Java Derusting Practice

src/ArrayPractice.java

@@ -1,18 +1,34 @@
 public class ArrayPractice {
   public static void main(String[] args) {
     // Create an array of Strings of size 4
+    String[] array = new String[4];
 
     // Set the value of the array at each index to be a different String
     // It's OK to do this one-by-one
+    array[0] = "Dog";
+    array[1] = "Cat";
+    array[2] = "Duck";
+    array[3] = "Wombat";
 
     // Get the value of the array at index 2
+    String index2Value = array[2];
+    System.out.println("Index 2 contains the word: " + index2Value);
 
     // Get the length of the array
+    int arrayLength = array.length;
+    System.out.println("Length of the array: " + arrayLength);
 
     // Iterate over the array using a traditional for loop and print out each item
+    System.out.println("FOR-LOOP:");
+    for (int i = 0; i < array.length; i++) {
+      System.out.println(array[i]);
+    }
 
     // Iterate over the array using a for-each loop and print out each item
-
+    System.out.println("FOR-EACH LOOP: ");
+    for (String word : array) {
+      System.out.println(word);
+    }
     /*
      * Reminder!
      * 

src/ListPractice.java

@@ -1,27 +1,50 @@
+
+import java.util.ArrayList;
+import java.util.Collections;
+import java.util.List;
+
 public class ListPractice {
 
 
   public static void main(String[] args) {
     // Create an empty ArrayList of Strings and assign it to a variable of type List
+    List<String> list = new ArrayList<>();
 
     // Add 3 elements to the list (OK to do one-by-one)
+    list.add("Dog");
+    list.add("Cat");
+    list.add("Duck");
 
     // Print the element at index 1
+    System.out.println("Element at index 1 is: " + list.get(1));
 
     // Replace the element at index 1 with a new value
     // (Do not insert a new value. The length of the list should not change)
+    list.set(1, "Wombat");
 
     // Insert a new element at index 0 (the length of the list will change)
+    list.add(0, "Goat");
 
     // Check whether the list contains a certain string
+    boolean containsCat = list.contains("Cat");
+    System.out.println("Does the list contain the word 'Cat'? " + containsCat); //false since the word was replaced
 
     // Iterate over the list using a traditional for-loop.
     // Print each index and value on a separate line
+    System.out.println("FOR-LOOP:");
+    for (int i = 0; i < list.size(); i++) {
+      System.out.println("Index " + i + ": " + list.get(i));
+    }
 
     // Sort the list using the Collections library
+    Collections.sort(list);
 
     // Iterate over the list using a for-each loop
     // Print each value on a second line
+    System.out.println("FOR-EACH LOOP:");
+    for (String word : list) {
+      System.out.println(word);
+    }
 
     /*
      * Usage tip!

src/MapPractice.java

@@ -1,28 +1,56 @@
-
+import java.util.HashMap;
+import java.util.Map;
 
 public class MapPractice {
   public static void main(String[] args) {
     // Create a HashMap with String keys and Integer values and
     // assign it to a variable of type Map
+    HashMap<String, Integer> maps = new HashMap<>();
 
     // Put 3 different key/value pairs in the Map
     // (it's OK to do this one-by-one)
+    maps.put("coin", 2);
+    maps.put("battery", 5);
+    maps.put("chair", 12);
 
     // Get the value associated with a given key in the Map
+    int batteryKey = maps.get("battery");
+    System.out.println("Value for battery: " + batteryKey);
 
     // Find the size (number of key/value pairs) of the Map
+    int mapSize = maps.size();
+    System.out.println("The Map size is: " + mapSize);
 
     // Replace the value associated with a given key (the size of the Map shoukld not change)
+    maps.put("battery", 10);
 
     // Check whether the Map contains a given key
+    String containsKey = "battery";
+    System.out.println("Contains '" + containsKey + "' key? " + maps.containsKey(containsKey));
 
     // Check whether the Map contains a given value
+    int containsValue = 4;
+    System.out.println("Contains '" + containsValue + "' value? " + maps.containsValue(containsValue));
 
     // Iterate over the keys of the Map, printing each key
+    System.out.println("KEYS:");
+    for (String key : maps.keySet()) {
+      System.out.println(key);
+    }
 
     // Iterate over the values of the map, printing each value
+    System.out.println("VALUES:");
+    for (int value : maps.values()) {
+      System.out.println(value);
+    }
 
     // Iterate over the entries in the map, printing each key and value
+    System.out.println("Here are entries in the Map" + " (key and its value)");
+    for (Map.Entry<String, Integer> entry : maps.entrySet()) {
+      String key = entry.getKey();
+      Integer value = entry.getValue();
+      System.out.println(key + ": " + value);
+    }
 
     /*
      * Usage tip!

src/NumberPractice.java

@@ -1,17 +1,28 @@
 public class NumberPractice {
   public static void main(String args[]) {
     // Create a float with a negative value and assign it to a variable
+    float negativeValue = -8.2f;
 
     // Create an int with a positive value and assign it to a variable
+    int positiveValue = 10;
 
     // Use the modulo % operator to find the remainder when the int is divided by 3
+    int remainder = positiveValue % 3;
+    System.out.println("The remainder when divided by 3: " + remainder); // output 1
 
     // Use the modulo % operator to determine whether the number is even
     // (A number is even if it has a remainder of zero when divided by 2)
     // Use an if-else to print "Even" if the number is even and "Odd"
     // if the number is odd.
+    if (positiveValue % 2 == 0) {
+      System.out.println("Even");
+    } else {
+      System.out.println("Odd");
+    }
 
     // Divide the number by another number using integer division
+    int number = positiveValue / 4;
+    System.out.println("Integer Division is: " + number);
 
     /*
      * Reminder!

src/Person.java

@@ -1,52 +1,69 @@
 /*
  * In this file you will follow the comments' instructions to complete
- * the Person class.
+ * the Person class. 
  */
 
 public class Person {
   // Declare a public String instance variable for the name of the person
   // Declare a private int instance variable for the age of the person
-
+  public String name;
+  private int age;
 
   // Create a constructor that takes the name and age of the person
   // and assigns it to the instance variables
-
+  public Person(String name, int age) {
+    this.name = name;
+    this.age = age;
+  }
 
   // Create a toString method that gives the name and age of the person
+  public String toString() {
+    return "Name: " + name + ", Age: " + age;
+  }
 
 
   // Implement the below public instance method "birthYear"
   // There should NOT be any print statement in this method.
   /**
    * birthYear returns the year the person was born.
    * 
-   * The birth year is calculated by subtracting the person's age from currentYear
+   * The birth year is calculated by subtracting  the person's age from currentYear
    * that's passed in as an int. It assumes that the person's birthday has already
    * passed this year.
    * 
    * @param currentYear an int for the current year
    * @return The year the person was born
    */
   // (create the instance method here)
+  public int birthYear(int currentYear) {
+    return currentYear - age;
+  }
 
 
   public static void main(String[] args) {
     // Create an instance of Person
-
+    Person person1 = new Person("Bud", 13);
     // Create another instance of Person with a different name and age and
     // assign it to a different variable
+    Person person2 = new Person("Nelie", 45);
 
     // Print the first person
+    System.out.println(person1);
 
     // Print the second person
+    System.out.println(person2);
 
     // Get the name of the first person and store it in a local variable
+    String firstName = person1.name;
 
     // Using the birthYear method, get the birth year of the first person
     // and store it in a local variable. Input the actual current year (e.g. 2025)
     // as the argument.
+    int currentYear = 2025;
+    int birthYear = person1.birthYear(currentYear);
 
     // In a separate statement, print the local variable holding the birth year.
+    System.out.println(firstName + "'s birth year is " + birthYear + ".");
 
     /**
      * Terminology!

src/SetPractice.java

@@ -1,17 +1,34 @@
+
+import java.util.HashSet;
+
 public class SetPractice {
   public static void main(String[] args) {
     // Create a HashSet of Strings and assign it to a variable of type Set
+    HashSet<String> wordSet = new HashSet<>();
 
     // Add 3 elements to the set
     // (It's OK to do it one-by-one)
+    wordSet.add("Good");
+    wordSet.add("Morning");
+    wordSet.add("Everyone");
 
     // Check whether the Set contains a given String
+    String checkString = "Hello";
+    boolean contains = wordSet.contains(checkString);
+    System.out.println("Does this set contains the word " + " '" + checkString + "'? " + contains);
 
     // Remove an element from the Set
+    wordSet.remove("Good");
 
     // Get the size of the Set
+    int size = wordSet.size();
+    System.out.println("Size of Set: " + size);
 
     // Iterate over the elements of the Set, printing each one on a separate line
+    System.out.println("Here are the elements in the set:");
+    for (String word : wordSet) {
+      System.out.println(word);
+    }
 
     /*
      * Warning!

src/StringPractice.java

@@ -1,25 +1,51 @@
+
+import java.util.ArrayList;
+
 public class StringPractice {
   public static void main(String[] args) {
     // Create a string with at least 5 characters and assign it to a variable
+    String word = "Computer";
 
     // Find the length of the string
+    int length = word.length();
+    System.out.println("Length of this word is: " + length);
 
     // Concatenate (add) two strings together and reassign the result
+    String word2 = "Table";
+    String addedWord = word + word2;
+    System.out.println("Concatenated Strings: " + addedWord);
 
     // Find the value of the character at index 3
+    char findChar = word.charAt(3);
+    System.out.println("Character at index 3 is: " + findChar);
 
     // Check whether the string contains a given substring (i.e. does the string have "abc" in it?)
+    boolean containsSubsting = word.contains("abc");
+    System.out.println("Does it contain 'abc'? " + containsSubsting); //false
 
     // Iterate over the characters of the string, printing each one on a separate line
+    System.out.println("-- Prints each letter in every line -- ");
+    for (int i = 0; i < word.length(); i++) {
+      System.out.println(word.charAt(i));
+    }
 
     // Create an ArrayList of Strings and assign it to a variable
+    ArrayList<String> strList = new ArrayList<>();
 
     // Add multiple strings to the List (OK to do one-by-one)
+    strList.add("Camera");
+    strList.add("Pen");
+    strList.add("Book");
 
     // Join all of the strings in the list together into a single string separated by commas
     // Use a built-in method to achieve this instead of using a loop
+    String allStrings = String.join(", ", strList);
+    System.out.println("Joined string: " + allStrings);
 
     // Check whether two strings are equal
+    String word3 = "Paper";
+    boolean areEqual = word.equals(word3);
+    System.out.println("Are they equal? " + areEqual);
 
     /*
      * Reminder!

toRefresh.md

@@ -2,4 +2,24 @@
 
 As you work through this exercise, write down anything that you needed to look up or struggled to remember here. It can be just a word or two (e.g. "joining strings"). You can use this as a guide of what to make extra sure you're refreshed on before exams and interviews.
 
-- 
+ListPractice 
+"https://www.geeksforgeeks.org/arraylist-set-method-in-java-with-examples/"
+- List<String> list = new ArrayList<>(); I always get confused how to set this up
+- get() function access the element
+- set() function that replaces a word in the list
+- contains(): https://www.geeksforgeeks.org/list-contains-method-in-java-with-examples/?ref=header_outind 
+- size(): https://stackoverflow.com/questions/4438710/using-collection-size-in-for-loop-comparison 
+- Collections.sort(): https://www.geeksforgeeks.org/collections-sort-java-examples/?ref=header_outind 
+
+StringPractice
+- How to find the value of character at index by using chartAt(): "https://stackoverflow.com/questions/11229986/get-string-character-by-index"
+- Joining all the strings by using join(): https://www.w3schools.com/java/ref_string_join.asp 
+
+SetPractice
+- remove(): https://www.w3schools.com/java/java_hashset.asp
+
+MapPractice
+- A good review for different methods - keySet, values, keys, boolean, and how to loop Map: https://www.w3schools.com/java/java_hashmap.asp
+
+Person
+- Almost forgot how to manually do this, there is also a shortcut that was shown in SDEV 217(?) before. Here is a good review: https://www.youtube.com/watch?v=VhErIOUHwuw