Finding an array within a list in Java

Question

I am trying to check if an array lies within a LinkedList or even Stack structure, as so:

LinkedList<int[]> list = new LinkedList();
int foo = new int[]{1,2,3};
int bar = new int[]{1,2,3};
list.addLast(bar);
if(list.contains(foo)) {
   System.out.print("I found foo!");
} else {
   System.out.print("I didn't find foo.");
}

this returns "I didn't find foo". Same result when I try if (Stack.search(foo) != -1) which would succeed if foo were found in the Stack.

How do I search a LinkedList or other Vector/List-type structure for a specific array?

That way you can compare primitive values not an array.

Sigismundus
– Sigismundus

2015-04-14 04:01:10 +00:00
Commented Apr 14, 2015 at 4:01 — Sigismundus
– Sigismundus, Commented Apr 14, 2015 at 4:01

Zealous System · Accepted Answer · 2015-04-14 03:59:56Z

1

Currently as per the above code. You haven't added foo in the linked list. and you haven't initialized int array properly.

        LinkedList<int[]> list = new LinkedList();
        int[] foo = new int[]{1,2,3};
        int[] bar = new int[]{1,2,3};
        list.addLast(foo);
        list.addLast(bar);
        if(list.contains(foo)) {
           System.out.print("I found foo!");
        } else {
           System.out.print("I didn't find foo.");
        }

answered Apr 14, 2015 at 3:59

Zealous System

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Comments

Community · Accepted Answer · 2017-05-23 11:57:04Z

1

Check out these questions: Comparing two integer arrays in java and equals vs Arrays.equals in Java

When dealing with arrays, == and equals() in Java both return whether the two arrays being compared are the same object, not whether their contents are the same. Arrays.equals() will compare the contents of the arrays.

The contains() method uses equals() to check if it has found the given object in the list, so that won't work here. You'll have to manually search the list like so:

boolean foundArray = false;
for (int[] array : list) {
    if (Arrays.equals(array, foo)) {
        foundArray = true;
        break;
    }
}

edited May 23, 2017 at 11:57

CommunityBot

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answered Apr 14, 2015 at 4:01

SmashMaster

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Comments

Ravi K Thapliyal · Accepted Answer · 2015-04-14 04:10:02Z

0

Since arrays inherit the default equals() implementation from Object, using contains() won't work as expected. You'll need to iterate over your collection and use the Arrays utility class to find a match like

LinkedList<int[]> list = new LinkedList();

int bar = new int[] {1,2,3};
list.addLast(bar);

boolean found = false;
int foo = new int[] {1,2,3};

for (int[] arr : list) {
   if (Arrays.equals(foo, arr)) {
     found = true;
     break; // search no more
   }
}

if (found) {
     System.out.print("I found foo!");
} else {
   System.out.print("I didn't find foo.");
}

Using arr1.equals(arr2) is similar to doing arr1 == arr2. To compare the actual array contents you should use Arrays.equals(arr1, arr2) instead.

edited Apr 14, 2015 at 4:10

answered Apr 14, 2015 at 4:04

Ravi K Thapliyal

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2 Comments

gentlefish Over a year ago

After seeing previous responses, this is exactly how I went through and implemented my code.

Ravi K Thapliyal Over a year ago

Note that someone had also posted about Arrays.deepEquals() (but removed the post later on). While it's not required here but if you ever start saving multi-dimensional arrays in your list or stack, use that to compare your nested arrays correctly as well.

camelsaucerer · Accepted Answer · 2015-04-14 04:02:39Z

0

You are searching if the array reference within your LinkedList is the same as the reference to foo. Instead, you need to compare the contents of the array using:

for (int[] array : list) {
    if (Arrays.equals(array, foo)) {
        System.out.print("I found foo!");
    }
}

edited Apr 14, 2015 at 4:02

answered Apr 14, 2015 at 3:55

camelsaucerer

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