Extremely overengineered Swift solution:

func lonelyinteger(a: [Int]) -> Int {
    a.reduce(0, ^)
}

function lonelyinteger(a) { // Write your code here let result = 0; for (let i = 0;i< a.length;i++){ result = result ^ a[i]; } return result; }

JAVA8 With stream (need stack to store a previus Item)

final Stack<Integer> stack=new Stack<>();
a.stream().sorted().filter((i)->{
     if (stack.isEmpty()){stack.push(i);return false;}
     else if (stack.get(0).equals(i)){stack.pop();return false;}
            else return true;
            }).findFirst();   
return stack.pop();

JAVA

    // Write your code here
    int res = 0;
    for(int i=0; i<a.size(); i++) {
        if(a.indexOf(a.get(i)) == a.lastIndexOf(a.get(i))) {
            res = a.get(i);
        }
    }
    return res;

JAVA

    // Write your code here
    int res = 0;
    for(int n : a) {
        res ^= n;
    }
    return res;