Section 3: Data Structures

Lesson 1: Arrays and Collections

1.1 Working with Arrays

Arrays in Java are used to store multiple values of the same type. They have a fixed size and are indexed from 0 to length-1.

Example (Arrays):

public class ArraysExample {

    public static void main(String[] args) {

        // Declaration and initialization of an array

        int[] numbers = {1, 2, 3, 4, 5};

        // Accessing elements

        System.out.println("Element at index 2: " + numbers[2]);

        // Iterating through the array

        for (int i = 0; i < numbers.length; i++) {

            System.out.println("Element at index " + i + ": " + numbers[i]);

        }

    }

}

1.2 Introduction to Java Collections (ArrayList, HashMap)

Java Collections provide dynamic data structures that can store and manipulate data. Two commonly used collections are ArrayList and HashMap.

Example (ArrayList):

import java.util.ArrayList;

public class ArrayListExample {

    public static void main(String[] args) {

        // Creating an ArrayList

        ArrayList<String> fruits = new ArrayList<>();

        // Adding elements

        fruits.add("Apple");

        fruits.add("Banana");

        fruits.add("Orange");

        // Accessing elements

        System.out.println("Element at index 1: " + fruits.get(1));

        // Iterating through the ArrayList

        for (String fruit : fruits) {

            System.out.println("Fruit: " + fruit);

        }

    }

}

Example (HashMap):

import java.util.HashMap;

public class HashMapExample {

    public static void main(String[] args) {

        // Creating a HashMap

        HashMap<String, Integer> ages = new HashMap<>();

        // Adding key-value pairs

        ages.put("John", 25);

        ages.put("Alice", 30);

        ages.put("Bob", 22);

        // Accessing values

        System.out.println("Age of Alice: " + ages.get("Alice"));

        // Iterating through the HashMap

        for (String name : ages.keySet()) {

            System.out.println(name + "'s age: " + ages.get(name));

        }

    }

}

Lesson 2: Generics

2.1 Understanding Generic Types in Java

Generics allow you to create classes and methods that work with different data types while providing type safety.

Example (Generic Class): 

// Generic class

class Box<T> {

    private T content;

    public Box(T content) {

        this.content = content;

    }

    public T getContent() {

        return content;

    }

}

public class GenericClassExample {

    public static void main(String[] args) {

        // Creating a Box with Integer content

        Box<Integer> integerBox = new Box<>(42);

        // Creating a Box with String content

        Box<String> stringBox = new Box<>("Hello, Generics!");

        // Accessing content

        System.out.println("Integer Box Content: " + integerBox.getContent());

        System.out.println("String Box Content: " + stringBox.getContent());

    }

}

2.2 Using Generics in Classes and Methods

Generics can be used in both classes and methods to create flexible and reusable code.

Example (Generic Method):

// Generic method

class Printer {

    public <T> void printElement(T element) {

        System.out.println("Element: " + element);

    }

}

public class GenericMethodExample {

    public static void main(String[] args) {

        Printer printer = new Printer();

        // Calling the generic method with different data types

        printer.printElement(42);

        printer.printElement("Hello, Generics!");

        printer.printElement(3.14);

    }

}

With these data structures and the power of generics, you can efficiently handle collections of data in a type-safe manner. Practice these examples and explore the versatility of arrays, collections, and generics in Java.