Classes - Multimedia Introduction to Programming Using Java - page 115

Java syntax: New-expression
new class-name ( arguments )
Purpose . Create a folder (instance, object).

Evaluation . Create a folder of class class-name , exe-
cute the constructor call class-name ( arguments ), and
yield the name of the new folder as the value.

The values of type Employee are the names of folders of class Employee .
When a variable of type Employee is first declared, it contains the value
null , which means that it does not contain the name of a folder.

3.2.2 The new-expression

The new-expression is used to create new folders, or instances, of a class.
Consider this expression, where class Employee is given in Fig. 3.1:

Activities
3-4.1, 3-5.2

new Employee("Gries", 1966)
Its evaluation is done in three steps:

1. Create a new folder of class Employee and place it in Employee 's file
drawer. Its fields are initialized according to the declarations of the fields.
The first part of the new-expression, “ new Employee ” tells us to do this.
2. Execute the constructor call Employee("Gries", 1966); where the con-
structor Employee that is called is the one in the newly created folder
whose parameter types match the argument types of the call.
3. Yield as the value of the call the name of the new folder.

We evaluate the new-expression new Employee("Gries", 1966) . Step 1 is
to create a new folder of class Employee; the folder is shown in the left of Fig.
3.3. The fields have initial values as given by their declarations in the class —
default values are used if an initializing declaration is not used.
Step 2 is to execute the constructor call Employee("Gries", 1966) . We do
this here by stepping over the call. According to the specification of the con-
structor, the first argument is stored in field name and the second in field start .
We show the result on the right side of Fig. 3.3.
Step 3 is to yield the name a0 as the result of the expression evaluation.
Here is some terminology. The manilla folder created by evaluating a new-
expression is called an instance of the class, or an object of the class. Evaluating
a new-expression instantiates the class by creating an instance of it.

Placement of a new-expression
The main use of the new-expression is on the righthand side of an assign-
ment statement, for example, as in: