Checking if ArrayList contains anything but one element

Question

What is the most elegant way to check if an arraylist contains one (and just one) value?

For example, how can I check if my arraylist contains just zeroes:

        [0,0,0,0]; [0,0,0,0,0]; [0,0,0,0,0,0,0,0]; <-- All should return true.

I know there are lots of ways to do this, including a very basic for-loop but is there an elegant, possibly one-line solution that uses the standard API?

Answer 1

You could add the elements to a set, and check the size with something like this -

public static void main(String[] args) {
  if (new java.util.HashSet<Integer>(
      Arrays.asList(new Integer[] { 0, 0, 0, 0 }))
      .size() == 1
      && new java.util.HashSet<Integer>(
          Arrays.asList(new Integer[] { 0, 0, 0, 0,
              0 })).size() == 1
      && new java.util.HashSet<Integer>(
          Arrays.asList(new Integer[] { 0, 0, 0, 0,
              0, 0, 0, 0 })).size() == 1) {
    System.out.println("All true");
  } else {
    System.out.println("Not true");
  }
}

Which will output All true , unless you add a non zero value to any of the Integer[] s.

Or, as a generic method

public static <T> boolean oneValue(java.util.List<T> in) {
   // A one-liner.
   return (in == null) ? false : new java.util.HashSet<T>(in).size() == 1;
}

It is true that these are less efficient then simply looping through the List , like this -

public static <T> boolean oneValue(java.util.List<T> in) {
  if (in == null)
    return false;
  T o = in.get(0);
  for (int i = 1; i < in.size(); i++)
    if (!in.get(i).equals(o))
      return false;
  return true;
}

Answer 2

I would recommend the following:

public boolean isAllSame(ArrayList<?> list){
    Object first=list.get(0);
    for(int i=1;i<list.size();i++){
        if(!list.get(i).equals(first))return false;
    }
    return true;
}

(Yes, it's a for loop. But, as was previously stated, it's not all that inelegant - runs in O(n) worst case and O(1) best. And only 4 lines.)

Answer 3

Put all array elements in a single arraylist and then put it in hashset and check size. If size is 1 then it will return/print true.

    Set<Integer> val= new HashSet<Integer>();
    List<Integer> allVal= new ArrayList<Integer>();

    allVal.addAll(Arrays.asList(new Integer[] { 0, 0, 0, 0 }));
    allVal.addAll(Arrays.asList(new Integer[] { 0, 0, 0, 0,0 }));
    allVal.addAll(Arrays.asList(new Integer[] { 0, 0, 0, 0,0, 0, 0, 0 }));

    val.addAll(allVal);
    if(val.size()==1)
        System.out.println("All true");
    else
        System.out.println("More than 1 element present");

Answer 4

You can try this:

    List<Integer> lst = Arrays.asList(new Integer[] { 0, 0, 0, 0 });

    int temp = Integer.parseInt(lst.toString().replaceAll(", ", "").replaceAll("\\[", "").replaceAll("]", "").trim());
    if(temp == 0) {
        System.out.println("All zeros");
    }