CmSc150 Fundamentals of Computing I
Homework 06 due 02/17 by 5 pm SOLUTION
1. Programming exercise
Download the project HW06-Errors.zip. It contains two classes – Account and
Transactions. The Transactions class has several syntax and semantic errors. Your task is to
correct them. When compiling the class you will get an error message for each syntax error.
For each error message do the following:
1. Write down the error message
2. Write down the reason for the message
3. Write down the necessary corrections
4. Correct the program and compile again.
When there are no more error messages, run the program and examine the results. Examine
the code and find out the logical (semantic) errors. Write down the semantic errors and
explain they can be corrected. Correct the program.
Zip the folder, change the extension to .zi and send it back.
Errors:
1. amount = scan.nextDouble();
Cannot find symbol “scan”
Scanner object not defined
import java.util.Scanner;
Scanner scan = new Scanner(System.in);
2. int bal = Account.deposit (amount);
Non-static method deposit(double) cannot be referenced from a static context
int bal = acc1.deposit (amount);
3. int bal = acc1.deposit (amount);
possible loss of precision
double bal = acc1.deposit (amount);
4. Account.withdraw (acc1);
withdraw(double) in Account cannot be applied to Account
Account.withdraw (amount);
5. Account.withdraw (amount);
Non-static method deposit(double) cannot be referenced from a static context
acc1.deposit (amount);
Logical error: after withdrawal the balance is not recorded
Correction:
bal = acc1.withdraw (amount);
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2. Questions and answers
True/False (Write T or F immediately after the number of the question)
1. Java methods can return only primitive types (int, double, float, char, boolean, etc).
.
Answer: False. Explanation: Java methods can also return (references to) objects, such as a
String.
2. Formal parameters are those that appear in the method call and actual parameters are those that
appear in the method header
Answer: False. Explanation: The question has the two definitions reversed. Formal parameters
are those that appear in the method header, actual parameters are the parameters in the method
call (those being passed to the method).
3. All Java classes must contain a main method which is the first method executed when the Java
class is called upon.
Answer: False. Explanation: Only the driver program requires a main method. The driver
program is the one that is first executed in any Java program (except for Applets), but it may call
upon other classes as needed, and these other classes do not need main methods
4. Java methods can return more than one item
Answer: False. Explanation: All Java methods return a single item, whether it is a primitive data
type an object, or void. The reserved word continue is used to exit the remainder of a loop and
test the condition again.
5. The following method header definition will result in a syntax error: public void aMethod( );
Answer: True. Explanation: The reason for the syntax error is because it ends with a “;”
symbol. It instead needs to be followed by { } with 0 or more instructions inside of the brackets.
An abstract method will end with a “;” but this header does not define an abstract method.
6. A method defined in a class can access the class’ instance data without needing to pass them as
parameters or declare them as local variables
Answer: True. Explanation: The instance data are globally available to all of the class’ methods
and therefore the methods do not need to receive them as parameters or declare them locally. If
variables of the same name as instance data were declared locally inside a method then the
instance data would be “hidden” in that method because the references would be to the local
variables.
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7. Defining formal parameters requires including each parameters type
Answer: True. Explanation: In order for the compiler to check to see if a method call is correct,
the compiler needs to know the types for the parameters being passed. Therefore, all formal
parameters (those defined in the method header) must include their type. This is one element that
makes Java a Strongly Typed language.
8. While multiple objects of the same class can exist, in a given program there can be only one
version of each class
Answer: True. Explanation: A class is an abstraction, that is, it exists as a definition, but not as
a physical instance. Physical instances are created when an object is instantiated using new.
Therefore, there can be many objects of type String, but only one String class.
Multiple Choice (use boldface or underline to answer)
1. The behavior of an object is defined by the object’s
a)
b)
c)
d)
e)
instance data
constructor
visibility modifiers
methods
all of the above
Answer: d. Explanation: The methods dictate how the object reacts when it is passed messages.
Each message is implemented as a method, and the method is the code that executes when the
message is passed. The constructor is one of these methods but all of the methods combine
dictate the behavior. The visibility modifiers do impact the object’s performance indirectly.
2. The relationship between a class and an object is best described as
a. classes are instances of objects
b. objects are instances of classes
c. objects and classes are the same thing
d. classes are programs while objects are variables
e. objects are the instance data of classes
Answer: b. Classes are definitions of program entities that represent classes of things/entities in
the world. Class definitions include instance data and methods. To use a class, it is instantiated.
These instances are known as objects. So, objects are instances of classes. Program code directly
interacts with objects, not classes.
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3. Which of the following reserved words in Java is used to create an instance of a class?
a.
b.
c.
d.
e.
class
public
public or private, either could be used
import
new
Answer: e. Explanation: The reserved word “new” is used to instantiate an object, that is, to
create an instance of a class. The statement new is followed by the name of the class. This calls
the class’ constructor. Example: Car x = new Car( ); will create a new instance of a Car and set
the variable x to it.
4. If a method does not have a return statement, then
a.
b.
c.
d.
e.
it will produce a syntax error when compiled
it must be a void method
it can not be called from outside the class that defined the method
it must be defined to be a public method
it must be an int, double, float or String method
Answer: b. Explanation: All methods are implied to return something and therefore there must
be a return statement. However, if the programmer wishes to write a method that does not return
anything, and therefore does not need a return statement, then it must be a void method (a method
whose header has “void” as its return type).
5. A class’ constructor usually defines
a.
b.
c.
d.
e.
how an object is initialized
how an object is interfaced
the number of instance data in the class
the number of methods in the class
if the instance data are accessible outside of the object directly
Answer: a. Explanation: The constructor should be used to “construct” the object, that is, to set
up the initial values of the instance data. This is not essential, but is typically done. The interface
of an object is dictated by the visibility modifiers used on the instance data and methods.
6. Instance data for a Java class
a.
b.
c.
d.
e.
are limited to primitive types (e.g., int, float, char)
are limited to Strings
are limited to objects(e.g., Strings, classes defined by other programmers)
may be primitive types or objects, but objects must be defined to be private
may be primitive types or objects
Answer: e. Explanation: The instance data are the entities that make up the class and may be
any type available whether primitive or object, and may be public or private. By using objects as
instance data, it permits the class to be built upon other classes. This relationship where a class
has instance data that are other classes is known as a has-a relationship.
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7. Consider a method defined with the header: public void foo(int a, int b).
following method calls is legal?
a.
b.
c.
d.
e.
Which of the
foo(0, 0.1);
foo(0 / 1, 2 * 3);
foo(0);
foo( );
foo(1 + 2, 3 * 0.1);
Answer: b. Explanation: The only legal method call is one that passes two int parameters. In
the case of answer b, 0 / 1 is an int division (equal to 0) and 2 * 3 is an int multiplication. So this
is legal. The answers for a and e contain two parameters, but the second of each is a double. The
answers for c and d have the wrong number of parameters.
8. Consider a method defined with the header: public void doublefoo(double x). Which of the
following method calls is legal?
a.
b.
c.
d.
e.
doublefoo(0);
doublefoo(0.555);
doublefoo(0.1 + 0.2);
doublefoo(0.1, 0.2);
all of the above are legal except for d
Answer: e. Explanation: In the case of a, the value 0 (an int) is widened to a double. In the case
of c, the addition is performed yielding 0.3 and then doublefoo is called. The parameter list in d
is illegal since it contains two double parameters instead of 1.
9. The value of the Java expression 10 - 25 % 4 is:
a. 6
b. 5
c. 9
d. 7
e. none of the above
10. If the int variables int1 and int2 contain the values 9 and 5, respectively, then the value of the
expression (double)(int1 / int2) is:
a.
b.
c.
d.
e.
0.8
0
1.8
1.0
1
Answer: d. Explanation: First the integer division is performed, because the expression is in
parentheses. The result is 1. Then the type casting is applied and the result is real 1, i.e. 1.0
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11. If the int variables int1 and int2 contain the values 9 and 5, respectively, then the value of the
expression (double)(int1) / int2 is:
a.
b.
c.
d.
e.
0.8
0.5
1.8
1.0
1
Answer: c. Explanation: First the type casting operation is applied to the value in int1 (note that
the contents of int1 is not changed) . As a result the expression now involves a double value,
therefore the value in int2 is also transformed to double, and only then the division is performed.
12. If the int variables int1 and int2 contain the values 10 and 5, respectively, then the value of
the expression int2 / (double)(int1) is:
a.
b.
c.
d.
e.
0.8
0.5
1.8
2.0
1
Answer: b. Explanation: First the type casting operation is applied to the value in int1 (note that
the contents of int1 is not changed) . As a result the expression now involves a double value,
therefore the value in int2 (however not changing the contents of int2) is also transformed to
double, and only then the division is performed.
How to turn in the assignment:
The program: Zip the project and send it as attached file by e-mail. Follow the naming
conventions.
The questions: write your answers into this document, write your name, then save and
send as attached .doc document by e-mail
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