twoSum.java

class TwoSum {

    HashMap<Integer, Integer> map = new HashMap<>();

    /** Initialize your data structure here. */
    public TwoSum() {

    }

    /** Add the number to an internal data structure.. */
    public void add(int number) {
        if(!map.containsKey(number))
            map.put(number,1);
        else
            map.put(number, map.get(number)+1);       
    }

    /** Find if there exists any pair of numbers which sum is equal to the value. */
    public boolean find(int value) {
        for(int key: map.keySet())
        {
            int num1 = key;
            int num2 = value - num1;

            if( (num1 == num2 && map.get(num1)>1) || (num1 != num2 && map.containsKey(num2)))
                return true;
        }
        return false;
    }
}

/**
 * Your TwoSum object will be instantiated and called as such:
 * TwoSum obj = new TwoSum();
 * obj.add(number);
 * boolean param_2 = obj.find(value);
 */