Java Reference
In-Depth Information
Java syntax: Referencing a field
expr . field-name
Example : ( new Date()).x
Purpose : To reference field field-
name of the folder whose name is
given by the value of expression expr .
Java syntax: Calling an instance method
expr . method-name ( arguments )
Example : ( new Date()).getyear()
Purpose : To call instance method method-name
that appears in the folder whose name is given by
the value of expression expr . If the method is a pro-
cedure, the call needs a semicolon.
Employee e= new Employee("Hall", 2000);
If evaluation of the new-expression in this assignment created the folder that is
on the right side of Fig. 3.3, the value a0 would be stored in e.
But an Employee new-expression can appear in many other places —wher-
ever an expression of type Employee can appear. For example, the following
statement creates an Employee and prints its description using its toString
function; thereafter, the newly created folder cannot be referenced anymore:
System.out.println( new Employee("Gries", 1966));
This example may not seem so useful. Here is a more useful one. We would
like to obtain the current year (or month, day, minute, etc.). We can do this by
constructing a new instance of class Date (in package java.util ), calling its
function getYear , and adding 1900 to the result:
int year= ( new Date()).getYear() + 1900;
In this example, a desired value is retrieved from the new Date folder, and the
Date folder is never used again.
In this text, you will come across other cases where a new-expression is used
in places other than the righthand side of an assignment. Get used to the fact that
the new-expression is just that, a new kind of expression.
a0
a0
Employee
Employee
getName()
setName(String)
getStart()
setStart( int )
getCompensation()
addToSalary( double )
toString()
Employee()
getName()
setName(String)
getStart()
setStart( int )
getCompensation()
addToSalary( double )
toString()
Employee()
null
name
name
Gries
0
1966
start
start
50000.
50000.
salary
salary
Figure 3.3:
Evaluating a new-expression
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