Advice to beginners might include:

• don't be too intimidated by the large number of classes. Some are used often, but most are used relatively rarely.
• the most widely used packages are java.lang and java.util.
• for working with data, see java.sql, javax.sql, java.io, and java.nio.file.
• for graphical applications, see the Swing classes (javax.swing, and so on).
• for server applications, see the servlet and Java Server Page APIs.
• for other packages, you should skim through their documentation just to get an idea of what is available. Later, when a specific need arises, you will often know which packages might be helpful.

As well, the standard JDK libraries:

• are generally of high quality
• often improve their performance over time
• are widely used, and usually form defacto standards

Significant changes and additions to the standard libraries occur in each major release, and it pays to keep current.

The Java 8 release includes:

• java.time
• interfaces with static helper methods, and default methods
• functional interfaces (interfaces with a single method)
• lambda expressions
• method references
• for collections: streams, pipelines and aggregate operations

The Java 7 release includes:

• type inference
• try-with-resources
• java.nio.file - new ways to interact with the file system
• multi-catch - catching multiple exceptions at once
• JavaFX
• binary literals

• scripting
• the JConsole monitoring tool
• various Swing improvements

• generics
• an enhanced for loop
• enums
• autoboxing
• and annotations

The Java 1.4 release includes:

• regular expressions
• assertions
• logging services