When use a static class instead of the singleton pattern?

Question

我已经阅读了这个问题和一些类似的问题，我想知道是否有任何情况我应该在单例模式上使用静态类？

Answer 1

Use a static "utility" class (a class with nothing but static methods) when you have "just code" methods - methods where you don't need any particular implementation of a base class or interface. The key indicator is that the code is stateless - ie there are no (static) fields in the class to give it state. Being stateless also means the methods are automatically thread safe - another benefit.

There are many examples in the JDK ( Collections being one notable example) and the apache commons libraries of this pattern working very well.

Also, to avoid "class bloat", rather than have a class for a particular trivial implementation, you can have static (abstract) factory methods that return a particular implementation, for example:

public static Comparator<String> createCaselessStringCompatator() {
    return new Comparator<String> () {
        public int compare(String o1, String o2) {
            return o1.toUpperCase().compareTo(o2.toUpperCase());
        }
    }; 
}

public static Comparator<String> createNumericStringCompatator() {
    return new Comparator<String> () {
        public int compare(String o1, String o2) {
            return new Double(o1).compareTo(new Double(o2));
        }
    }; 
}

This pattern avoids creating a whole new class file for what amounts to just a single line of actual useful code (ie a " closure ") and it bundles them up so if you know your utility class name, your IDE will prompt you for which impl you want to choose:

Collections.sort(myStringList, MyComparators.|<-- ctrl+space to offer options

Whereas without this pattern, you'd have to remember the class name of each of the implementations.

Answer 2

我认为这将是一个需要一些实用功能的地方，这又是静态的。