CS-140 Fall 2017 Test 2 Version A Nov. 29, 2017

Transcription

1 CS-140 Fall 2017 Test 2 Version A Nov. 29, 2017 Name: 1. (10 points) For the following, Check T if the statement is true, the F if the statement is false. (a) T F : An interface defines the list of fields that must be provided by a class that implements that interface An interface defines the methods that must be implemented, not the fields. (b) T F : In Java there is an expectation that if two objects are equal, they will have the same hashcode. (c) T F : The primary feature of functional programming is the ability to treat functions the same way procedural programming uses data. (d) T F : The name of an anonymous inner class should start with a capital letter. An anonymous inner class is anonymous... it has no name. (e) T F : A lambda expression consists of one or more arguments, followed by -> followed by an expression or list of Java statements enclosed by curly braces. (f) T F : An abstract class can implement an interface without implementing all the methods in the interface. The methods not yet implemented are considered abstract methods. (g) T F : It is possible to create a class in Java which does not support the tostring method. Every object inherits Object which has a tostring method which is always inherited. (h) T F : An abstract class can be instantiated, but the abstract methods in that class cannot be dispatched. An abstract class cannot be instantiated. If you try to do so, you will get a compiler error. (i) T F : When Java expects a reference to a Comparator object, the compiler allows a lambda expression instead. The lambda expression arguments are the same as the arguments to compareto. (j) T F : A child class cannot inherit the class methods of its parent. Class methods (or static methods) are not inherited. 2. (10 points) Check the best answer to fill in the blank in the following statements: (a) When you define a class, use the implements keyword to indicate that the class supports an interface. private extends implements interface (b) If a constructor in class MyClass invokes super(field1);, it must do so before any other instructions in the constructor. before any other instructions after all other instructions anywhere trick question - you can t invoke super (c) You can check to see if a reference variable points to a superclass by using the instanceof operator. ref method instanceof operator this.class.getname() method extends keyword (d) If a class has an abstract method, then that class is an abstract class. a private a concrete an abstract a parent Page 1 of 8

2 (e) If a method is declared with a final keyword, then that method cannot be overridden in a child class. a final an abstract a private a nooverride (f) When a method throws an exception it causes normal execution of that method to stop, and control is transferred to a catch block. an error a segmentation violation a temper tantrum an exception (g) A checked exception is checked by the compiler to make sure that either the exception is caught, or the method uses the throws keyword. at run-time by the programmer by the compiler trick question - a checked exception is not checked (h) An object of a sub-type class can be used anywhere an object of the super-type can occur. a sub-type a parent a public an inherited (i) The static type of a reference variable is determined at compile time, based on how the variable is declared. dynamic private checked static (j) When you specify that the compiler should treat a reference variable as a reference to a sub-type of its static type, that is called down-casting. forecasting down-casting broadcasting up-casting 3. (15 points) Given the following Java lambda expressions and argument values, what result will the lambda expression produce? (a) x >x+3 with argument 3.0, (b) x > x x + 2 x + 1 with argument 3, (c) (x,y) >(x y) (x y) with arguments 9 and 2 (a) 6.0 (b) 16 (c) 49 (d) (x,y) >x.lst.compareto(y.lst)==0? x.lst.compareto(y.lst) : x. fst. compareto(y.fst) with arguments n1 and n2 where n1 and n2 are objects with public fst and lst fields with values: n1[ fst= Tom lst= Smith ], n2[fst= Tom lst= Smith ] (d) 0 (e) (x) > { if (0==x%2) return 0; if (0==x%3) return 3; return 1; with argument 21 (e) 3 Page 2 of 8

3 4. (20 points) Given the following Java classes: public class Subconscious { private S t r i n g name ; public Subconscious ( S t r i n g name) { this. name = name ; public S t r i n g getname ( ) { return name ; public void pr omi se ( ) { System. out. p r i n t l n (name + promises not to keep any promises. ) ; public void claim ( ) { System. out. p r i n t l n (name + w i l l r u l e the world! ) ; public class Red extends Subconscious { public Red( S t r i n g name) { super (name ) public void pr omi se ( ) { System. out. p r i n t l n ( super. getname()+ w i l l b u i l d a wall. ) public void claim ( ) { System. out. p r i n t l n ( super. getname()+ w i l l make America g r e a t again. ) ; public class Blue extends Subconscious { public Blue ( S t r i n g name) { super (name ) public void pr omi se ( ) { System. out. p r i n t l n ( super. getname()+ w i l l pay your c o l l e g e t u i t i o n. ) public void claim ( ) { System. out. p r i n t l n ( super. getname()+ says we re s t r o n g e r t o g e t h e r! ) ; public class Debate { public static void main ( S t r i n g [ ] args ) { Blue dem = new Blue ( H i l l a r y ) ; Red rep = new Red( Donald ) ; Subconscious [ ] p a r t i e s = { rep, dem ; for ( Subconscious party : p a r t i e s ) { party. promise ( ) ; for ( Subconscious party : p a r t i e s ) { party. claim ( ) ; Page 3 of 8

4 (a) If you execute until just before the first for loop in the Debate main method, complete the following graphical display of memory: (b) If you run java -cp test2.debate, what will get printed to the screen? Solution: Donald will build a wall. Hillary will pay your college tuition. Donald will make America great again. Hillary says we re stronger together! Page 4 of 8

5 5. (25 points) Given the following Java classes: public class Fraction { private long numerator ; private long denominator ; public Fraction ( long numerator, long denominator ) { this. numerator = numerator ; this. denominator = denominator ; normalize ( ) ; public double todouble ( ) { return ( double ) numerator / denominator ; public void normalize ( ) { i f ( denominator <0) { denominator= denominator ; numerator= numerator ; long g = gcd (Math. abs ( numerator ), denominator ) ; i f ( g>1) { numerator /=g ; denominator /=g ; public Fraction add ( Fraction that ) { // Code not s u p p l i e d public Fraction mult ( Fraction that ) { return new Fraction ( this. numerator that. numerator, this. denominator that. denominator ) public int hashcode ( ) { f i n a l int prime = 3 1 ; int r e s u l t = 1 ; r e s u l t = prime r e s u l t + ( int ) ( denominator ˆ ( denominator >>> 3 2 ) ) ; r e s u l t = prime r e s u l t + ( int ) ( numerator ˆ ( numerator >>> 3 2 ) ) ; return r e s u l t public boolean e q u a l s ( Object obj ) { i f ( this == obj ) return true ; i f ( obj == null ) return f a l s e ; i f ( g e t C l a s s ( )!= obj. g e t C l a s s ( ) ) return f a l s e ; Fraction other = ( Fraction ) obj ; i f ( denominator!= other. denominator ) return f a l s e ; i f ( numerator!= other. numerator ) return f a l s e ; return true public S t r i n g t o S t r i n g ( ) { return Fraction [ numerator= + numerator +, denominator= + denominator + ] ; static private long gcd ( long x, long y ) { i f ( x==y ) return x ; i f ( x<y ) return gcd ( y x, x ) ; return gcd ( x y, y ) ; Page 5 of 8

6 (a) What is the string produced by Frac f=new Frac(1,2); f.mult(new Frac(1,3)).toString()? Fraction [numerator=1 denominator=6] (b) Write the code for the add method of the Fraction class so that fraction addition is performed using a the formula: b + c d = ad+bc bd. Solution: public Fraction add ( Fraction that ) { long num = ( this. numerator that. denominator ) + ( that. numerator this. denominator ) ; long den = this. denominator that. denominator ; return new Fraction (num, den ) ; (c) Given two instances of class Fraction, Fraction frac1 and Fraction frac2, such that f rac1.hashcode() == f rac2.hashcode(), will f rac1.equals(f rac2) be true? Why or why not? Solution: Probably, but not required. hashcode maps the values of numerator and denominator to an integer, and it is possible that two unequal fractions map to the same hashcode. (d) Given two instances of class Fraction, Fraction frac1 and Fraction frac2, such that f rac1.equals(f rac2), will frac1 == frac2 be true? Why or why not? Solution: Probably not. frac1 == frac2 implies frac1 and frac2 point to the same object, but equals allows two different objects with the same numerator and denominator to return true. (e) (5 points (bonus)) If you create a new Fraction(3,0), will the Fraction class work? If not, what is the first code that will break? Solution: When the creator invokes normalize, and it invokes gcd, gcd will get in an endless loop. Page 6 of 8

7 6. (20 points) Given the following Java class: public class Square extends JButton implements A c t i o n L i s t e n e r { private static f i n a l long serialversionuid = 1L ; int row ; int c o l ; TicTacToe game ; public Square ( TicTacToe game ) { this. game = game ; setfont ( getfont ( ). derivefont ( ( float ) ) ) ; s e t P r e f e r r e d S i z e (new Dimension ( 7 0, 7 0 ) ) ; setmargin (new I n s e t s ( 30, 30, 30, 30)); settext ( ) ; addactionlistener ( this ) ; public void choose ( S t r i n g val ) { settext ( val ) ; setenabled ( f a l s e ) public void actionperformed ( ActionEvent arg0 ) { choose ( X ) ; i f ( game. checkwinner ( ) ) return ; game. computerturn ( ) ; game. checkwinner ( ) ; (a) The constructor for the Square class invokes the getfont routine to get the value of the Font field associated with this square. How can the value for the font field be anything other than null, since this Square object has just been created? Solution: Since Square extends JButton, and the Square constructor does not invoke the JButton constructor explicitly with the super keyword, Java automatically invokes the JButton constructor with no arguments. The no-argument JButton constructor initializes the fields of JButton, including the font field. (b) The actionperformed method does not use the argument ActionEvent arg0. Why is it valid for the actionperformed method to ignore its argument? Solution: The only action registered to the Square object is a press of the button represented by this. Therefore, arg0 can ıonly be this. (c) Once the choose method is invoked on a specific Square object, can the actionperformed method ever get invoked? Solution: No. The choose method invokes setenabled(false) which prevents the button associated with this object from ever getting pressed again, and if the button cannot get pressed, the actionperformed method will not be invoked. Page 7 of 8

8 (d) The actionperformed method returns immediately instead of allowing the computer to have a turn if the TicTacToe checkwinner method returns a true value. Can you speculate what the return value from checkwinner means? Solution: The TicTacToe checkwinner method must return a true value if someone has won the TicTacToe game (gotten three in a row.) If the human player won by putting an X in this square, there is no need for the computer to take a turn. Question: Total Points: Bonus Points: Page 8 of 8