The Package Explorer that is shown by default in the Java perspective shows the Java element hierarchy of the Java projects in your workbench. It provides you with a Java-specific view of the resources shown in the Navigator. The element hierarchy is derived from the project's build paths.
For each project, its source folders and referenced libraries are shown in the tree. You can open and browse the contents of both internal and external JAR files. Opening a Java element inside a JAR opens the CLASS file editor, and if there is a source attachment for the JAR file, its corresponding source is shown.