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An assertion is a statement
containing a boolean expression that is assumed to be true when the statement
is executed. The system reports an
AssertionError if the expression
evaluates to false. It is used for debugging purposes:
assert(a > 0); // throws an AssertionError if a <= 0
Assertions can be in two
forms:
assert Expression1 ;
assert Expression1 : Expression2 ;
Expression1
should always result in a boolean value.
Expression2
can be anything that results in a value. The value is used to generate a
String message that displays more
debugging information. For instance:
class AssertionTest
{
int a ;
public void func()
{
assert (a < 0): "a is positive" ;
// Remaining code
}
}
In the above code, the
assertion expression is true if a is
negative. If a is positive, an
AssertionError is thrown with the
message "a is positive."
Assertions are disabled by
default. To compile with assertions enabled, you need to use the
source 1.4 flag:
javac -source 1.4 Test.java
To enable assertions at
runtime, use the -enableassertions or
-ea flags.
For selective disabling of
assertions at runtime, use the -da or
-disableassertions flags.
For enabling assertions in the
system classes, use the -esa or
-dsa flags. Assertions can also be
enabled or disabled on a package basis.
Appropriate and
inappropriate use of assertions
You can place an assertion at any location that you don't expect to be reached
normally. Assertions can be used to validate the parameters passed to a
private method. However, assertions should not be used to validate parameters
passed to public methods because a public method must check its arguments
regardless of whether assertions are enabled or not. However, you can test
postconditions with assertions in both public and non-public methods. Also,
assertions should not change the state of a program in any manner.
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350 Mock Questions on SCJP 1.5 - JUST Rs.200 or 7 USD
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