java - 如何编写静态类持有者习惯用法单例

Question

我必须以延迟初始化“静态类持有者习语单例”模式编写此代码

 public class DateComparator{
    private static DateComparator instance = null;
    public static  DateComparator getInstance()
    {
       if(instance == null)
          instance = new DateComparator();
       return instance;
    }
 }

这是我更改的代码:

public class DateComparator{
    private static DateComparator container = null;
    private static class DateComparatorHolder {
        private static DateComparator instance=createInstance();
    }
    private static DateComparator createInstance() {
        if(container == null)
            //container = new DateComparator();
        return new DateComparator();
    }
    public static DateComparator getInstance()
    {       
        return DateComparatorHolder.instance;
    }

Answer 1

不需要container变量或createInstance():

public class DateComparator{
    private static class DateComparatorHolder {
        private static final DateComparator instance= new DateComparator();
    }

    public static DateComparator getInstance()
    {       
        return DateComparatorHolder.instance;
    }
}

仅此而已。