Overview

Repeat
  Repeat
    You ask questions
    I attempt answers
  Until we run out of time or you run out of questions
  Repeat
    I ask questions
    You attempt answers
    I provide followup comments
  Until we run out of time or you start asking questions again
Until we run out of time

Sam: What do you see as the difference between int, Integer and BigInteger?

• Student: Boy Sam, that's in the reading. I know that one.

Student: So, how does BigInteger work?

• Yay! We distinguish implementation from interface. You don't have to know how it accomplishes its task.
• And that's normal. Do you know how 3 * 12 works in C (or on the chip)?
  • You'll learn this in 211.
• But we can hypothesize, and we could implement our own ArbitraryLengthInteger class
• Implementation questions
  • Linked or area of memory (array)?
  • What fields do we need?
  • Other general principles?
• An integer is just a sequence of digits (plus a sign)
• To add: Starting from the right of each sequence, add each digit, keeping track of the carry
• To multiply: You know the algorithm!
• Of course, BigIntegers probably use unsigned ints for each digit, rather than a number in the range 0-9.

Sam: What's the most confusing part of 207? Student: Organizing all the files. Testing.

• It's somewhat painful that we have so many things to keep track of.
  • GitHub repository
  • Directory on your filesystem
  • Eclipse Project
  • One or more packages
  • One or more classes
• Should be a one-one correspondence between the first three.
  • Three different ways of thinking about the same set of files.

• Package

  • Logical grouping of classes
  • Encapsulation idea: Different groups for different purposes
    • One set for my Ushahidi code edu.grinnell.rebelsky.ushahidi
    • Another set for string utilities (or all utilities) edu.grinnell.rebelsky.utils
  • Encapuslation enforcement: Java's protection levels let you say "All classes in the same package can access a (field, method, constructor, class, etc.); things in different classes can't"

  • E.g.

    package foo; public class Foo { int x; ...; } // class Foo

    package foo; public class Bar { ... f = new Foo(...); f.x = 3; // permitted } // class Bar

    package qux; import foo.Foo; public class Qux { ... f = new Foo(...); f.x = 3; // not permitted } // Qux

  • As the example above suggests, classes in one package can still refer to classes in another package, but you need to import or use the full name.
  • We also make packages to avoid problems with repeated names.

What does the import foo.Foo or import java.io.PrintWrinter mean?
(Or any import.)

• It doesn't quite mean what it sounds like (which is "it sounds like C's include")
• It really means "I don't want to have to give the fully qualified class name for Foo", so accept that the class name implies the fully qualified class name.
• I don't want to type java.io.PrintWriter pen = new java.io.PrintWriter(...)
  • This lets me type PrintWriter pen = new PrintWriter(...)
• We can't use import for two classes with the same base name. import edu.grinnell.walker.Utilities; import edu.grinnell.rebelsky.Utilities;
  • Ambiguious! Ambiguity is bad. Java/Eclipse will complain.
• For this reason, Sam discourages import java.io.*;.

Can you have more than one class in a Java file?

• You can have only one public class in a Java file.
• Many pundits discourage you from putting multiple classes in a file (with a few exceptions, such as anonymous classes or inner classes)

How should we deal with parsing BigIntegers?

• new BigInteger(string)

Advice about testing?

• Start with known result and work backwards to generate input.
  • E.g., to test average, pick a random number, r, and another smaller random number, s, then use r+s and r-s.