Completed Files & added Hackerrank Section

src/main/java/Collections/Practice/CollectionBasics.java

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+package Collections.Practice;
+
+import java.util.ArrayList;
+import java.util.Collection;
+
+public class CollectionBasics {
+    public static void main(String[] args) {
+
+        Collection<Integer> numbers = new ArrayList<>();
+
+        numbers.add(4);
+        numbers.add(8);
+        numbers.add(15);
+        numbers.add(16);
+        numbers.add(23);
+        numbers.add(42);
+
+        System.out.println("Sum of numbers: " + sum(numbers));
+        System.out.println("Count of even numbers: " + countEven(numbers));
+        System.out.println("Largest number: " + findMax(numbers));
+        System.out.println("Contains duplicates? " + hasDuplicates(numbers));
+    }
+
+
+    /*
+     PROBLEM 1
+     Return the sum of all numbers in the collection
+    */
+    public static int sum(Collection<Integer> numbers) {
+
+        int total = 0;
+
+        // TODO:
+        // Loop through the collection
+        // Add each number to total
+
+        return total;
+    }
+
+
+    /*
+     PROBLEM 2
+     Count how many numbers are even
+    */
+    public static int countEven(Collection<Integer> numbers) {
+
+        int count = 0;
+
+        // TODO:
+        // Loop through the collection
+        // If the number is even, increase count
+
+        return count;
+    }
+
+
+    /*
+     PROBLEM 3
+     Find the largest number in the collection
+    */
+    public static int findMax(Collection<Integer> numbers) {
+
+        int max = Integer.MIN_VALUE;
+
+        // TODO:
+        // Loop through numbers
+        // Update max if current number is larger
+
+        return max;
+    }
+
+
+    /*
+     PROBLEM 4
+     Return true if the collection contains duplicates
+     Return false otherwise
+    */
+    public static boolean hasDuplicates(Collection<Integer> numbers) {
+
+        // TODO:
+        // Hint:
+        // Compare the size of a collection with the size of a Set
+
+        return false;
+    }
+
+
+    /*
+     PROBLEM 5
+     Count how many times a target value appears
+    */
+    public static int countOccurrences(Collection<Integer> numbers, int target) {
+
+        int count = 0;
+
+        // TODO:
+        // Loop through numbers
+        // If number equals target, increase count
+
+        return count;
+    }
+
+
+    /*
+     BONUS PROBLEM
+     Create and return a new collection
+     that only contains numbers greater than 20
+    */
+    public static Collection<Integer> filterGreaterThanTwenty(Collection<Integer> numbers) {
+
+        Collection<Integer> result = new ArrayList<>();
+
+        // TODO:
+        // Loop through numbers
+        // Add numbers greater than 20 to result
+
+        return result;
+    }
+}

src/main/java/Collections/Practice/CommonMethodsDemo.java

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+package Collections.Practice;
+
+import java.util.ArrayList;
+import java.util.Collection;
+import java.util.List;
+
+public class CommonMethodsDemo {
+    public static void main(String[] args) {
+
+        System.out.println("=== COLLECTION COMMON METHODS DEMO ===\n");
+
+        /*
+         IMPORTANT CONCEPT:
+         We are using the Collection interface as the reference type.
+         The object type is ArrayList.
+        */
+
+        Collection<String> fruits = new ArrayList<>();
+
+        // ------------------------------
+        // add()
+        // ------------------------------
+
+        System.out.println("Adding elements using add()");
+
+        fruits.add("Apple");
+        fruits.add("Banana");
+        fruits.add("Orange");
+        fruits.add("Mango");
+
+        System.out.println("Fruits: " + fruits);
+
+
+        // ------------------------------
+        // size()
+        // ------------------------------
+
+        System.out.println("\nChecking size of collection");
+
+        int size = fruits.size();
+        System.out.println("Size: " + size);
+
+
+        // ------------------------------
+        // contains()
+        // ------------------------------
+
+        System.out.println("\nChecking if collection contains certain items");
+
+        boolean hasApple = fruits.contains("Apple");
+        boolean hasGrape = fruits.contains("Grape");
+
+        System.out.println("Contains Apple? " + hasApple);
+        System.out.println("Contains Grape? " + hasGrape);
+
+
+        // ------------------------------
+        // remove()
+        // ------------------------------
+
+        System.out.println("\nRemoving an element");
+
+        fruits.remove("Banana");
+
+        System.out.println("After removing Banana:");
+        System.out.println(fruits);
+
+
+        // ------------------------------
+        // addAll()
+        // ------------------------------
+
+        System.out.println("\nAdding multiple elements using addAll()");
+
+        List<String> moreFruits = new ArrayList<>();
+        moreFruits.add("Pineapple");
+        moreFruits.add("Strawberry");
+        moreFruits.add("Peach");
+
+        fruits.addAll(moreFruits);
+
+        System.out.println("After addAll:");
+        System.out.println(fruits);
+
+
+        // ------------------------------
+        // Iterating through Collection
+        // ------------------------------
+
+        System.out.println("\nLooping through collection:");
+
+        for (String fruit : fruits) {
+            System.out.println(fruit);
+        }
+
+
+        // ------------------------------
+        // isEmpty()
+        // ------------------------------
+
+        System.out.println("\nChecking if collection is empty");
+
+        boolean empty = fruits.isEmpty();
+        System.out.println("Is empty? " + empty);
+
+
+        // ------------------------------
+        // clear()
+        // ------------------------------
+
+        System.out.println("\nClearing the collection");
+
+        fruits.clear();
+
+        System.out.println("After clear:");
+        System.out.println(fruits);
+
+
+        System.out.println("\nCheck if empty after clear:");
+
+        System.out.println("Is empty? " + fruits.isEmpty());
+
+
+        // ------------------------------
+        // TODO Exploration Section
+        // ------------------------------
+
+        /*
+         TODO 1:
+         Create a new Collection called numbers
+         Add the following values:
+         10, 20, 30, 40, 50
+        */
+
+
+        /*
+         TODO 2:
+         Print the size of the numbers collection
+        */
+
+
+        /*
+         TODO 3:
+         Check if the collection contains 30
+        */
+
+
+        /*
+         TODO 4:
+         Remove the number 20
+        */
+
+
+        /*
+         TODO 5:
+         Loop through the numbers collection
+         and print each value
+        */
+
+
+        /*
+         REFLECTION QUESTIONS:
+
+         1. Why can we use Collection as the reference type?
+         2. What methods are available because of the Collection interface?
+         3. What methods are NOT available when using Collection instead of List?
+         */
+    }
+
+}