HackerRank Java- Singleton Pattern

Complete the Singleton class in your editor which contains the following components:

A private Singleton non parameterized constructor.
A public String instance variable named str.
Write a static method named getSingleInstance that returns the single instance of the Singleton class.
Once submitted, our hidden Solution class will check your code by taking a String as input and then using your Singleton class to print a line.

import java.io.*;
import java.util.*;
import java.text.*;
import java.math.*;
import java.util.regex.*;
import java.lang.reflect.*;


class Singleton {
    private Singleton() {}
    public String str;
    private static Singleton instance = null;
    public static Singleton getSingleInstance() {
        if (instance == null) {
            instance = new Singleton();
        }
        return instance;
    }
}