UNIT 3 ARRAYS, RECURSION, AND COMPLEXITY CHAPTER 11 CLASSES CONTINUED EXERCISE 11.1 1. static public final int DEFAULT_NUM_SCORES = 3; 2. Java allocates a separate set of memory cells in each instance for the instance variables of a class. By contrast, only one set of cells is allocated for the class variables. 3. First, a static method can be included to allow the user to call the method by sending a message to the class. This method can serve as a public accessor or mutator of a class variable. Second, a static method can be included to perform some useful calculation. The Math class methods are good examples. Third, a static method can be included in a set of cooperating helper methods to decompose responsibilities for tasks in a class. 4. The reference to instvar in the static method is a syntax error because static methods cannot reference instance variables. 5. A public constant is accessible to all clients, and a static constant is accessible by using the class name as a reference. EXERCISE 11.3 1. The purpose of an interface is to provide clients with enough information to be able to use the methods of a class. An interface includes just a set of method signatures, whereas a class includes the implementations of these methods and perhaps some data as well. 2. static private void drawsquare(pen p, int x, int y, int length){ p.up(); p.move(x, y); p.down(); p.setdirection(270); for (int i = 1; i <= 4; i++){ p.move(length); p.turn(90); EXERCISE 11.4 1. public interface Account{ public double getbalance(); public void deposit(double amount); public void withdraw(double amount); 1
2. The expression implements <an interface> causes the compiler to check the implementing class for the presence of all the methods in the specified interface. If any of them are missing, a syntax error occurs. EXERCISE 11.5 1. A class hierarchy is a set of classes that are organized by the relationship of subclass and superclass. An example is the hierarchy Object/Circle/Wheel, where Circle is a subclass of Object and Wheel is a subclass of Circle. 2. The expression extends <a class> is used to inherit the data and behavior of another class. 3. To run a constructor of the same form in the superclass, one uses the form super(<parameters>) at the beginning of the corresponding constructor in the subclass. 4. With methods other than constructors, one uses the form super.<method name> (<parameters>). 5. The visibility modifier protected allows subclasses in a hierarchy to access instance and class variables or methods of superclasses without allowing other clients in the system to access these items. 6. There is a syntax error. The method setspokes is not included in the Shape interface. The programmer attempts to send this message to an instance of Wheel, but this object is masquerading as a Shape. The variable s must be cast to a Wheel before sending it the setspokes message. EXERCISE 11.6 1. a. Syntax error: the array element, as an Object, does not understand the indexof message. b. Syntax error: same as a. c. Correct. d. Runtime error: ClassCastException because a Student is cast to a String. EXERCISE 11.7 1. An abstract class serves as a repository of data and behavior common to a set of subclasses. Because this class is abstract, it cannot be instantiated. An example is the AbstractShape class, which is abstract and defines the behavior common to circles and rectangles. 2. We declare abstract methods for two possible reasons. First, the abstract class in which they are declared implements an interface, but the implementation of these methods is deferred to the concrete classes. Second, even if there is no interface, 2
these methods must be implemented in all subclasses, so declaring them abstract enforces this. 3. A final method cannot be overridden in a subclass. Examples are the accessors for variables that are defined in an abstract class. These methods should not be overridden by subclasses. 4. A protected variable is accessible throughout the defining class and any of its subclasses. An example is a balance for all subclasses of an abstract bank account class. EXERCISE 11.8 1. Generally, to locate the right method to execute, the JVM first looks in the class of the receiver object. If the method is not found there, the JVM look at this class s superclass, if there is one. This process is repeated until a method is found or an exception is thrown (if the method is not eventually found in the Object class). In this example, the server s method somemethod is executed first. Within this method, the JVM executes a call to the method somemethod in the superclass. 2. Student answers will vary, but here are some examples. The main application class in a terminal-based program depends on the Scanner class for input operations. A phone book class aggregates objects that consist of a name, address, and phone number. A colored point inherits data and behavior from a point. EXERCISE 11.9 1. The rule for passing a parameter object is the same as the rule for assignment, namely, the class of the actual parameter must be the same as or less inclusive than the class of the formal parameter. 2. The answer is b. EXERCISE 11.10 1. Preconditions describe assumptions that must be true before a method can perform its task correctly. For example, a method that uses a parameter as a divisor must expect that number to not be zero. 2. Postconditions describe assumptions that must be true after a method executes correctly. For example, a correctly executed method to deposit money in a bank account will leave behind a balance that is the sum of the balance before the deposit was made and the amount deposited. EXERCISE 11.11 3
1. A method should throw an exception when its preconditions are violated. For example, if an integer parameter must be less than a certain value, the method should throw an exception if the parameter is greater than or equal to that value. 2. When a method throws an exception, control is passed immediately to the calling method and from there goes on up the call chain to the main method, where the JVM halts execution with a trace of this call chain. 3. An exception can be caught and handled by using a try-catch statement. 4. The @param tag marks a parameter of a method. The @return tag marks the returned value of a method. The @throws tag marks an exception thrown by the method. EXERCISE 11.12 1. The programmer should use equals instead of == when comparing two objects because == is too restrictive. In cases where two distinct objects are structurally the same (have the same values for all their instance variables), == would return false because == returns true only when the operands refer to one object. 2. true false false 3. When an object is assigned to a variable, that variable contains a reference to the object. 4. To obtain a true copy of an object, you must create a new instance of that object s class and then copy the original object s instance variables to the new object s variables. EXERCISE 10.13 1. The advantages of using turtle graphics are that it s thoroughly object-oriented and there is no forgetful bitmap to worry about. The disadvantages are that shapes cannot be filled and the drawing is a bit slow. The advantages of using the standard graphics classes are that the methods are very fast and the shapes can be filled. The main disadvantage is that the programmer must refresh the forgetful bitmap. REVIEW QUESTIONS Fill in the Blank 1. class, static 2. super 3. protected 4. abstract 5. abstract 4
6. preconditions, postconditions 7. equals, clone 5