1 Java assert statement Java assert (cont.) an assert statement - - PowerPoint PPT Presentation

1

1

Assertions

2

Assertions

• assertions communicate assumptions about the

state of the program, and stop processing if they turn out to be false

• very often comments are statements about the state

the program should be in

• but assertions can tell the run-time system what the

programmer believes is going on – if these assumptions turn out not to be the case, the run-time system can stop processing

• Java assert uses exceptions as its mechanisms

3

Assertions (cont.)

• assertion checking can be switched on and off as

required

• assertions can be used during the test phase for a

product, and then be switched off for production deployment

• assertions may be seen as an extension to the safety

features provided by the language – built-in checks ascertain the validity of primitive

• perations, say, array indexing, casts, etc.

– programmer-written checks do the same at a higher level for user-defined abstractions, e.g., new data structures

4

Assertions (cont.)

• seeing checks as a safety system, you may not want

to turn them off (completely)

• turning on checking only during development and

testing is sometimes described as: "wearing a life jacket when close to shore, and throwing it overboard once you are in the middle

• f the ocean"

2

5

Java assert statement

• an assert statement can be placed anywhere you

normally place an exutable statement

• assert has two forms:

assert BooleanExpression assert BooleanExpression : expression

• if assertions are enabled, these statements behave

almost similarly to the following Java statements: if (! BooleanExpression) throw new Java.lang.AssertionError (); if (! BooleanExpression) throw new Java.lang.AssertionError ("" + expression);

• Note. A Throwable expression is set as a cause.

6

Java assert (cont.)

• technically, AssertionError has constructors that

turns expression values into a string – the original expression value becomes lost, and so cannot be (mis)used for further processing

• by Java compiler default, assertions are disabled at

runtime; – command-line switches allow you to selectively enable or disable assertions – may selectively set assertion state for a class and its inner classes, and the same for packages

• programming environments often set assertion state
• n (as a more appropriate default value)

7

Assertion idioms

• test impossibilities (never happens):

switch (a) { case 0: return "Red"; case 1: return "Green"; case 2: return "Blue"; default: assert false; }

• the compiler may sometimes deduce that code

cannot be reached, and won't allow even an (unnecessary) assert there.

8

Assertion idioms (cont.)

Algorithm Verification:

• slow but reliable method to determine the result,

compared to the outcome of the efficient but complicated method public Data fastMethod (E [ ] array) { Data result; // efficient algorithm calculates result.. assert result.equals (slowMethod (array)); return result; }

3

9

Assertion idioms (cont.)

Class invariant checking:

• true when the method starts, and the method ensures

it will be true after the method finishes

• invariants relate to the private state of an object and

they should be stated as assertions. public void put (E element) { assert Invariant (); elements [size++] = element; assert Invariant (); } public boolean Invariant () { return size >= 0 . .; // state valid and consistent }

10

Assertion idioms (cont.)

Precondition checking:

• it is responsibility of the code calling the method to

meet any preconditions the method has

• if preconditions are violated, in principle, all bets are
• ff: no need to fulfill responsibilities
• however, to support early detection of errors and

minimize damage, not assertions but exceptions are used to report the violations of preconditions public E getElement () { if (isEmpty ()) // not: assert throw new NoSuchElementException("empty"); return ...; }

• precondition checks are used for public methods

11

Assertion idioms (cont.)

Postcondition checking:

• may relate both to the return value from the

method, and to the whole state of system public void putElement (E element) { // .. put it into container assert ! isEmpty (); }

• postcondition checks performed using asserts in

any kind of method (public or private)

12

Summary

• practice defensive programming: check for the

unexpected

• exceptions are be used to

– return "alternative" results that presumable can be handled by the caller: checked exceptions – report run-time failures (memory exhaustion, invalid index argument, numerical overflow, etc.): unchecked exceptions

• throw exceptions for precondition errors
• asserts makes checks to reveal internal errors (bugs)
• prefer predefined standard exceptions
• remember to clean up/release resources
• write exception-safe components (failure atomicity)