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.

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 code like if (c == '0').

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. However, it is possible to assign each element in your register array the same index that the corresponding 1 or 0 has in the initial String fill argument. Just make sure to adjust your tap position accordingly.

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.

I notice that toString() is not explicitly called in the test client code. What is the explanation?

      LFSR test1 = new LFSR("01101000010", 8);

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

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 be used to set the size of the register and the tap position.

I get an ArrayOutOfBounds or NullPointerException error. What could cause this? Do your constructors initialize all of the data members (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 read in the .png files? Use our data type, described in Section 3.1 of the textbook. You will only need the constructors and methods of the Picture API summarized on page 330.

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. If you download, you will also want to download Look at to see how to retrieve the r, g, and b values for the Color.


Be sure to thoroughly test each piece of your code as you write it. We offer some suggestions below.