Java Assertion
Assertion facility is added in J2SE 1.4. In order to support
this facility J2SE 1.4 added the keyword assert to the language,
and AssertionError class. An assertion checks a boolean-typed
expression that must be true during program runtime execution.
The assertion facility can be enabled or disable at runtime.
Declaring Assertion
Assertion statements have two forms as given below
assert expression; assert expression1 : expression2;
The first form is simple form of assertion, while second form
takes another expression. In both of the form boolean expression
represents condition that must be evaluate to true runtime.
If the condition evaluates to false and assertions are enabled,
AssertionError will be thrown at runtime.
Some examples that use simple assertion form are as follows.
assert value > 5 ; assert accontBalance > 0; assert
isStatusEnabled();
The expression that has to be asserted runtime must be boolean
value. In third example isStatusEnabled() must return boolean
value. If condition evaluates to true, execution continues
normally, otherwise the AssertionError is thrown.
Following program uses simple form of assertion
//AssertionDemo.java Class AssertionDemo{
Public static void main(String args[]){
System.out.println( withdrawMoney(1000,500) );
System.out.println( withdrawMoney(1000,2000) );
}
public double withdrawMoney(double balance , double amount){
assert balance >= amount; return balance - amount;
} }
In above given example, main method calls withdrawMoney method
with balance and amount as arguments. The withdrawMoney method
has a assert statement that checks whether the balance is grater
than or equal to amount to be withdrawn. In first call the
method will execute without any exception, but in second call it
AssertionError is thrown if the assertion is enabled at runtime.
Enabling Assertion
By default assertion are not enabled, but compiler complains if
assert is used as an identifier or label. The following command
will compile AssertionDemo with assertion enabled.
javac -source 1.4 AssertionDemo.java
The resulting AssertionDemo class file will contain assertion
code.
By default assertion are disabled in Java runtime environment.
The argument -eanbleassertion or -ea will enables assertion,
while -disableassertion or -da will disable assertions at
runtime.
The following command will run AssertionDemo with assertion
enabled.
Java -ea AssertionDemo or Java -enableassertion AssertionDemo
Second form of Assertion
The second for of assertion takes another expression as an
argument.
The syntax is,
assert expression1 : expression2;
where expression1 is the condition and must evaluate to true at
runtime.
This statement is equivalent to
assert expression1 : throw new AssertionError(expression2);
Note: AssertionError is unchecked exception, because it is
inherited from Error class.
Here, expression2 must evaluate to some value.
By default AssertionError doesn’t provide useful message so this
form can be helpful to display some informative message to the
user.
