CSCI A201/A597 and I210 Lab Notes Two Second semester 2000-2001

• 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

The minute paper is exercise 2.17 (p. 94) in your book:

Minute Paper for A201/A597/I210 Week Two (R2.17, p. 94)
What are the values of the following expressions? In each line, assume that double x = 2.5; double y = -1.5; int m = 18; int n = 4; String s = "Hello"; String t = "World"; x + n * y - (x + n) * y m / n + m % n 5 * x - n / 5 Math.sqrt(Math.sqrt(n)) (int)Math.round(x) (int)Math.round(x) + (int)Math.round(y) s + t s + n 1 - (1 - (1 - (1 - (1 - n)))) s.substring(1, 3) s.length() + t.length()

Here now is the interactive conversion program announced in Lecture Notes Four.

First a sample session of working with the program (on Unix). I marked the user 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.

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 for the illustration of reading input with
   the ConsoleReader class */ 

public class Conversion {
    public static void main(String[] args) {
	ConsoleReader console = new ConsoleReader(System.in); 
	// now we can read from the 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 ints)
	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 and echo the info 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
   (that needs user input) resides. */ 

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

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

Here is also a diagrammatic description of the fundamental difference between primitive and reference types.

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.

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 difference, and it will be useful for you to remember it from now on when you reason about and design programs.