The assert Statement and the AssertionError Class in Java

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The assert Statement and the AssertionError Class
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The following two forms of the assert statement can be used to specify assertions:
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assert assert
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<boolean expression> ; <boolean expression> : <message expression> ;
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// the simple form // the augmented form
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The <boolean expression> can be a boolean or a Boolean expression In the latter case, its value is unboxed to a boolean value If assertions are enabled (see p 269), the execution of an assert statement proceeds as shown in Figure 614 The two forms are essentially equivalent to the following code, respectively:
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if (<assertions enabled> && !<boolean throw new AssertionError(); if (<assertions
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expression>)
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// the simple form
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enabled>
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&& !<boolean
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throw new AssertionError(<message
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expression>) expression>);
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// the augmented form
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CHAPTER 6: CONTROL FLOW
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If assertions are enabled, then the <boolean expression> is evaluated If its value is true, execution continues normally after the assert statement However, if it is false, an AssertionError is thrown and propagated In the simple form, the AssertionError does not provide any detailed message about the assertion failure
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Execution of the Simple assert Statement (with Assertions Enabled)
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Evaluate boolean expression [true ]
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[false ]
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Throw an AssertionError
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Normal execution continues
Execution is aborted and AssertionError propagated
The augmented form specifies a <message expression> that can be used to provide a detailed error message In the augmented form, if the assertion is false, the <message expression> is evaluated and its value passed to the appropriate AssertionError constructor The <message expression> must evaluate to a value (ie, either a primitive or a reference value) The AssertionError constructor invoked converts the value to a textual representation In particular, the <message expression> cannot call a method that is declared void The compiler will flag this as an error The augmented form is recommended, as it allows a detailed error message to be included in reporting the assertion failure Example 618 illustrates using assertions Statements at (2), (3), and (4) in class Speed are all assert statements In this particular context of calculating the speed, it is required that the values satisfy the assumptions at (2), (3), and (4) in the private method calcSpeed() The simple form of the assert statement is used at (2) and (4)
assert distance >= 00; assert speed >= 00; // (2) // (4)
The augmented form is used at (3)
assert time > 00 : "Time is not a positive value: " + time; // (3)
The augmented form at (3) is equivalent to the following line of code, assuming assertions have been enabled at runtime:
if (time <= 00) throw new AssertionError("Time is not a positive value: " + time);
The javalangAssertionError class is a subclass of javalangError (see Figure 69) Thus AssertionErrors are unchecked They can be explicitly caught and handled using the try-catch construct, and the execution continues normally, as one would
610: ASSERTIONS
expect However, Errors are seldom caught and handled by the program, and the same applies to AssertionErrors Catching these errors would defeat the whole purpose of the assertion facility In addition to the default constructor (invoked by the simple assert form), the AssertionError class provides seven single-parameter constructors: six for the primitive data types (byte and short being promoted to int) and one for object references (type Object) The type of the <message expression> used in the augmented assertion statement determines which of the overloaded constructors is invoked It is not possible to query the AssertionError object for the actual value passed to the constructor However, the method getMessage() will return the textual representation of the value
Example 618 Using Assertions public class Speed { public static void main(String[] args) { Speed objRef = new Speed(); double speed = objRefcalcSpeed(-120, 30); // double speed = objRefcalcSpeed(120, -30); // double speed = objRefcalcSpeed(120, 20); // double speed = objRefcalcSpeed(120, 00); Systemoutprintln("Speed (km/h): " + speed); }
// // // //
(1a) (1b) (1c) (1d)
/** Requires distance >= 00 and time > 00 */ private double calcSpeed(double distance, double time) { assert distance >= 00; // (2) assert time > 00 : "Time is not a positive value: " + time; // (3) double speed = distance / time; assert speed >= 00; // (4) return speed; } }