Exam 1 Average: 27.5 / 40 Exam 1 Grade Range: 14 - 39 Question - - PDF document

1

Exam 1 Exam 1

• Average: 27.5 / 40
• Grade Range: 14 - 39

Question 1

• All true

– A class with an abstract method must be declared as abstract – An abstract class cannot be instantiated. – A subclass of an abstract class may be instantiated if it overrides all abstract methods and provides a body for each

Question 2

• FileReader input = new FileReader(args[0])

– Reader reads characters – FileReader reads characters from a File – The argument to FileReader is a filename

• In this case the filename is the first argument found on the

command line java className foo bar moo 7

args

args[0] args[1] args[2] args[3]

foo bar moo 7

Question 3

• Both false

– A finally block will always execute regardless

• f whether the exception is caught or not

– A try block can have an unlimited number of catch clauses, not a maximum of 2.

Question 4

• Answer is (b)

– The catch blocks are tested in turn, the first one that matches the subclass of Exception caught will be executed

• In the example, ArithmeticException is caught
• The first catch block is executed, the others are ignored
• “5” is printed

– The finally block is always executed

• “2” is printed

– After the try/catch/finally is done, execution continues with next statement.

• “1” is printed

2

Question 5

• Answer is (a)

A B C

max(int x, int y) max(int x, int y) max(int x, int y)

Question 5

• Code will compile

– C.max overrides B.max which overrides A.max

• This is good subclass behavior and is okay

– Switching the arguments in B.max is okay since both arguments are ints

• May not run as expected but it will pass the

compiler

Question 5

• Let’s step through code

– C c = new C(); – c.max (13, 29); – In C.max(13, 29):

• return (super.max (x + 10, y + 10)
• Call B.max (23, 39)

Question 5

• Let’s step through code

– In B.max(23, 39)

• return super.max (y, x) – 10;
• call A.max (39, 23)
• A.max (39, 23) will return 39
• B.max(23, 39) will return 39 – 10 = 29
• C.max(13, 29) returns 29
• “29” gets printed

Question 6

• Answer is (c)

A B C

int f() { return 0;} int f() { return 1;} int f() { return 2;}

Question 6

• In main

– A ref1 = new C(); // okay since C is subclass of B // which is a subclass of A – B ref2 = (B)ref1; // Cast ok since C is subclass of B – print ref2.f();

– Ref2 is referencing an object of type C

• Even though it is declared as class B

– This is okay – Polymorphism

• C.f() is called.
• “2” is printed.

3

Question 7

Mother Daughter

identify() identify()

Question 7

• In main

– Mother m = new Daughter(); – Daughter d = new Daughter(); – m.identify(); – d.identify();

– Both m and d reference objects of type Daughter

• This is okay since Daughter is a subclass of Mother
• Polymorphism
• Daughter.identify is called in both cases

– Output

• This is a Daughter speaking
• This is a Daughter speaking

Question 8

• To get data from the keyboard

– You read from System.in – Gave 1 point if you gave correct instantiation of some InputStream.

• new FileInputStream(…)

Question 9

• Could be interpreted as what happens

during compile time or runtime:

– Compile time:

• Will cause compile error
• Why: exception not caught and not declared
• Recall:

– If method A calls method B and method B throws an exception, A can either: » Catch it » Declare that it can throw the same kind of Exception.

Question 9

• Could be interpreted as what happens

during compile time or runtime:

– Runtime:

• Exception will be passed to the one who called the

method

• Passing up will continue until main is reached at

which point the program will terminate.

Question 10

• Reader/Writer

– Reads and writes characters

• InputStream/OutputStream

– Reads and writes bytes

4

Question 11

• Truck Class:

class Truck extends Auto { private int towingWeight; public Truck (int year, int doors, int weight){ super (year, doors); // must be first towingWeight = weight; } public String report () { return (“Vehicle is a “ + getYear() + “ Truck “); } }

Question 11

• Alternately, since Auto data is protected

class Truck extends Auto { private int towingWeight; public Truck (int year, int doors, int weight){ super.year= year; super.doors = doors; towingWeight = weight; } public String report () { return (“Vehicle is a “ + year + “ Truck “); } }

Question 12

• BufferedWriter output = new BufferedWriter (

new OutputStreamWriter ( new FileOutputStream (“myTest”)))

• - Program writes character data (BufferedWriter)
• - This data is buffered for eficiency (BufferedWriter)
• - Converted to bytes (OutputStreamWriter)
• - written to file “myTest” (FileOutputStream)

Question 13

• We subclass Exceptions

– So that we can distinguish a specific exception condition – Method catching exceptions can take different actions based on type of exception.

Question 14

• The program will not compile

– try block without a corresponding catch – The fact that f is private is okay

• Since main is a method of X7, it can access its

private members.

Exam

• Any questions?