Internal Invariants
Assertions
can be used within programs to make sure the program behaves in a predetermined
manner and will throw an error when violated. For instance, an assertion can be
placed in the code below to declare that interest will never be negative.
if (interest > 5000){
principal = principal + 500;
}else if (interest < 5000 ){
principal = principal +10;
}else {
assert interest >0:"Interest cannot be negative"
}
It can also be used to check the ability to reach statements in a method. In
the example below the assert statement is included at a place where it would not
be reached. If, however, for some reason the code is executed, we know that a serious
problem exists.
public int returnSomeValue(int d){
if (d > =0)
return d;
if (d<0)
return Math.abs(d);
assert false: "This statement should not be reached Since it has reached
and you are seeing this error there is some serious problem".
}
Preconditions:
Following the principles of "Design by Contract," assert statements can be used
to check for preconditions. Preconditions are values or parameters passed to a
method, to be used for the functioning of the program. Assert statements can be
used to check the validity of the parameters passed before they get used in the
body of the method.
However java warns that assert statements should not be used to check
parameters of a public method. In the sample above an assert statement could have
been used if the method were public, to check that the interest was always greater
than zero.
private int checkInterest (int interest) {
assert interest > 0:"Interest cannot be negative."
Do computation with interest.
}
Postconditions:
Just like preconditions, there may be instances where a program needs to execute
some postconditions. Postconditions need to be evaluated before each exit point
in the method. For instance assert statements can be used to check for the
validity of the returned values in a method that has multiple return
statements.
Class Invariants:
A class
invariant specifies the values of attributes across multiple instances of a
class. Assertions can be used to check on a method of a class if it is accessed
and modified by multiple instances. Assert statements can be used to make sure
that internal structures in a class are intact or according to the specification
even after multiple manipulations on them.
Where not to use Assertions:
Assertions
are not suitable for all occasions. A fact that needs to be remembered is that
assertions cannot be used wherever the normal course of a program is affected
when assertions are disabled. Assertions can be disabled and affect the
outcome of the program. Normally, assertions are used to check conditions during
programming. The idea is to have them on while developing and testing.
Assertions are turned off during production, when you feel the system (program)
is running normally and you do not need the extra overhead of assertions in the
code. The memory saving is particularly helpful in wireless applications where
memory is a constraint.
Assertions
should not be used to check arguments on a public method. According to the spec
this should be done by the method regardless of whether assertions are enabled
or not. Moreover incorrect arguments to methods need to throw an appropriate
IllegalArgumentException instead of a generic AssertionError.
Compiling programs with Assertions:
By default, assertions are disabled during compilation using J2SE 1.4 compiler,
so the command javac somefile.java will compile without assertions. To compile a
java program with assertions we need to include the command line option –source
1.4.
javac –source 1.4 AssertionsFile.java
Assert is a new keyword included in J2SE 1.4, so compiling a program with
assertions using an earlier version of the compiler will complain that assert
is not defined. This could cause compatibility problems with existing code that
might have used assert as an identifier. However existing class files may not be
affected. A java file needs to be compiled with assertions enabled so that the
program can be run with assertions enabled.
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