A loop to compare all string values of an array

Question

Let's say you have a string array arr with 3 strings in it. To compare its values, you would simply do the following:

if (arr[0].equals(arr[1]) && arr[0].equals(arr[2] && arr[1].equals(arr[2]) {
 return true;
}

But what if that array had hundreds of strings? What is the best way to compare all the values?

I thought of using for loops but Java does not allow loops inside a conditional. Any ideas?

• azurefrog: But it does allow conditionals inside a loop...
• azurefrog: Also, your third check is not needed. IF a == b AND a == c, then we already know that b == c, due to transitivity. It's enough to check whether each element is equal to the first.
• Tdorno: A better approach would be to just not put duplicate strings in your data structure. Which means you should look to use a hashtable or something similar. Ends up with O(1) efficiency.
• Jason: What version of Java are you using?
• Dragneel: @azurefrog I am trying to put a loop inside a conditional, not a conditional inside a loop. For instance, if(for loop){}

Answer 1

How about this 1-liner:

return Arrays.stream(arr).distinct().count() == 1;

This code neatly handles empty (but not null) arrays, returning false if empty.

If you want to return true when the array is empty, change the test to:

return Arrays.stream(arr).distinct().count() < 2;

• Dragneel: What about in the case of a 2-dimensional array, and let's say you intend on only comparing the values of the second row for an example.
• Blue Boy: would 'return Arrays.stream(arr[1]).distinct().count() == 1;' work? or no?
• Dragneel: Nope. I just tested this with a 1D array and works fine. But it doesn't work with a 2D array.
• Blue Boy: Did you change the 'arr' to 'arr[1]'?
• Dragneel: I have. The result was return Arrays.stream(arr[1]).distinct().count() == 1

Answer 2

A brute force method to do this is to compare the 1st string with every other string in the array.

public boolean allUnique(String[] arr){
 //Assuming the array has at least 1 element. 
 String s = arr[0];
 for(int i = 1; i < arr.length; i++){
 if(!arr[i].equals(s)){
 return false;
 } 
 }
 return true;
}

• Dragneel: This approach was one of my attempts, but I actually need the method to return true rather than false after checking all values.
• Atri: @LawrenceLelo you can return true after the loop. If all comparisons match then it will return true.

Answer 3

If the array could be of any dimension, then the Objects.deepEquals() method might be of help:

boolean allEqual = Arrays.stream(arr).allMatch(a -> Objects.deepEquals(arr[0], a));

Even better:

boolean allEqual = Arrays.stream(arr, 1, arr.length) // bounds check left
 .allMatch(a -> Objects.deepEquals(arr[0], a)); // to the reader :)

Test:

String[][] arr = {
 {"a", "a"},
 {"a", "a"},
 {"a", "a"}};
boolean allEqual = Arrays.stream(arr, 1, arr.length)
 .allMatch(a -> Objects.deepEquals(arr[0], a));
System.out.println(allEqual); // true

• Dragneel: I just tested this code with a 2D array. It compiles but it is returning false.
• fps: @LawrenceLelo What's your check? It's returning true to me for a 2d array.
• Dragneel: I was testing the code with an array of objects rather strings. Though, the objects do return a string with the toString() method. Perhaps my results are different because I'm using an array of objects instead of strings.
• fps: @LawrenceLelo If you are using objects, make sure they implement equals() consistently.

Answer 4

for(int i = 0; i < arr.length-1; i++){
 if(!arr[i].equals(arr[i+1])){
 return false;
 }
}
return true;