Spring Semester 2004

• Work with integer and floating-point numbers

• Write arithmetic expressions in Java

• Define and initialize variables and constants

• Recognize limitations of the int and double types and the overflow and roundoff errors that can result

• Be able to change the values of variables through assignment

• Use String type to define and manipulate character strings

First a sample session of working with the program (on Unix). I marked the user's answers with blue, but this just for illustration purposes in these lab notes. Don't expect the program to actually print in color when you compile and run it, of course.

frilled.cs.indiana.edu%javac Conversion.java
frilled.cs.indiana.edu%java Conversion
Hi my name is Hal. What is your name?
Dave
Hello, Dave! How many dollars do you want to convert?
(Please type an integer value, no decimal part)
40
I see, you want to convert 40 dollars in British pounds. Very well...
What is the conversion rate today?
0.65
Well, Dave for 40 dollars you can get: 26.0 pounds.
Thank you for using Hal! Good-bye.
frilled.cs.indiana.edu%

/* A conversion program that illustrates how one can read input
   from the keyboard using the textbook's ConsoleReader class */ 

class Conversion {
    public static void main(String[] args) {
	ConsoleReader console = new ConsoleReader(System.in); 
	// now we can read from 'console' 
	System.out.println("Hi my name is Hal. What is your name?"); 
	// greet the user
	String name = console.readLine(); 
	// wait for input and collect it (a String, for the name) 
	System.out.print("Hello, " + name + "! "); 
	// echo the name to the user, raising user's confidence in us 
	System.out.println("How many dollars do you want to convert?"); 
	// approach the user directly, offer your services 
	System.out.println("(Please type an integer value, no decimal part)"); 
	// instruct the user what limitations you have (accepting only int's)
	int amount = console.readInt(); 
	// wait for the user to provide the sum to be converted 
	System.out.println("I see, you want to convert " + amount + 
			   " dollars in British pounds. Very well..."); 
	// talk to the user, be friendly, echo information frequently 
	System.out.println("What is the conversion rate today?"); 
	// ask the user for the last piece of input 
	double rate = console.readDouble(); 
	// wait for the conversion rate, which can have a fractional part
        System.out.println("Well, "+ name + " for " + amount + " dollars " + 
			   "you can get: " + rate * amount + " pounds.");
	// do the calculation and report it to the user 
	System.out.println("Thank you for using Hal! Good-bye."); 
	// thank the user for interest and say good-bye
    } 
}

/*******************************************************
   ConsoleReader class is used for keyboard input. 
   Just keep the source code of this class (the file)
   in the same directory in which your other program
   (the one that needs user input) resides. 

   In this particular case keep it in the same folder
   with Conversion.java that has been presented above. 
********************************************************/ 

import java.io.*; // I/O package needed 

class ConsoleReader { 
    ConsoleReader(InputStream inStream) { // constructor 
	reader = new BufferedReader(
                   new InputStreamReader(
                     inStream));     
    }
    String readLine() { // instance method 
	String inputLine = "";
	try {
	    inputLine = reader.readLine(); 
	} catch (IOException e) {
	    System.out.println(e); 
	    System.exit(1); 
	} 
	return inputLine; 
    }
    int readInt() { // instance method 
	String inputString = readLine(); 
	int n = Integer.parseInt(inputString); 
	return n; 
    }
    double readDouble() { // instance method 
	String inputString = readLine(); 
	double x = Double.parseDouble(inputString); 
	return x; 
    } 
    private BufferedReader reader; // instance method 
}

Reference types (using Rectangle)

Rectangle a = new rectangle(10, 20, 30, 40); 
Rectangle b = a; 

Now if you have

a.translate(3, 3); 
System.out.println(b);

Primitive types (using int)

int a = 3;
int b = a; 

The big difference is that the primitive value is copied into the storage location. So each location has its own copy. It just works that way with the numbers (as primitive types) but doesn't with objects (reference types). That's how it works. With reference types, what the variable holds is a reference, the arrow. With primitive types, the variable holds a value, a copy of the actual value.

Now if you have

a = 10; 
System.out.println(b);

Each location has its own (copy of the) value, and changing one does not affect the other. This is an important difference, we refer to it as the by value vs. by reference contrast, and it will be useful for you to remember it from now on when you reason about and design programs.

Your Lab Assignment Two could be similar to

• Problem 19 on page 143.

• But also to any of the problems listed below.

And now the practice problems:

Try to solve these problems to practice some of the things you learned this week. In the examples that follow, your program's answers are always in blue, to distinguish them from what you would type as a user. Remember: the resulting programs are elementary, and the problems are interesting.

1.	Write a program that prompts the user for two integers and then prints The sum The difference The product The average The distance (absolute value of the difference) The maximum (the larger of the two ) The minimum (the smaller of the two)
	Here's a sample run with your program: frilled.cs.indiana.edu%java One Please enter your first integer number, then press Enter. 3 Please enter your second integer number, then press Enter. 6 3 + 6 = 9 3 - 6 = -3 3 * 6 = 18 avg(3, 6) = 4.5 dist(3, 6) = 3 max(3, 6) = 6 min(3, 6) = 3 frilled.cs.indiana.edu%