Check if an arrayList contains an array

Question

I have an arrayList that contains arrays. How do I check if the arrayList contains a specified array? I used .contains method and it returns false instead of expected true.

import java.util.ArrayList;
import java.util.Arrays;

public class main {
    public static void main(String[] args) {
        ArrayList<String[]> action = new ArrayList<String[]>();
        action.add(new String[]{"appple", "ball"});
        String[] items = new String[]{"appple", "ball"};
        if (action.contains(new String[]{"appple", "ball"})) {
            System.out.println("Yes");
        }
        System.out.println(action.contains(items)); // False
    }
}

Answer 1

As you are creating different arrays (even if the contents are the same), contains will result false.

However, if you do this:

 List<String[]> action = new ArrayList<String[]>();
 String[] items = new String[]{"apple","ball"};   
 action.add(items);
 if (action.contains(items)) 
     System.out.println("Yes");

This will print Yes. Also, some examples of the behaviour:

 String[] items = new String[]{"apple","ball"};   
 action.add(items);
 String[] clone = items.clone();
 String[] mirror = items;

 action.contains(clone); // false 
 action.contains(mirror); // true

 items[0]="horse";
 System.out.println(mirror[0]);        // "horse"
 System.out.println(clone[0]);         // "apple"
 System.out.println(action.get(0)[0]); // "horse"

 mirror[1]="crazy";
 System.out.println(clone[1]);         // "ball"
 System.out.println(action.get(0)[1]); // "crazy"
 System.out.println(items[1]);         // "crazy"

 clone[1]="yolo";
 System.out.println(action.get(0)[1]); // "crazy"
 System.out.println(items[1]);         // "crazy"
 System.out.println(mirror[1]);        // "crazy"

 System.out.println(action.get(0).hashCode());    //2018699554
 System.out.println(items.hashCode());            //2018699554
 System.out.println(clone.hashCode());            //1311053135
 System.out.println(mirror.hashCode());           //2018699554

---

Custom "contains"

The issue here is that if you want to search for an specific array afterwards, you'd lose the references and searching an item wouldn't be possible, not even replicating the array with the same exact values.

As a workaround, you could implement your own contains method. Something like:

If you wish to get the index:

static int indexOfArray(List<String[]> list, String[] twin)
{        
   for (int i=0;i<list.size();i++)
      if (Arrays.equals(list.get(i),twin))
           return i;
   return -1;
}

And then, call it like:

String[] toSearch = new String[]{"apple","ball"};
int index = indexOfArray(action, toSearch); 

if (index>0) 
    System.out.println("Array found at index "+index);
else
    System.out.println("Array not found");

If the index is bigger than -1, you can get your original array by just:

String[] myArray = action.get(index);

---

HashMap + identifier

An alternative would be storing the arrays into a HashMap by declaring an identifier for each array. For example:

Base64 ID

This will give the same result for the same values, as the encoded value is based on the entries, not the Object's reference.

 static String getIdentifier(String[] array)
 {
    String all="";
    for (String s : array)
        all+=s;
    return Base64.getEncoder().encodeToString(all.getBytes());
 }

And then you could:

Map<String, String[]> arrayMap= new HashMap<>();
String[] items = new String[]{"apple","pear", "banana"}; // *[1234] 
action.add(items);
arrayMap.put(getIdentifier(items), items); // id = QUJDYWFh
//....
//Directly finding the twin will fail
String[] toSearch = new String[]{"apple","pear", "banana"}; // *[1556]
System.out.println(action.contains(toSearch)); // false

//But if we get the identifier based on the values
String arrId = getIdentifier(toSearch); // id = QUJDYWFh
System.out.println(action.contains(arrayMap.get(arrId)));  //true

//arrayMap.get(arrId)->  *[1234]
//.....

Name.

Choose a representative name and use it as Id

Map<String, String[]> arrayMap= new HashMap<>();
String[] items = new String[]{"apple","pear", "banana"};
action.add(items);
arrayMap.put("fruits", items);
//...
System.out.println(action.contains(arrayMap.get("fruits"))); // true