Versions of Java.

Keywords: class, interface, package, public, protected, private, abstract, static, final, synchronized, native, strictfp, transient, ...

Statements: if, else, switch, default, for, while, do, break, continue, return, throw, try (with resources), catch, finally, synchronized.

Primitive types: boolean, char, byte, short, int, long, float, double.

General use: Object, Class.

Numbers: Byte, Short, Integer, Long, Float, Double, BigInteger, BigDecimal.

Strings: Character, String, StringBuilder.

Collections: Collection, List, ArrayList, Set, HashSet, Map, HashMap.

Layout: BorderLayout.

Special-purpose classes: AutoCloseable.

Applets: Applet, AppletContext, AppletStub.

Recommended links:

The Java® Language Specification
documentation of the Java programming language
Java™ Platform, Standard Edition 7 API Specification
documentation of Java SE classes
JavaTM Platform, Enterprise Edition 6 API Specification
documentation of Java EE classes