At this point, we have seen all of the elements that make up Java programs. Figure 3.3.1 shows these organizational units.
To review, a package is a collection of classes, which define methods. Methods contain statements, some of which contain expressions. Expressions are made up of tokens, which are the basic elements of a program, including numbers, variable names, operators, keywords, and punctuation like parentheses, braces and semicolons.
The standard edition of Java comes with several thousand classes you can
import, which can be both exciting and intimidating. You can browse this library at http://docs.oracle.com/javase/8/docs/api/. Most of the Java library itself is written in Java.
Note there is a major difference between the Java language, which defines the syntax and meaning of the elements in Figure 3.3.1, and the Java library, which provides the built-in classes.