Java class library is contained in packages

• Java provides several standard classes available to all programs.

• This collection of classes is called the Java API (Application Programming Interface).

• The Java API is organized into packages.

• The main package in the package hierarchy is java.

• There are several subpackages that descend from the java package.

• Since the beginning of the book, the java.lang package has been used.

• The java.lang package contains several classes, including the System class, used to display output with println().

• The java.lang package is unique because it is automatically imported into all Java programs.

• Therefore, it was not necessary to import java.lang in the previous examples.
  Other packages need to be imported explicitly.

• Various packages will be examined in the following chapters.

Useful links:
https://docs.oracle.com/javase/8/docs/api/java/lang/package-summary.html
https://docs.oracle.com/javase/8/docs/api/java/net/package-summary.html
https://docs.oracle.com/javase/8/docs/api/java/net/package-summary.html
https://docs.oracle.com/javase/8/docs/api/java/applet/package-summary.html
https://docs.oracle.com/javase/8/docs/api/java/awt/package-summary.html

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