false;
}

If you want an exact match, you can make use of Arrays.equals(). I don't think there's a library function to do a contains all on an array though, so you would have to write that yourself if that's what you wanted.

edited Jan 31, 2011 at 9:10

answered Jan 31, 2011 at 9:00

Mark Peters

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Comments

Tushar Tarkas · Accepted Answer · 2011-01-31 09:11:12Z

0

"contains" contract checks for equality. So in your case what is failing is equality of int[]. Since Array does not override the equals method from Object you will need a workaround to check for containment.

If you need to check for containment within the Array then you are left with no choice but to iterate through the ArrayList and do the comparison yourself.

answered Jan 31, 2011 at 9:11

Tushar Tarkas

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Comments

Sean Patrick Floyd · Accepted Answer · 2011-01-31 09:13:24Z

0

from java api:

public boolean contains(Object o)

Returns true if this list contains the specified element. More formally, returns true if and only if this list contains at least one element e such that (o==null ? e==null : o.equals(e)).

since int[] is a primitive, im pretty sure no .equals method exists so its my guess it would always return false.

I would recommend a different way of storing the data? maybe with a key of some sort?

edited Jan 31, 2011 at 9:13

Sean Patrick Floyd

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answered Jan 31, 2011 at 9:04

eosgood

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3 Comments

Mark Peters Over a year ago

int[] isn't actually a primitive. All arrays are an Object and inherit its implementation of equals() which returns true if the operands are the same instance (==)

Sean Patrick Floyd Over a year ago

int[] is not primitive. int is. int[] extends Object, but doesn't override equals()

eosgood Over a year ago

ah my b. i was trying to get at the fact that it doesn't override the equals() method and thusly cannot be used for contains.

Andreas Dolk · Accepted Answer · 2011-01-31 09:13:42Z

0

Two java array arrays are equal iff they have the same object reference. Content doesn't matter.

You're looking for a way to check if they have an equal content. This could help:

 Arrays.equals(new int[]{1,2}, new int[]{1,2});   // evaluates to true
 Arrays.equals(new int[]{1,2}, new int[]{2,1});   // evaluates to false (!)

If order shouldn't affect equality, then you will have to implement a static equals method by yourself.

answered Jan 31, 2011 at 9:13

Andreas Dolk

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Akintayo Olusegun · Accepted Answer · 2011-01-31 10:36:51Z

0

First they are not the same Object reference, so they are not equal. equals() will return false. For your condition, you will need to implement a method to compare them yourself.

answered Jan 31, 2011 at 10:36

Akintayo Olusegun

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Adarsh Singh · Accepted Answer · 2022-02-23 14:15:19Z

0

If you have one array and want to compare that all elements of array are present in list:

Long[] array1 = {1111L, 1112L};
Long[] array2 = {1111L, 1114L};
List<Long> list = new ArrayList<>();
list.add(1111L);
list.add(1112L);
list.add(1113L);

Arrays.asList(array1).stream().allMatch(val -> list.contains(val)); //return true
Arrays.asList(array2).stream().allMatch(val -> list.contains(val)); //return false

answered Feb 23, 2022 at 14:15

Adarsh Singh

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