Adam Blank Lecture 2 Winter 2019 CS 2. Introduction to Programming Methods

Transcription

1 Adam Blank Lecture 2 Winter 2019 CS 2 Introduction to Programming Methods

2 CS 2: Introduction to Programming Methods File I/O, Object Oriented Programming, and Lists

3 Questions From Last Time 1 What are static and void? We ll talk about this in much more detail today. What is setscale exactly? It changes the coordinate system of the drawing window. It s really not important to understand how we used it last lecture beyond that it allowed us to center the picture easily. How do lab partners work? You may choose your lab partner. If you can t find one, we will help you. How does grading work? See syllabus. Do I need a VM? How do I turn things in? This will all be covered in the first lab. How did the int become a String? What? Java provides a way to turn any variable into a String which gets called automatically. We ll cover this later.

4 Pictures 2

5 Outline 1 File I/O 2 Object Oriented Programming 3 Lists

6 Where We Are 3 Last Time Welcome to Java-land (home of the semicolons) We wrote small stand-alone programs, mostly directly in the main method We learned vaguely what the new keyword does This Time We will learn how to use file I/O in Java We will learn what all those pesky keywords (public, private, static, void) actually mean We will write our first object oriented program that represents a real-world object and learn the terminology associated with it

7 Scanner API 4 A Scanner object can be used to look through text from various sources such as Strings and Files. It is meant to be consumed piece by piece. Creating a Scanner Scanner scan = new Scanner("I am a String") Scanner scan = new Scanner(new File("file.txt")); Scanner console = new Scanner(System.in); Using a Scanner next() hasnext() nextline() hasnextline() nextint() hasnextint() nextdouble() hasnextdouble() Returns the next token or word Returns true iff another token exists Returns the next line Returns true iff another line exists Returns the next int Returns true iff another int exists Returns the next double Returns true iff another double exists

8 Scanner Examples 5 CAPITALIZE Capitalize every word of an input String and print each on its own line. For example, if the input string is: "I am a banana." Then, our program would output: OUTPUT >> I >> AM >> A >> BANANA. Profanity Filter Read in the contents of a file, line by line, and replace all instances of common curse words with less objectionable words. Example omitted from slides for... no reason.

9 Classes and Instances 6 Definitions A class is code that defines the properties and functionality of a type An instance is a realization of a class that can be manipulated Student Example A Student class might define... properties students have (e.g., name, alertness) actions students can take (e.g., a student can study, sleep) We can simultaneously have two actual students: Imagine a student named Eve who is currently very alert AND a student named Dave who is currently asleep Classes are really good at modelling real-world objects and systems.

10 Collections and Lists 7 Collections Collections store many pieces of data of the same type. In Java, collections are in the util package: import java.util.*; Different collections have different properties: Data ordered by indices Sorted data Data without duplicates etc. Lists A list is a collection of elements ordered by a 0-based index. It supports add/remove from anywhere! The size isn t fixed! There are multiple implementations; first, ArrayList

11 ArrayList API 8 Creating an ArrayList ArrayList<Integer> intlist = new ArrayList<Integer>(); ArrayList<String> stringlist = new ArrayList<String>(); Using a Scanner add(val) add(idx, val) get(idx) set(idx, val) remove(idx) size() Appends val to the end of the list Puts val at index idx; all elements at indices idx and larger get shifted forward Returns the value at index idx Replaces the value at index idx with val Removes and returns the value at index idx; all elements at higher indices get shifted backward Returns the number of elements in the list