public class Singleton {
    // 私有構(gòu)造方法
    private Singleton() {};
    // 創(chuàng)建私有靜態(tài)實例
    private static Singleton instance = new Singleton();
    public static Singleton getInstance() {
        return instance;
    }
}

public class Singleton {
    // 私有構(gòu)造方法
    private Singleton() {}
    // valitale 修飾
    private static volatile Singleton instance = null;
    public static Singleton getInstance() {
    	// 雙重檢查是否為 null
        if (instance == null) {
            // 加鎖
            synchronized (Singleton.class) {
                if (instance == null) {
                    instance = new Singleton();
                }
            }
        }
        return instance;
    }
}

public class Singleton {
    private Singleton() {}
    // 利用嵌套類可以訪問外部類的屬性和方法
    private static class Holder {
        private static Singleton instance = new Singleton();
    }
    public static Singleton getInstance() {
        return Holder.instance;
    }
}

public enum Singleton {
     INSTANCE;
}

public abstract class Enum<E extends Enum<E>>
        implements Comparable<E>, Serializable {
    // 枚舉項的名字 , 程序員用 toString 方法而不是這個
    private final String name;
    public final String name() {
        return name;
    }
	// 程序員用不到的，給使用枚舉類型的 API 使用，比如  java.util.EnumSet
    private final int ordinal;
    public final int ordinal() {
        return ordinal;
    }
	// 程序員用不到的，給編譯器發(fā)出的代碼，告訴編譯器實例化這個枚舉類
    protected Enum(String name, int ordinal) {
        this.name = name;
        this.ordinal = ordinal;
    }
    public String toString() {
        return name;
    }
    public final boolean equals(Object other) {
        return this==other;
    }
    public final int hashCode() {
        return super.hashCode();
    }
	// 克隆枚舉類拋出異常，這保證了枚舉永遠不會被克隆，這對于保持它們的 "單例 "狀態(tài)是必要的。
    protected final Object clone() throws CloneNotSupportedException {
        throw new CloneNotSupportedException();
    }
    public final int compareTo(E o) {
        Enum<?> other = (Enum<?>)o;
        Enum<E> self = this;
        if (self.getClass() != other.getClass() && // optimization
            self.getDeclaringClass() != other.getDeclaringClass())
            throw new ClassCastException();
        return self.ordinal - other.ordinal;
    }
    @SuppressWarnings("unchecked")
    public final Class<E> getDeclaringClass() {
        Class<?> clazz = getClass();
        Class<?> zuper = clazz.getSuperclass();
        return (zuper == Enum.class) ? (Class<E>)clazz : (Class<E>)zuper;
    }
	// 返回這個枚舉常量的枚舉類型所對應(yīng)的Class對象
    public static <T extends Enum<T>> T valueOf(Class<T> enumType,
                                                String name) {
        T result = enumType.enumConstantDirectory().get(name);
        if (result != null)
            return result;
        if (name == null)
            throw new NullPointerException("Name is null");
        throw new IllegalArgumentException(
            "No enum constant " + enumType.getCanonicalName() + "." + name);
    }
    protected final void finalize() { }
    private void readObject(ObjectInputStream in) throws IOException,
        ClassNotFoundException {
        throw new InvalidObjectException("can't deserialize enum");
    }
    private void readObjectNoData() throws ObjectStreamException {
        throw new InvalidObjectException("can't deserialize enum");
    }
}