实例化多个类可以读写的单个类的正确方法是什么？

public class Singleton {
    private static Singleton instance;
    private String data;

    private Singleton() {
        // 私有构造函数
    }

    public static Singleton getInstance() {
        if (instance == null) {
            instance = new Singleton();
        }
        return instance;
    }

    public String getData() {
        return data;
    }

    public void setData(String data) {
        this.data = data;
    }
}

Singleton singleton = Singleton.getInstance();
singleton.setData("Hello, World!");
System.out.println(singleton.getData());  // 输出：Hello, World!