Solution

SingleNumber.java

package singleNumber;

import java.util.*;

public class SingleNumber {
	public static void main(String[] args) {
		int arr[]= {4,5,6,5,6,4,7};  
		
		HashMap<Integer, Integer> h = new HashMap<>();
	
        for (int i=0; i<arr.length;i++) {
            if (h.containsKey(arr[i])==false) { 
            	h.put(arr[i], 0);
            	}
            else { 
            	h.remove(arr[i]);
            	}
        }
        System.out.println(h.keySet().iterator().next()); 
	}
		
		

}