CollectionBasics.java

package Collections.Practice;

import java.util.*;

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
        for(int num: numbers) {
            total += num;
        }
        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

        for(int num: numbers) {
            if(num % 2 == 0) {
                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

        for(int num: numbers) {
            if(num > max) {
                max = num;
            }
        }
        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
        HashSet<Integer> set = new HashSet<>();

        for(int num: numbers) {
            //if false, then return true
            if(!set.add(num)) {
               return true;
            }
        }

        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

        for(int num : numbers) {
            if(num == target) 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

        Iterator<Integer> iterator = numbers.iterator();

        while (iterator.hasNext()) {
            int element = iterator.next();
            if (element > 20) {
                result.add(element);
            }
        }
        return result;
    }
}