Java Reference
In-Depth Information
You can use assertions to implement class invariants. Class invariants are conditions that always hold true about
the values that determine the state of an object of a class. Class invariants may not be true for brief moments when an
object is transitioning from one state to another. Suppose you have a
BankAccount
class with four instance variables:
name
,
dob
,
startDate
, and
balance
. The following class invariants must be true for a
BankAccount
object:
The
•
name
on the account must not be
null
.
•
The
dob
on the account must not be
null
and must not be a date in future.
•
The
startDate
on the account must not be
null
.
•
The
startDate
on the account must not be before
dob
.
balance
on the account must be greater than zero.
You can pack all these conditions checks in one method, say
validAccount()
method.
•
The
private boolean validAccount() {
boolean valid = false;
// Check for class invariants here. Return true if it is true. Otherwise, return false.
return valid;
}
You can use the following assertion in methods and constructors to make sure that the class invariants are
enforced. You assume that the
toString()
method of the
BankAccount
class returns enough pieces of information to
help programmers debug the error.
assert validAccount(); this.tostring();
You can use the above
assert
statement in the beginning of every method and before you return from the
method. You do not need to check for class invariants inside a method if it does not modify the object's state. You
should use it only at the end in a constructor because class invariants will not hold when the constructor starts
executing.
Checking for Assertion Status
How do you know in your program if assertions are enabled? It is easy to check for the assertion status using an
assert
statement. Consider the following snippet of code:
boolean enabled = false;
assert enabled = true;
/* Check the value of enabled here */
This code uses the first form of the
assert
statement. Note that it uses the assignment operator (=), not the
equality comparison operator (
==
) in the expression
enabled = true
. The expression will assign
true
to the
enabled
variable and it will evaluate to
true
. Note that the
enabled
variable has been initialized to
false
. If assertion is
enabled, the
enabled
variable will have a value of
true
after the
assert
statement is executed. If assertion is disabled,
the variable
enabled
will have a value of
false
. Therefore, checking for the value of the
enabled
variable after the
assert
statement will give you a clue whether assertions are enabled for your class. Listing 10-2 shows the complete
code for checking if assertions are enabled for the
AssertionStatusTest
class. Note that assertion can be enabled or
disabled on a class basis, too. If assertions are enabled for a specific class, it does not guarantee that it is also enabled
for all other classes.
