Java程序辅导

COM6050 Lab Sheet 1 September 2010 Page 1 
COM6050: Java and UML for programmers: Lab 1  
 
Dr Richard Clayton 
 
The three aims of this practical are (i) to introduce you to the online resources of the 
module, (ii) to ensure you can edit and compile a simple Java program, and (iii) to make 
sure you understand the ideas introduced in the first lectures. 
 
You should work through this worksheet on your own. You should aim to complete the first 
three parts of the lab sheet in the Thursday session, leaving time to complete the final part 
on Monday. If you experience any problems then ask a Demonstrator or Lecturer to help 
you. If you don’t finish the practical during the lab sessions on Thursday and Monday, then 
try to complete it in your own time. You do not need to hand the completed sheet in, and 
this work will not be marked. 
 
Locating online resources 
 
 Open a web browser and go to http://www.sheffield.ac.uk, click on the MUSE link at 
the top of the page. 
 Log in to MUSE using your username. You should find a link to My Online Learning 
Environment (MOLE).  
 Login to MOLE. If your MOLE page does not include a link to COM6050 then tell a 
Lecturer or Demonstrator. 
 The COM6050 MOLE page has several links, and this is the medium we will use for 
delivering course materials. There will also be some example multiple choice 
questions already available for you to practice with in the Assessment folder. 
 Follow the link to COM6050 Teaching Schedule. Here you will find links to the lecture 
notes, and lab exercises. 
 
A simple Java program 
 
 Create a folder where your Java code for the COM6050 module will be stored. At this 
point you may wish to think about a suitable way to organize your code, so that you 
can find your work easily. 
 From the Lab Sheets and Java Code folder, open the file HelloWorld.java. You may 
wish to create a new folder called COM6050, with a subfolder called Week1 in which 
you will store your work. 
 You will need to copy the Java code from your browser window into a text editor, and 
then save the file as HelloWorld.java. You could use Windows Notepad, or any 
other text editor of your choosing. jEdit is an open source code editor written in Java, 
and can be downloaded from www.jedit.org. If you are comfortable with an IDE, then 
Eclipse is installed on the DCS desktop.  
 Modify HelloWorld.java as indicated by the comments in the file, and save it. 
 Compile the modified file using the java compiler in a command window (dos 
prompt> javac HelloWorld.java), and run the result (dos prompt> java 
HelloWorld). 
 If you experience any problems at this stage then ask. 
 
A more detailed programming task 
 
Hopefully you will complete the first and second parts of the practical very quickly, and can 
get on to the more interesting part. Here you will modify a more complex program, and 
COM6050 Lab Sheet 1 September 2010 Page 2 
implement some of the ideas about variables, types and operators that were covered in the 
first lecture. 
 
 From the Lab Sheets and Java Code folder, download the file Quadratic.java, 
compile it, and run it. 
 This program solves a quadratic equation ax2 + bx + c = 0, giving the two solutions 
for x, where the coefficients a, b, and c are known, using the formulae 
a
acbb
x
2
)4(
1
2 
 and 
a
acbb
x
2
)4(
2
2 
  
 The program Quadratic.java uses both integer and double types to store a, b, and c, 
and prints out the solution (x1 and x2) for the each case. The coefficients are initially 
set to be a=1, b=2, and c=1, and then we subtract 0.5 from a before working out 
the solution. Why are the solutions for integer and double types different, and which 
is correct? (Hint: you may wish to refer to the program Typecast.java). 
 Remove (or comment out) the code for the incorrect variable type. 
 Add some code to Quadratic.java to insert the solutions x1 and x2 back into the 
quadratic equation so we can check if x1 and x2 are indeed solutions to the initial 
quadratic equation, (i.e. calculate a*x12 + b*x1 + c, and a*x22 + b*x2 + c). These 
values should be exactly 0 if the solutions x1 and x2 are correct. 
 Now change coefficient b to have the value 20,000 and obtain your solution for x1 
and x2. Are these solutions still correct? If not, why not? 
 
Second programming task 
 
In the following task you will use the Scanner class to read data items from the file. We 
covered the Scanner class briefly in Lecture 1, and you should take a look at the lecture 
notes to remind yourself how Java handles input and output. See also 
http://download.oracle.com/javase/6/docs/api/. 
 
You should write a Java program that first reads data from the file DataFile.txt (available on 
the MOLE pages – see below for a description), which is a file of integers, into an array of 
integers and then calculates each of the following: 
(i) The average of all the numbers stored in the array. 
(ii) The standard deviation of all the numbers stored in the 
array, calculated using the equation on the right, where 
N is the number of values in the array, aj is the jth value 
in the array, and ā is the average. 
The symbol Σ means summation. Thus, the above 
summation could be rewritten as: 
 
 
 
 
Note: Java uses 0 for the first index of an array, so you will need to take this into 
account in your program. 
 
The data file for the program is DataFile.txt, which can be found in the folder for 
lab 1 linked from MOLE. This file contains a list of integers, with one integer on 
each line of the file. The first integer in the file is not part of the data and instead 
states how many integers there are to follow. Thus, in the very simple example to 
the right, the first line, containing the integer 4, indicates that there are 4 integer 
data items in the file (45, -56, 0 and 23), which should be read into an array 
4 
45 
-56 
0 
23 
 



N
j
j aa
N
aSD
1
2)(
1
)(
22
3
2
2
2
1
2
1
)...()()()()( aaaaaaaaaa N
N
j
j 

COM6050 Lab Sheet 1 September 2010 Page 3 
before calculations can begin. 
 
How to proceed 
The following is just one way that you might choose to develop your program. It suggests a 
cautious and controlled way to develop a small program such as the one in this exercise. 
 
(a) Assume that the data file exists and that the data in it is well formed. Thus there will be 
no need for explicit error checking in the program, except for the try-catch block we 
mentioned in Lecture 1. 
try { 
 Scanner inputFile = new Scanner(new File("DataFile.txt")); 
 // you will need some extra code here  
 }  
catch (FileNotFoundException e) { 
 e.printStackTrace(); 
 } 
(b) Write a simple Java program that reads data from the file, one item at a time, and 
displays each item on the screen as it is read, using System.out.println(). You need a loop 
structure for this. Since the first line of the file states how many items are to follow, a for 
loop can be used. You can read an integer from a Scanner object using the nextInt() 
method using int numberReadFromFile = inputFile.nextInt(); 
Visually check that the display of data matches the data in the text file, and make sure 
that you have imported java.io.*, and java.util.Scanner. 
(c) Next, declare an array after you have read the first line of the file. The first line of the file 
will give the size of the array to be declared (but remember that array indexing starts at 
0 in Java). Using the for loop, read each subsequent data item into the array. Then write 
a second for loop that displays each item in the array. Visually check that the display of 
data matches the data in the text file. Now you have successfully read the data into an 
array, you can complete the exercise. 
(d) Use a for loop (or for-each loop) to sum the items in the array and divide by the number 
of items in the array (i.e. the size of the array) to give the average as a real number. 
(Hint: Dividing one integer by another will not produce the correct result. Why not?) Make 
sure you display a message telling the user that you are displaying the average of the 
data. 
(e) Split the calculation of the standard deviation into parts. First, use another for loop to 
calculate the summation, storing this in a temporary variable. Then use this in the final 
calculation of the standard deviation. Java has a Math class that provides all of the static 
Math methods you are likely to need. You will need the Math.sqrt() method for calculating 
the standard deviation, so make sure you have imported java.math.*. Again be careful 
about mixing integers and floats in the same calculation. Also, make sure you tell the 
user what you are displaying using a string message when you display the results.