Assertion in Java

An assertion is a statement in the Java programming language that enables you to test your assumptions about your program. For example, if you write a method that calculates the speed of a particle, you might assert that the calculated speed is less than the speed of light.

Each assertion contains a boolean expression that you believe will be true when the assertion executes. If it is not true, the system will throw an error. By verifying that the boolean expression is indeed true, the assertion confirms your assumptions about the behavior of your program, increasing your confidence that the program is free of errors.

Experience has shown that writing assertions while programming is one of the quickest and most effective ways to detect and correct bugs. As an added benefit, assertions serve to document the inner workings of your program, enhancing maintainability.

The assertion statement has two forms. The first, simpler form is:

assert Expression1 ;

where Expression1 is a boolean expression. When the system runs the assertion, it evaluates Expression1 and if it is false throws an AssertionError with no detail message.

The second form of the assertion statement is:

assert Expression1 : Expression2 ;

where:

Expression1 is a boolean expression.

Expression2 is an expression that has a value.

Use this version of the assert statement to provide a detail message for the AssertionError. The system passes the value of Expression2 to the appropriate AssertionError constructor, which uses the string representation of the value as the error's detail message.

The purpose of the detail message is to capture and communicate the details of the assertion failure. The message should allow you to diagnose and ultimately fix the error that led the assertion to fail. Note that the detail message is not a user-level error message, so it is generally unnecessary to make these messages understandable in isolation, or to internationalize them. The detail message is meant to be interpreted in the context of a full stack trace, in conjunction with the source code containing the failed assertion.

Like all uncaught exceptions, assertion failures are generally labeled in the stack trace with the file and line number from which they were thrown. The second form of the assertion statement should be used in preference to the first only when the program has some additional information that might help diagnose the failure. For example, if Expression1 involves the relationship between two variables x and y, the second form should be used. Under these circumstances, a reasonable candidate for Expression2 would be "x: " + x + ", y: " + y.

In some cases Expression1 may be expensive to evaluate. For example, suppose you write a method to find the minimum element in an unsorted list, and you add an assertion to verify that the selected element is indeed the minimum. The work done by the assert will be at least as expensive as the work done by the method itself. To ensure that assertions are not a performance liability in deployed applications, assertions can be enabled or disabled when the program is started, and are disabled by default. Disabling assertions eliminates their performance penalty entirely. Once disabled, they are essentially equivalent to empty statements in semantics and performance. See Enabling and Disabling Assertions for more information.

The addition of the assert keyword to the Java programming language has implications for existing code.

References : Oracle share on :        :  

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