CS 199 EMP

Even More Practice

Attendance Link:

https://forms.gle/8yto1LvJ4hntiKNu7

2 meetings every week Tuesday and Thursday

  • Early CST - Early risers and Eastern timezones

  • Evening CST - Western timezones (and really late Eastern folks)

  • Calendar contains zoom links

  • Attend 7 sessions for credit, attendence via google form.

Format

  • 3 practice problems every session

  • focused on practical understanding of concepts covered recently in class

  • slides and material available after class

How to follow along

  • Small breakout rooms of working together

  • Work for 10-15 minutes together, feel free to use paper, whiteboards, online share tools.

  • Use the code playground on the 125 homepage for the interactive running

Libraries and Imports

Libraries

  • Libraries are code other people write that you can use

  • Programs are shorter and easier to read.

  • They are also well tested so you don't have to worry as much about bugs.

  • We use import name.of.package

import

import java.util.Date;

var date = new Date();
System.out.println(date);
// print today's date and time

Documentation

  • to understand what a library is doing we use documentation and web search

  • For example for the last slide we can use Official Javadoc for Date

  • It's often helpful to look for example code of usage of the library and trying out simple examples first

External vs Java Standard Library

  • Java itself has an extensive library written by the language creators. To import them all you need to do is import java.whatever

  • External libraries are more complicated since you need to install them on your system. After that the import will look something like import com.fasterxml.jackson.databind.ObjectMapper;

Problem (10 minutes)

  • Import a class UUID from java.util. UUID is a form of unique id

  • Reading the documentation and other online sources to create a single UUID (either random or fixed amount) and print it out on the screen.

  • No starter code but also don't overthink it. This should be 3-4 lines of code.

Type Inference

  • Type inference is Java compiler smartly understanding data types without having to specify it.

  • At first glance it can seem similar to scripting languages like Python, JavaScript and Ruby which guess the data type.

  • Unike scripting languages, type inference is mathematically correct so it doesn't innvolve guess work and retains type safety.

  • This is especially helpful for people learning to program, since it makes code simpler and faster to write.

  • Becoming more popular in newer languages like C++, Rust, Kotlin, Swift, TypeScript

  • Note: Type inference is only for local variables not functions paramters, returns or class/instance variables.

  • To use type inference use var instead of the variable type.
var integer = 5; // int
var doubleBoi = 3.14159; // double
var intArray = new int[5]; // int[]
var array3d = new float[5][5][5]; // float[][][]
var randomObject = new StringBuilder(); // StringBuilder
var netid = "hdeep2"; // String
  • It can also infer the result of computations
var radiusWithoutBorder = Integer.parseInt("3"); // int
var radius = radiusWithoutBorder + 1; // int
var area = 3.14 * radius * radius; // double
System.out.println(area);

int[][][] array3d = {{{0, 1},
                      {1, 0}},
                     {{1, 0},
                      {0, 1}}};  
var sum = 0;
// useful in for loops
for (var matrix: array3d) { // int[][]
  for (var vector: matrix) { // int[]
    for (var num: vector) {
      sum += num;
    }
  }
}
System.out.println(sum);

Problem (10 minutes)

  • Convert the following class to use as much type inference as possible

  • Remember that local variables can use var but instance+class variables and function params+returns can't

class Sphere {
  public double r = 0.0;
  private static double pi = 3.14159;
  
  double volume() {
    double frac = 4.0 / 3;
    double cubed = r * r * r;
    return frac * pi * cubed;
  }
  
  double increaseRadius(int inc) {
    double oldRadius = r;
    r = r + inc;
    return oldRadius;
  }
}

Serialization

Deserialization problem

  • Note: The Sept 24 Covers CSV Deserialization (Parsing) as well as two different homework problems.

  • Feel free to use that for practice.

  • Here I will focus on serialization

Serialized Data Formats

  • Over the internet and communicating between applications, we use serialized data formats to talk between programs and computers. Often between different programming languages too.

  • The data format is just text, so often represented by String.

  • Each format has it's own rules and limitations, for example CSV has each entry in a new line and fields seperated by commas.

  • Popular formats exist like JSON, CSV, XML, YAML.

  • Libraries exist for all these formats, and usually you'd just use those.

Serialization

  • Process of converting data in our program to some format so we can transmit it somewhere.

  • In Java it can mean converting some variable or object into the data format String

Deserialization

  • It's the reverse process of taking the data (in JSON, CSV, YAML etc.) and parsing it for our program.

  • This means taking the String input and converting it to our variables and objects.

Problem (10 minutes)

  • Serialize an array of QuizScore into CSV

  • Return it as a String.

  • You can use + to add Strings together.

public class QuizScore {
  String name;
  int score;
  public QuizScore(String setName, int setScore) {
    name = setName;
    score = setScore;
  }
  
  public static String toCSV(QuizScore[] scores) {
    return null; // TODO:
  }
}

QuizScore[] results = {new QuizScore("Quiz 1", 97), new QuizScore("Quiz 2", 9)};
System.out.println(QuizScore.toCSV(results));

Solutions

Spoilers

1)

import java.util.UUID;

var date = UUID.randomUUID();
System.out.println(date);

2)

class Sphere {
  public double r = 0.0;
  private static double pi = 3.14159;
  
  double volume() {
    var frac = 4.0 / 3;
    var cubed = r * r * r;
    return frac * pi * cubed;
  }
  
  double increaseRadius(int inc) {
    var oldRadius = r;
    r = r + inc;
    return oldRadius;
  }
}

public class QuizScore {
  String name;
  int score;
  public QuizScore(String setName, int setScore) {
    name = setName;
    score = setScore;
  }
  public static String toCSV(QuizScore[] results) {
    var csv = "";
    for (var result: results) {
      var lineCSV = result.name + ", " + result.score + "\n";
      csv += lineCSV;
    }
    return csv;
  }
}