Classes - Multimedia Introduction to Programming Using Java - page 136

3.6 Key concepts

•Class . A class is a file drawer that contains two kinds of items: manila folders
of the class and components that were declared with modifier static .

• Instances and objects. The terms folder , instance , and object are used inter-
changeably.

• Components: fields and methods . The components of an object of a class are
instance variables (or fields ) and instance methods —variables and methods that
are defined in the class without modifier static . Thus, the class definition is a
template for the folders of the class.

• Procedures, functions, constructors. There are three kinds of method. A call
to a function produces a value. A call to a procedure performs some task but does
not produce a value. A call to a constructor initializes the fields of a newly cre-
ated object. A method call can have arguments , which are expressions.

• New-expression. Evaluation of a new-expression new class-name ( arguments )
creates a new object of class class-name , initializes its fields by executing the
call class-name ( arguments ), and yields the name of the new object as its value.

• Calling a constructor . A constructor is called either in a new-expression or as
the first statement of another constructor of the class; in the latter case, keyword
this is used instead of the class name.

• Class as a type. A class name may be used as a type, and a variable declared
with that type can contain the name of a folder of that type.

• Null. The value null represents the absence of the name of an instance. If a
variable u contains null , attempting to access a component using u. component-
name is an error.

• Information hiding. The principle of information hiding says to hide informa-
tion from those who do not need it. We usually hide fields of an object by declar-
ing them to be private , although in rare cases, making them public is pre-
ferred.

• Inside-out rule. The inside-out rule, used in most programming languages,
says that a program part can reference items declared or referenceable in sur-
rounding constructs, unless a redeclaration hides them.

• toString. Put a function toString in (almost) every class to provide a descrip-
tion of the object in which the function appears.

• Equality testing. A relation c==b tests whether the names (of objects) that are
in c and b are the same, not whether the contents of the objects are the same.
Write a function equals to test whether the contents of the objects are the same.

• Static items. Make a variable static if it is a constant or if only one copy is