在Java中實現單例模式有多種方式，以下是其中幾種常用的實現方式：

1. 餓漢式（線程安全）：

public class Singleton {
    private static Singleton instance = new Singleton();

    private Singleton() {}

    public static Singleton getInstance() {
        return instance;
    }
}

2. 懶漢式（線程不安全）：

public class Singleton {
    private static Singleton instance;

    private Singleton() {}

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

3. 懶漢式（線程安全，雙重檢查鎖定）：

public class Singleton {
    private static volatile Singleton instance;

    private Singleton() {}

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

4. 靜態內部類（線程安全）：

public class Singleton {
    private Singleton() {}

    private static class SingletonHolder {
        private static final Singleton INSTANCE = new Singleton();
    }

    public static Singleton getInstance() {
        return SingletonHolder.INSTANCE;
    }
}

以上是幾種常用的單例模式實現方式，選擇哪種方式取決于具體的需求和線程安全性要求。