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| No. | Answer | Remark | |
|---|---|---|---|
| 1 | c | Prints: C | The boolean type variable, b, is initialized to true. The boolean expression of the first if statement, b = false, is a simple assignment expression that sets b to false. Always look carefully at the boolean expression of an if statement to verify that the expected equality operator (==) has not been replaced by the simple assignment operator (=). |
| 2 | c | Prints: 0,1,1,0 | The first try block contains two statements. The first invokes method m1, and the subsequent statement contains a post increment expression with the variable, a, as the operand. Method m1 throws a WhiteException exception, so variable a is not incremented as control passes to the catch block where b is incremented. The throws clause of m1 declares a ColorException, so the body may throw a ColorException or any subclass of ColorException. The second try block also contains two statements. The first invokes method m2, and the subsequent statement contains a post increment expression with the variable, d, as the operand. Method m2 does not throw an exception, so d is incremented, and the try block completes normally. Although the throws clause of m2 declares a WhiteException, there is no requirement to throw any exception. |
| 3 | d | Prints: D | The expression, b = false, appears to be testing the value of b, but it is really setting the value of b. Always look carefully at the boolean expression of an if statement to verify that the expected equality operator (==) has not been replaced by the simple assignment operator (=). |
| 4 | f | Compile-time error | The throws clause of White.m2 declares a WhiteException, so the body of m2 may throw a WhiteException or any subclass of WhiteException. Instead, the body of m2 throws a superclass of WhiteException. The result is a compile-time error. |
| 5 | c | Prints: C | The first if statement initializes the value of b. The expression, b = false, appears to be testing the value of b, but it is really setting the value of b. Always look carefully at the boolean expression of an if statement to verify that the expected equality operator (==) has not been replaced by the simple assignment operator (=). |
| 6 | f | Compile-time error | The throws clause of White.m1 declares a ColorException, but the catch clause in the main method catches only a subclass of ColorException. The result is a compile-time error. |
| 7 | a | Prints: 0,0,0,1,1 | The first catch clause has a parameter e of type Level3Exception, so the first catch clause is able to catch any exception type that is assignable to type Level3Exception. Since Level2Exception is the superclass of Level3Exception, an instance of Level2Exception is not assignable to a catch clause parameter of type Level3Exception. Similarly, Level1Exception is also a superclass of Level3Exception, so an instance of Level1Exception is not assignable to a catch clause parameter of type Level3Exception. The only exception type that can be caught by the first catch clause is a Level3Exception. The second catch clause has a parameter e of type Level2Exception, so the second catch clause is able to catch a Level2Exception. The Level1Exception is the superclass of Level2Exception. An instance of Level1Exception is not assignable to a catch clause parameter of type Level2Exception, so the second catch clause can not catch a Level1Exception. Since a Level3Exception is a subclass of Level2Exception an exception of type Level3Exception is assignable to a catch clause parameter type Level2Exception. All exceptions of type Level3Exception will be caught by the first catch clause, so the second catch clause in this program will not have an opportunity to catch a Level3Exception. The third catch clause has a parameter e of type Level1Exception, so the third catch clause is able to catch a Level1Exception. The exceptions of type Level2Exception and Level3Exception are assignable to the catch clause parameter of the third catch clause, but the exceptions of those subclass types will be caught by the first two catch clauses. The switch statement throws a Level1Exception. The try block completes abruptly as control passes to the third catch block where d is incremented. The finally block is also executed, so f is incremented. |
| 8 | e | Prints: 0,0,1,0,1 | The first catch block is able to catch a Level3Exception or any subclass of Level3Exception. The second catch block is able to catch a Level2Exception or any subclass of Level2Exception. The switch statement throws a Level2Exception. The try block completes abruptly as control passes to the second catch block where c is incremented. The finally block is also executed, so f is incremented. |
| 9 | g | Prints: 1,0,0,0,1 | The switch statement does not throw an exception; so the switch completes normally. The subsequent statement increments the variable, a; and the try block completes normally. The finally block is also executed, so f is incremented. |
| 10 | d | Prints: 0,1,1 | The try block contains two statements. The first invokes method m1, and the subsequent statement contains a post increment expression with the variable, a, as the operand. Method m1 throws a WhiteException exception, so variable a is not incremented as control passes to the catch block where b is incremented. Although Color.m1 declares a ColorException in the throws clause, a subclass of Color is free to declare only a subclass of ColorException in the throws clause of the overriding method. |
| 11 | a d f | With assertions enabled it prints an AssertionError message. With assertions disabled it prints nothing. The assert statement is being used to check a control-flow invariant to verify that the control flow never reaches a point in the program. | The assert statement indicates that the programmer believes that b1 and b2 will never be true simultaneously, and the assert statement should not be reached under normal operating conditions. |
| 12 | a d f h | If assertions are enabled at run time it prints an error message. With assertions disabled it prints 1112. The assert statements are being used to check a precondition--something that must be true when the method is invoked. Method m2 is an example of an improper use of an assert statement: an assert statement should not be used for argument checking in a public method. | Assertions may be enabled or disabled at run time. Since assertions are not always enabled, they should not be used to validate the parameters of public methods. Parameter checking is typically published in the API specification of a method and must be enforced even when assertions are not enabled. Rather than use an assertion, an appropriate runtime exception should be thrown such as IllegalArgumentException, IndexOutOfBoundsException, or NullPointerException. However, an assertion may be used to validate the parameters of a nonpublic method. |
| 13 | b c e | If assertions are not enabled at run time it prints nothing. With assertions enabled it prints an error message. The assert statement is being used to check a postcondition--something that must be true when the method completes successfully. | Variable c equals 200 when the assertion is checked. |
| 14 | b d e | If assertions are not enabled at run time it prints nothing. With assertions enabled it prints nothing. The assert statement is being used to check a class invariant--something that must be true about each instance of the class. | This question is an example of using assertions to check a class invariant--something that must be true about each instance of the class. Although a class invariant must be true before and after the execution of each public method, the invariant is typically only checked at the end of each method and constructor. |
| 15 | a d f h | With assertions enabled it prints an error message. With assertions disabled it prints: true,true,false The combination of the if/else statements and the assert statement indicate that the programmer expects no more than one boolean, b1, b2 or b3, to be true. The assert statement is being used to check an internal invariant--something that the programmer assumes to be true at a particular point in the program. | Method m1 has a series of if/else statements. The first if statement is processed if none of the booleans are true. The second is processed if only b1 is true. The third is processed if only b2 is true. A set of three booleans can exist is eight states. The three if statements account for three of those states; so five more states remain. The assert statement indicates that the programmer assumes that only one of those five remaining states is valid--that is the state where only b3 is true. The combination of the three if statements and the assert statement indicate that the programmer believes that no more than one of the booleans will be true at that point in the program. That assumption is called an internal invariant. |
| 16 | a c e | Prints "Only b1 is true" followed by an error message. The combination of the if/else statements and the assert statement indicate that the programmer expects no more than one boolean, b1, b2, or b3, to be true. The assert statement is being used to check a control-flow invariant to verify that the control flow never reaches a point in the program. | Method m1 has a series of if/else statements. The first if statement is processed if none of the booleans are true. The second is processed if only b1 is true. The third is processed if only b2 is true. The fourth is processed if only b3 is true. A set of three booleans can exist in one of eight states. The first four if statements account for four of those states; so four more states remain. The combination of the three if statements and the fact that an AssertionError is thrown from the last else block indicates that the programmer believes that no more than one of the booleans will be true when method m1 is being processed. An assumption concerning the state of a set of variables is called an internal invariant. In this case, however, the assertion was tested by verifying that control never reached a particular point in the program. Based on the testing technique, we would say that the assertion tests a control-flow invariant. A throw statement is used in place of an assert statement, because the throw statement can not be disabled. As a result, the method is certain to generate an error once control passes beyond all of the return statements. The declared return type of method m1 is String. No return statement appears after the sequence of if statements; therefore, every if statement must either return a String or throw an exception. Assertions can be disabled at run time, so an assert statement in the final if block is no guarantee that an exception will be thrown. For that reason, an assert can not replace the throw statement. |
| 17 | b e f | The flow of control does not reach a particular point in the program. The default case of a switch statement is not reached. The else block of an if/else statement is not reached. | A control-flow invariant is placed at a point in the program that the programmer assumes will never be reached. Two examples are the default case of a switch statement or the else block of an if/else statement. It makes no sense to use an assert statement to verify that the flow of control does reach a particular point in the program, because it is unlikely that an assertion error is helpful when the program is found to be functioning correctly. An assert statement placed at the beginning of a method is generally used to check a precondition. An assert statement that is placed at the end of a method to check the state of some variables is generally said to be checking a post condition. However, it is also possible that an assert statement placed at the end of a method might also be checking a control-flow invariant. The correct term depends on the usage. |
| 18 | h | Prints: 121212 | This trick question has a for loop nested inside of a do loop. For each iteration, the values of i and j are as follows: (1,0)(2,0)(1,1)(2,1)(1,2)(2,2). |
| 19 | a | Prints: 0001 | This trick question has a do-loop nested inside of a for-loop. For each iteration, the values of i and j are as follows: (0,0)(0,1)(0,2)(1,3). |
| 20 | f | Prints: 1112 | This trick question has a do-loop nested inside of a for-loop. For each iteration, the values of i and j are as follows: (1,0)(1,1)(1,2)(2,3). |
| 21 | b | Prints: 012 | This trick question has a while-loop nested inside of a do-loop. For each iteration, the values of i, j and k are as follows: (1,0,0)(2,0,1)(3,0,2). |