Programming Assignment Checklist: LFSR

Frequently Asked Questions

What are the goals of this assignment? To introduce you to object-oriented programming and to reinforce the message of Lecture 1.

What preparations do I need to do before beginning this assignment? Review the Lecture 1 slides to reacquaint yourself with the basic ideas behind LFSRs. Also read Sections 3.1 and 3.2 of the textbook to learn the basics of object-oriented programming.

What are the APIs that I need to use as a client? How can I use them? Since you'll be using Picture and Color, you will eventually need to know some parts of the API for these classes.

Do the APIs that I define for LFSR and PhotoMagic need to be exactly the same as the prescribed ones? Yes, we will be testing the methods in the API directly. If your method has a different signature or does not behave as specified, you will lose a substantial number of points. You may not add public methods to the API; however, you may add private methods (which are only accessible in the class in which they are declared).

How should I represent the LFSR? There are several possible approaches. The approach outlined in the template is to use an int[] array of size N, each entry of which is either 0 or 1. If you follow this approach, the constructor amounts to creating the array and converting the char values in the string to int values for the array (and initializing your other instance variables). However, you are allowed to use any efficient representation that you like.

How do I create an instance variable which is an array if I don't know its length at compile time? Declare it as an instance variable, but initialize it in constructor (where you will know its length). For example, see Program 3.2.3 in the textbook.

What extra comments should I include when writing object-oriented programs? You must comment the purpose of every method, using the names of the argument variables in your description. Additionally, you must comment the purpose of each instance variable.

In the initial register, fill.charAt(0) is the leftmost bit. But in the Lecture 1 slides and the assignment page, bit 0 is the rightmost bit. Do I have to arrange the bits in my register array that way? Strings are always indexed from left to right (the way we read English). Traditionally, binary digits are indexed from right to left, with bit 0 as the lowest order or rightmost bit. You are welcome to use either approach here—just be careful to do it correctly and consistently.

How do I convert the char values in the string to int values for the array? The number zero is represented by the char value '0'. The char data type is a Java primitive type, so you can compare two of them with if (c == '0').

My step() method is producing 19 for the binary number 11001. What am I doing wrong? You are calculating the bits in reverse order. 19 is the decimal value of the binary number 10011.

My generate() works with the 11 bit fill and the tap at position 8, but when I try generate()with the 20 bit fill and the tap at position 16, I get a different answer from the one shown in the example. What am I doing wrong? Make sure you are not hardwiring 11 or 8 in your LFSR code. The LFSR constructor arguments should set the size of the register and the tap position.

The toString() is not explicitly called in the test client code, but it still gets called. How does this work?

      LFSR lfsr = new LFSR("01101000010", 8);

When a Java object is passed to any of the print methods, a call is automatically made to that object's toString() method.

I get an ArrayOutOfBounds or NullPointerException error. What could cause this? Do your constructors initialize all of the instance variables (e.g., N, reg, and tap)? Did you allocate memory for your array with new? Did you inadvertently redeclare int N or int[] reg in a method or constructor, thereby hiding the instance variable with the same name?

How do I do exclusive or (XOR) in Java? Use the ^ Java symbol. The operation a ^ b, where a and b are int values, does a bit-by-bit exclusive or of the values.

Why are we using the .png format instead of .jpg? It is a lossless format that preserves the precise bits in the colors.

How do I read in the .png files? Use the Picture data type, described in Section 3.1 of the textbook. You will only need the constructors and methods of the Picture API. To see it in action, look at the program which takes the name of a picture file as a command line argument, converts all pixels to gray, and displays all those pixels. See to see how to retrieve the r, g, and b values for the Color.

Sometimes my encrypted picture looks like a shadowy version of the original. How do I pick a good tap number? Here are suggested tap numbers for maximizing the cycle of different size linear feedback shift registers: 5-bit (tap 2), 6-bit (tap 4), 9-bit (tap 4), 10-bit (tap 6), 11-bits (tap 8), 20-bit (tap 16), 30-bit (tap 22), 36-bit (tap 24), 48-bit (tap 42), 60 bits (tap 58), 100 bits (tap 62), and 150 bits (tap 96).


Be sure to thoroughly test each piece of your code as you write it. We offer some suggestions in the assignment specification. You should also test your data type with other parameters.

Possible Progress Steps

These are purely suggestions for how you might make progress. You do not have to follow these steps.