charAt( int ) ...
Variable s1 contains the name of a folder of class String , and the folder con-
tains not only the string of characters but also many methods that are useful in
manipulating those characters. These methods are discussed in detail in Sec. 5.2.
Here is an important point: testing two String variables s1 and s2 using
s1 == s2
does not test whether they represent the same string of characters. Instead, it tests
whether s1 and s2 contain the same folder name, just like when two variables of
class JFrame are compared. To test whether the strings of characters in the fold-
ers named by s1 and s2 are the same, use the test s1.equals(s2) .
Equality is discussed in detail in Sec. 3.2.4.
• Class . A class is a file drawer of folders. Each folder in a drawer has the same
• Instances and objects. The terms folder , instance , and object are used inter-
• Components: fields and methods . The possible components of an object are
fields , which are variables and, thus, contain values, and methods , which are
called to perform tasks.
• 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 some or all fields of a
newly created object. A method call can have arguments , which are expressions.
The values of the arguments are used in some fashion when the method is called;
the specification of the method explains how they are used.
• New-expression . Evaluation of a new-expression new class-name ( arguments )
creates a new object of the class, initializes its fields by calling constructor class-
name , and yields as its value the name of the newly created object.
• 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. For example, vari-
able w of type JFrame can contain the name of a folder that goes in JFrame 's file
• Null . The value null represents the absence of the name of an instance. If a