ADS-Lecture 01 Introduction to Java and Algorithm

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概述

Lecture 01 Introduction to Java and Algorithm

Introduction

What is this Course 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, and various command line tools.
- Java (not the focus of the course!)

Assumes solid foundation in programming fundamentals, including:

Object oriented programming, recursion, lists, and trees.

Why Study Algorithms or Data Structures?

Daily life is supported by them.
Major driver of current progress (?) of our civilization.
To become a better programmer.

“The difference between a bad programmer and a good one is whether [the programmer] considers code or data structures more important. Bad programmers worry about the code. Good programmers worry about data structures and their relationships.”

------ Linus Torvalds (Creator of Linux)
Being an efficient programmer means using the right data structures and algorithms for the job.
To understand the universe. Science is increasingly about simulation and complex data analysis rather than simple observations and clean equations.

Hello World!

Java and Object Orientation

public class HelloWorld {
public static void main(String[] args) {
System.out.println("hello world");
}
}
/*
1. In Java, all code must be part of a class.
2. Classes are defined with "public class CLASSNAME"
3. We use { } to delineate the beginning and ending of things.
4. We must end lines with a semicolon.
5. The code we want to run must be inside
public static void main(String[] args).
*/

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 main method using public static void main(String[] args)

*: This is not completely true, e.g. we can also declare interfaces in .java files that may contain code. We’ll cover these later.

Java and Static Typing

public class HelloNumbers {
public static void main(String[] args) {
int x = 0;
while (x < 10) {
System.out.println(x);
x = x + 1;
}
}
}
/*
1. Before Java variables can be used, they must be declared.
2. Java variables must have a specific type.
3. Java variable types can never change.
4. Types are verified before the code even runs!!!
*/

/** Demonstrates creation of a method 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));
}
}
/*
1. Functions must be declared as part of a class in Java.
A function that is part of a class is also called a "method".
So in Java, all functions are methods.
2. To define a function in Java, we use "public static".
We will see alternate ways defining functions later.
3. All parameters of a function must have a declared type,
and the return value of the function must have a declared type.
Functions in Java return only one value!
*/

Java is statically typed!

All variables, parameters, and methods must have a declared type.
That type can never change.
Expressions also have a type, e.g. larger(5, 10) + 3 has type int.
The compiler checks that all the types in your program are compatible before the program ever runs!
- e.g. String x = larger(5, 10) + 3 will fail to compile.
- This is unlike a language like Python, where type checks are performed DURING execution.

Reflections on Static Typing

The Good:

Catches certain types of errors, making it easier on the programmer to debug their code.
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: