CS125: Introduction to Programming Principles

You are to complete a class called Coder which will handle encoding and decoding a message using this technique. The class diagram for Coder is here

-String key

+Coder(String codeKey)
+String encode(String message)
+String decode(String message)
-char intToChar(int unicode)

In particular, you must implement the constructor, and the two methods: encode and decode. More details about these methods can be found in the starter file Coder.java. You have also been provided with another file called CoderTester.java that contains some test cases for your Coder class. You may add your own test cases to the main method if you wish.

In order to implement the methods properly, you need to remember that all characters have an integer code associated with them known as their Unicode value. In Java, you can add and subtract char values as you would numbers, but the result is an integer. For example in Java 'a' + 'b' is 195 since this the sum of the Unicode values of 'a' and 'b'. You can also add integers to char values. For example 'a' + 1 is 98, which also happens to be the Unicode value of 'b'.

While implementing the encode and decode methods, you can calculate the shifted value of the characters by using the Unicode integer values. For example, if keyCharacter is the current character in the key

• to compute the shift from 'A' you can compute shift = keyCharacter - 'A'
• to encode the i-th character in the message you can compute message.charAt(i) + shift
• to decode the i-th character in the message you can compute message.charAt(i) - shift
• if you go beyond 'Z' ('F' + 'X' for example) you should subtract 26 to get a letter between 'A' and 'Z'
• if you go below 'A' ('F' - 'X' for example) you should add 26 to get a letter between 'A' and 'Z'

However, you will need to convert the Unicode back to a character once you have figured out the new character's Unicode value. The Coder class has a private method called intToChar that will handle that conversion. You can call this method and pass an integer parameter, and the method will return the appropriate char value. For example if you call this method with the parameter value 98, the method will return 'b'.

Submit the completed file Coder.java below.

You have been provided with a class called Computer which represents computer attributes. The class has the following methods

Computer

- String brand
- double cpuSpeed
- int RAM

+ Computer(String type, double speed, int memory)  
+ void makeSameSpeed(Computer other)
+ void doubleSpeed()
+ boolean sameBrand(Computer other)
+ String getUpperBrand()
+ boolean equals(Computer other)
+ String toString()

Documentation for each of these methods can be found in the source code for the Computer class.

Question 1: toString()

Most classes should define a toString() method. This method returns a String version of the object using the instance variable(s). It is a method that can be useful in debugging. You can call this method in the same way that other methods are called, but the toString() method is called automatically whenever you use an object in a print/println statement. Java creates a default toString method if you do not create your own which cryptically prints out where the object is in memory. Currently the toString() method in the Computer class has been commented out.

• Run the main method in the Question1 class. Notice what is displayed by System.out.println statements. Why do you think the first and third outputs are the same and the second output is different?


• Uncomment the toString method in the Computer class. Run the main method in Question1 again. Briefly explain why the output looks different now.


• Change the body of the toString method so that it prints out the values of instance variables instead. Submit your new toString() method here.
  
  

Question 2: References

The main method in the Question2a class creates two computers that are the same, and then doubles speed of the first computer. Then it displays the speed of the two machines. The main method in the Question2b class appears to do the same thing in a different way. Do you get the same output in each case? Explain why.

Question 3: Passing parameters that are references

The makeSameSpeed(Computer other) method is supposed to change the speed of other to match the speed of this. When you run the main method in the Question3 class, do you get the expected result? What do you get? Why?

Rewrite the makeSameSpeed method to work correctly. Submit your new makeSameSpeed method here.