Java DataStructures, HashMaps, Arrays, Cheat Sheet

// Java Data structures
// different approaches of storing, accessing and bending data to your will

//Recursion
// https://www.udemy.com/java-tutorial/learn/#/lecture/320992

public class Application {
    public static void main(String[] args) {

        // Ex: 5! (5 x 4 x 3 x 2 x 1);
        System.out.println(factorial(5));
    }

    private static int factorial(int value) {
        System.out.println(value);

        // If the value is 1, stop and just return 1.
        if (value == 1) {
            return 1;
        }

        // if the value is > 1, we will subtract it by 1 and multiple it by
        // itself.
        return factorial(value - 1) * value; // (5-1) * 5

    }
}


 //Results:
 5
 4
 3
 2
 1
 120


Explanation:
fact(5) = fact(4) * 5;
= fact(3) * 4 * 5;
=(fact(2) * 3) * 4;
=((fact(1) * 2) * 3) * 4;
=((1 * 2) * 3) * 4;









//ArrayList print out everything in the Array (iteration)

import java.util.ArrayList;

public class TemperaturesC {

    public static void main(String[] args) {

        ArrayList<Integer> weeklyTemperatures = new ArrayList<Integer>();
        weeklyTemperatures.add(78);
        weeklyTemperatures.add(67);
        weeklyTemperatures.add(89);
        weeklyTemperatures.add(94);
        weeklyTemperatures.add(2, 111);

    for (int j = 0; j < weeklyTemperatures.size(); j++){
    System.out.println(weeklyTemperatures.get(j));
  }
    }
}

//Resuts:
78
67
111
89
94


//For each: array
for (Integer grade : quizGrades){
    System.out.println(grade);
}

// In the example above, the colon (:) can be read as "in".
// The for each loop altogether can be read as "for each Integer element (called grade) in quizGrades, print out the value of grade."

import java.util.ArrayList;

public class TemperaturesForEach {
    public static void main(String[] args) {

        ArrayList<Integer> weeklyTemperatures = new ArrayList<Integer>();
        weeklyTemperatures.add(78);
        weeklyTemperatures.add(67);
        weeklyTemperatures.add(89);
        weeklyTemperatures.add(94);

        for (Integer temperature: weeklyTemperatures) {
            System.out.println(temperature);
        }

    }
}


//Hash maps: like looking through a dictionary. HashMaps.
// Like if I was suppose to find "Max" for example. I'll go to the "M" section, etc etc
// process is very similar to Arrays, except has <String, Integer> instead of just <Integer>.

import java.util.HashMap;

public class Restaurant {
    public static void main(String[] args) {
 HashMap<String, Integer> restaurantMenu = new HashMap<String, Integer>();

     //add items to the HashMap
   restaurantMenu.put("Turkey Burger", 13);
   restaurantMenu.put("Naan Pizza", 11);
   restaurantMenu.put("Cranberry Kale Salad", 10);

    //print out the price of the Naan Pizza
    System.out.println( restaurantMenu.get("Naan Pizza") );


    // Loop through and print out the whole menu:
    //The keySet method of HashMap returns a list of keys.
    for (String item : restaurantMenu.keySet() ) {
        System.out.println("A " + item + " costs " + restaurantMenu.get(item) + " dollars.");

    }
}


  //Results:
  11
  3
  A Turkey Burger costs 13 dollars.
  A Cranberry Kale Salad costs 10 dollars.
  A Naan Pizza costs 11 dollars.