I was curious to learn what has been updated since I was a teaching assistant for COS 126, so I am reading through the lecture slides for the Spring 2025 semester

I use these notes to keep track of thoughts I have during reading, and anything that sticks out to me as interesting. My notes are incomplete as I have not yet finished reading the slides

0. Introduction

  • The everything is correctly in quotes in the statement “ ‘Everything’ can be encoded as a sequence of bits ” because continuous things, like real numbers, cannot always be encoded in a finite sequence of bits

  • The class continues to use Java as it did when I was a teaching assistant
    The upside is that Java’s verbosity and strong typing make it very explicit what you are doing, and provide a strong foundation of understanding. The downside is that those same features make Java a more difficult first language\ Some universites, like Cornell, use a looser language like Python for the first programming course to get students coding quickly, then Java for the second to learn the deeper details of programming
    I don’t know which system works best—it probably depends on the student

0. Hello, world

  • Machine language, natural language, and high-level programming language are contrasted, and it is pointed out that natural language is getting more understandable by computers since I was a teaching assistant for the course

0. Built-in data types

  • “ A data type (type) is a set of values and a set of operations on those values ” is a key concept in programming languages

  • Java overloading the division symbol (/) to perform both integer and floating point division causes confusion

  • The standard warning not to use equality testing (==) with floating point numbers is presented

  • Java is statically typed: the types of expressions are determined at compile time, and the compiler will use the type system to catch common errors

  • Java will concatenate (+) strings with other types automatically (e.g. using the toString method for objects)

  • Explicitly casting floating point numbers to integers rounds down

1. Conditionals

  • Local variables declared in a Java code block are not accessible outside the block

  • Even if the body of an if statement only has a single statement, it is recommended to put that single statement in a code block, which is good advice.

1. Loops

  • Java’s for-each / enhanced-for statements are not explored, because which limits the power of for statements, but avoids talking about Collections this early in the class.

2. Arrays

  • The example double[] a = new int[10]; is used to demonstrate a compile-time type mismatch error.
    What’s interesting is that arrays of reference types in Java are covariant, allowing Object[] a = new String[10];. However, this will cause a runtime ArrayStoreException if you try to store anything other than a String in array a.
    When generics (List<T>) were added to Java, they were correctly invariant–List<Object> l = new ArrayList<String>(); will fail to compile with a type-mismatch error.
  • I was uncertain if the shuffle algorithm presented was correct, because it was different from the traditional presentation, but apparently it is indeed correct so that was cool to learn

3. Functions

  • Functions can only return a single item. Unlike in languages that have tuples built-in, if you want to return more information from a function, you can create a class to hold the information, or you can use the syntactic sugar of record classes
  • Java chooses which overloaded function to call based on the arguments and their types, not based on the return type
  • The scope of a Java variable is until the end of the block in which it is declared

3. Libraries & Clients

4. Recursion

  • Fun use of M.C. Escher’s Circle Limit IV (Heaven and Hell)
  • Dynamic programming can be top-down memoization, or can build the solutions from the bottom-up. Top-down can be simpler to program (just add memoization to existing recursive code), but is usually slower because still involves recursive calls.

4. Performance

  • Algorithms topics focus on system-independent effects, while systems topics focus on system-dependent effects like hardware, systems software, and operating system. I always found the latter super-interesting
  • Reminder that (n choose k) = n!/k!(n-k)! = (n*…*(n-k+1))/k! This definition often comes in useful
  • There used to be confusion over the size of a kilobyte: 10^3 (1,000) or 2^10 (1,024). Now kilobytes are 10^3 (1,000) bytes, while kibibytes are 2^10 (1,024) bytes. The same holds for other prefixes: mega / mebi , giga / gibi …
  • In Java, bytes are 8 bits, short : 16 bits, int : 32 bits, long : 64 bits, float : 32 bits, double : 64 bits, char : 16 bit unicode, boolean s use an unspecified number of bits