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Assertions Questions

1. Which of the following statements is true?



SHOW ANSWER
Correct Ans:It is sometimes good practice to throw an AssertionError explicitly.
Explanation:
Option A is correct because it is sometimes advisable to thrown an assertion error even if assertions have been disabled.

Option B is incorrect because it is considered appropriate to check argument values in private methods using assertions.

Option C is incorrect; finally is never bypassed.

Option D is incorrect because AssertionErrors should never be handled.


2.
public class Test2 
{
    public static int x;
    public static int foo(int y) 
    {
        return y * 2;
    }
    public static void main(String [] args) 
    {
        int z = 5;
        assert z > 0; /* Line 11 */
        assert z > 2: foo(z); /* Line 12 */
        if ( z < 7 )
            assert z > 4; /* Line 14 */

        switch (z) 
        {
            case 4: System.out.println("4 ");
            case 5: System.out.println("5 ");
            default: assert z < 10;
        }

        if ( z < 10 )
            assert z > 4: z++; /* Line 22 */
        System.out.println(z);
    }
}
which line is an example of an inappropriate use of assertions?



SHOW ANSWER
Correct Ans:Line 22
Explanation:
Assert statements should not cause side effects. Line 22 changes the value of z if the assert statement is false.

Option A is fine; a second expression in an assert statement is not required.

Option B is fine because it is perfectly acceptable to call a method with the second expression of an assert statement.

Option C is fine because it is proper to call an assert statement conditionally.


3.
public class Test 
    public void foo() 
    {
        assert false; /* Line 5 */
        assert false; /* Line 6 */
    } 
    public void bar()
    {
        while(true)
        {
            assert false; /* Line 12 */
        } 
        assert false;  /* Line 14 */
    } 
}
What causes compilation to fail?



SHOW ANSWER
Correct Ans:Line 14
Explanation:
Option D is correct. Compilation fails because of an unreachable statement at line 14. It is a compile-time error if a statement cannot be executed because it is unreachable. The question is now, why is line 20 unreachable? If it is because of the assert then surely line 6 would also be unreachable. The answer must be something other than assert.

Examine the following:

A while statement can complete normally if and only if at least one of the following is true:

- The while statement is reachable and the condition expression is not a constant expression with value true.

-There is a reachable break statement that exits the while statement.


4.
Which three statements are true?

Assertion checking is typically enabled when a program is deployed.
It is never appropriate to write code to handle failure of an assert statement.
Assertion checking is typically enabled during program development and testing.
Assertion checking can be selectively enabled or disabled on a per-package basis, but not on a per-class basis.
Assertion checking can be selectively enabled or disabled on both a per-package basis and a per-class basis.



SHOW ANSWER
Correct Ans:2,3,5
Explanation:
(1) is wrong. It's just not true.

(2) is correct. You're never supposed to handle an assertion failure.

(3) is correct. Assertions let you test your assumptions during development, but the assertion code—in effect—evaporates when the program is deployed, leaving behind no overhead or debugging code to track down and remove.

(4) is wrong. See the explanation for (5) below.

(5) is correct. Assertion checking can be selectively enabled or disabled on a per-package basis. Note that the package default assertion status determines the assertion status for classes initialized in the future that belong to the named package or any of its "subpackages".

The assertion status can be set for a named top-level class and any nested classes contained therein. This setting takes precedence over the class loader's default assertion status, and over any applicable per-package default. If the named class is not a top-level class, the change of status will have no effect on the actual assertion status of any class


5. Which statement is true?



SHOW ANSWER
Correct Ans:Assertions can be enabled or disabled on a class-by-class basis.
Explanation:
Option A is correct. The assertion status can be set for a named top-level class and any nested classes contained therein. This setting takes precedence over the class loader's default assertion status, and over any applicable per-package default. If the named class is not a top-level class, the change of status will have no effect on the actual assertion status of any class.

Option B is wrong. Is there such a thing as conditional compilation in Java?

Option C is wrong. For private methods - yes. But do not use assertions to check the parameters of a public method. An assert is inappropriate in public methods because the method guarantees that it will always enforce the argument checks. A public method must check its arguments whether or not assertions are enabled. Further, the assert construct does not throw an exception of the specified type. It can throw only an AssertionError.

Option D is wrong. Because you're never supposed to handle an assertion failure. That means don't catch it with a catch clause and attempt to recover.


6.
public class Test2 
{
    public static int x;
    public static int foo(int y) 
    {
        return y * 2;
    }
    public static void main(String [] args) 
    {
        int z = 5;
        assert z > 0; /* Line 11 */
        assert z > 2: foo(z); /* Line 12 */
        if ( z < 7 )
            assert z > 4; /* Line 14 */

        switch (z) 
        {
            case 4: System.out.println("4 ");
            case 5: System.out.println("5 ");
            default: assert z < 10;
        }

        if ( z < 10 )
            assert z > 4: z++; /* Line 22 */
        System.out.println(z);
    }
}
which line is an example of an inappropriate use of assertions?



SHOW ANSWER
Correct Ans:Line 22
Explanation:
Assert statements should not cause side effects. Line 22 changes the value of z if the assert statement is false.

Option A is fine; a second expression in an assert statement is not required.

Option B is fine because it is perfectly acceptable to call a method with the second expression of an assert statement.

Option C is fine because it is proper to call an assert statement conditionally.

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Read more:
Assertions - Pointing out the correct statements
Assertions - Pointing out the errors



7.
What will be the output of the program (when you run with the -ea option) ?

public class Test 
{  
    public static void main(String[] args) 
    {
        int x = 0;  
        assert (x > 0) : "assertion failed"; /* Line 6 */
        System.out.println("finished"); 
    } 
}



SHOW ANSWER
Correct Ans:An AssertionError is thrown.
Explanation:
An assertion Error is thrown as normal giving the output "assertion failed". The word "finished" is not printed (ensure you run with the -ea option)

Assertion failures are generally labeled in the stack trace with the file and line number from which they were thrown, and also in this case with the error's detail message "assertion failed". The detail message is supplied by the assert statement in line 6.


8.
What will be the output of the program?

public class Test 
{
    public static int y;
    public static void foo(int x) 
    {
        System.out.print("foo ");
        y = x;
    }
    public static int bar(int z) 
    {
        System.out.print("bar ");
        return y = z;
    }
    public static void main(String [] args ) 
    {
        int t = 0;
        assert t > 0 : bar(7);
        assert t > 1 : foo(8); /* Line 18 */
        System.out.println("done ");
    }
}



SHOW ANSWER
Correct Ans:compilation fails
Explanation:
The foo() method returns void. It is a perfectly acceptable method, but because it returns void it cannot be used in an assert statement, so line 18 will not compile.


9.
public class Test 
    public void foo() 
    {
        assert false; /* Line 5 */
        assert false; /* Line 6 */
    } 
    public void bar()
    {
        while(true)
        {
            assert false; /* Line 12 */
        } 
        assert false;  /* Line 14 */
    } 
}
What causes compilation to fail?



SHOW ANSWER
Correct Ans:Line 14
Explanation:
Option D is correct. Compilation fails because of an unreachable statement at line 14. It is a compile-time error if a statement cannot be executed because it is unreachable. The question is now, why is line 20 unreachable? If it is because of the assert then surely line 6 would also be unreachable. The answer must be something other than assert.

Examine the following:

A while statement can complete normally if and only if at least one of the following is true:

- The while statement is reachable and the condition expression is not a constant expression with value true.

-There is a reachable break statement that exits the while statement.

The while statement at line 11 is infinite and there is no break statement therefore line 14 is unreachable. You can test this with the following code:

public class Test80
{
public void foo()
{
assert false;
assert false;
}
public void bar()
{
while(true)
{
assert false;
break;
}
assert false;
}
}


10.
What will be the output of the program?

public class Test 
{  
    public static void main(String[] args) 
    { 
        int x = 0;  
        assert (x > 0) ? "assertion failed" : "assertion passed" ; 
        System.out.println("finished");  
    } 
}



SHOW ANSWER
Correct Ans:compilation fails
Explanation:
Compilation Fails. You can't use the Assert statement in a similar way to the ternary operator. Don't confuse.


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