Java: We recommend that you select more containers than Jakarta common collection-JDK.

Recently, a tree structure needs to be efficiently organized in the memory, so I took a closer look at collection. sun has few Collection types and few functions. for example, the iterator of treemap is organized by binary trees, but only the itorator for backward search can be returned, and the itorator is private and difficult to use. many variables/methods in JDK are private, and it is difficult to inherit and extend some functions.

Fortunately, the common package of the Apache Jakarta project provides a variety of collection items, some of which are very interesting, such as lazymap. We suggest you browse them and use them later:
Http://jakarta.apache.org/commons/collections/apidocs/index.html