当我们编写JAVA应用程序时，主要是在第四层即应用层工作。一般说来，不必关心TCP/UDP层，这是因为java.net包已提供了系统无关的底层实现。然而，理解TCP和UDP对于决定选择使用哪种java类，还是十分必要的-JAVA applications, primarily in the fourth layer is the application layer work. Generally speaking, does not have to care about TCP / UDP layers. This is because the java.net package provides system unrelated to the underlying implementation. However, the understanding of TCP and UDP for the decision to choose which category of java, is very necessary