Java Reference
In-Depth Information
An assertion that precedes a statement is called a
precondition
of the state-
ment. An assertion that follows a statement is called a
postcondition
. Such a pre-
condition-statement-postcondition triple has the following meaning:
Style Note
13.2, 13.2.2
indenting
assertions
Execution of the statement begun with the precondition true is
guaranteed to terminate, and when it terminates, the postcondition
will be true.
When you see a relation enclosed in braces within a comment, you should
assume that the program author is asserting that that relation is true at that point.
The earlier code ensures that the relation is true; the later code relies on it being
true. In later sections, you will see hints from time to time about when and where
to use such assertions.
2.7
A model of execution
We now show you precisely how a method call is executed. Learning this mate-
rial, and being able to execute method calls yourself using our model of execu-
tion, will make writing programs much easier. You will
know
what is going on
inside the computer. Further, from time to time you will want to execute a
method call by hand in order to pinpoint a difficult-to-find error in your program.
This section requires knowledge of classes and our view of a class as a file
drawer of manila folders.
2.7.1
Frames for method calls
Whenever a method is called, some memory is set aside to contain information
related to the call: parameter values, the current statement being executed, and
so on. This memory is called the
frame for the call
. Figure 2.8 shows the format
of the frame that we use throughout this text. We discuss its components:
Activity 4-3.3
gives a pictorial
description of
this complete
subsection.
•
Method-name.
The method name appears in a box in the upper-left of
the frame.
method-name: program-counter
scope box
local variable
n
…
local variable
2
parameter
n
…
parameter
2
local variable
1
parameter
1
Figure 2.8:
Format of the frame for a method call
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