Lecture 1: Welcome to 61B

8/26/2020

Hello World

# In Python
print("hello world")

// In Java
public class HelloWorld {
    public static void main(String[] args) {
        System.out.println("hello world");
    }
}

// Output:
hello world

• In Java, all code must be part of a class
• Classes are defined with public class CLASSNAME
• We use { } to delineate the beginning and ending of things
• We must end lines with a semicolon
• The code we want to run must be inside public static void main(String[] args)

# In Python
x = 0
while x < 10:
    print(x)
    x = x + 1

// In Java
public class HelloNumbers {
    public static void main(String[] args) {
        int x = 0;  // Must declare variables, variable types can never change
        while (x < 10) {
            System.out.println(x);
            x = x + 1;
        }
    }
}

• Before Java variables can be used, they must be declared
• Java variables must have a specific type
• Java variable types can never change
• Types are verified before the code even runs!!!

# In Python
def larger(x, y):
    """Returns the larger of x and y"""
    if (x > y):
        return x
    return y

print (larger(-5, 10))


// In Java
public class LargerDemo {
    /** Returns the larger of x and y. */
    public static larger(int x, int y) {
        if (x > y) {
            return x;
        }
        return y;
    }

    public static void main(String[] args) {
        System.out.println(larger(-5, 10));
    }
}

• Functions must be declared as part of a class in Java. A function that is part of a class is called a "method". So in Java, all functions are methods.
• To define a function in Java, we use "public static". We will see alternate ways of defining functions later.
• ALL parameters of a function must have a declared type, and the return value of a function must have a declared type.
• Functions in Java return only one value!

Java and Object Orientation

• Java is an object oriented language with strict requirements:
  • Every Java file must contain a class declaration
  • All code lives inside a class, even helper functions, global constants, etc.
  • To run a java program, you typically define a min method using public static void main(String[] args)

Java and Static Typing

• Java is statically typed!
  • All variables, parameter,s and methods must have a declared type
  • That type can never change
  • Expressions also have a type
  • The compiler checks that all the types in your program are compatible before the program ever runs!
    • This is unlike Python, where type checks are performed DURING execution

Reflections on Static Typing

• The Good:
  • Catches certain types of errors, making debugging easier
  • Type errors can (almost) never occur on the end user's computer
  • Makes it easier to read and reason about code
  • Code can run more efficiently, e.g. no need to do expensive runtime type checks
• The Bad:
  • Code is more verbose
  • Code is less general (functions can only be applied to inputs of certain types)
    • There is a way around this in Java (generics)

Welcome to 61B 2019

What is 61B About?

• Writing code that runs efficiently
  • Good algorithms
  • Good data structures
• Writing code efficiently
  • Designing, building, testing, and debugging large programs
  • Use of programming tools
    • git, IntelliJ, JUnit
  • Java

Why Study Algorithms or Data Structures?

• Daily life is supported by them

Why Study Algorithms or Data Structures?

• Major driver of current progress of our civilization
• Self-driving cars
• AlphaGo
• To become a better programmer
• Being an efficient programmer means using the right data structures and algorithms for the job

Why Study Algorithms or Data Structures?

• To understand the universe. Science is increasingly about simulation and complex data analysis rather than simple observations and clean equations.
• To create beautiful things
• As an end unto itself