Data Structures and Algorithms in Java[1].pdf - Fulvio Frisone

Class modifiers are optional keywords that precede the class keyword. We have
already seen examples that use the public keyword. In general, the different class
modifiers and their meaning is as follows:
• The abstract class modifier describes a class that has abstract methods.
Abstract methods are declared with the abstract keyword and are empty (that
is, they have no block defining a body of code for this method). A class that has
nothing but abstract methods and no instance variables is more properly called an
interface (see Section 2.4), so an abstract class usually has a mixture of
abstract methods and actual methods. (We discuss abstract classes and their uses
in Section 2.4.)
• The final class modifier describes a class that can have no subclasses.
(We will discuss this concept in the next chapter.)
• The public class modifier describes a class that can be instantiated or
extended by anything in the same package or by anything that imports the
class. (This is explained in more detail in Section 1.8.) Public classes are declared
in their own separate file called classname. java, where "classname" is the
name of the class.
• If the public class modifier is not used, the class is considered friendly.
This means that it can be used and instantiated by all classes in the same package.
This is the default class modifier.
1.1.1 Base Types
The types of objects are determined by the class they come from. For the sake of
efficiency and simplicity, Java also has the following base types (also called
primitive types), which are not objects:
boolean
Boolean value: true or false
char
16-bit Unicode character
byte
8-bit signed two's complement integer
short
16-bit signed two's complement integer
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