JAVA中的让类成为singleton是什么意思？
A singleton is simply a class that is instantiated exactly once. Singletons typically represent a system component that is intrinsically unique, such as the window manager or file system. Making a class a singleton can make it difficult to test its clients, as it’s impossible to substitute a mock implementation for a singleton unless it implements an interface that serves as its type.表示不理解啊。。
