Lesson 3: Data Structure - Lists

Goals

Understand the difference between Arrays and Lists
Discuss different kind of Lists
Declare & Initialise ArrayList and Access its elements
Code, code, code 🤩

Recap & Assignment check

Let’s look into the assignments from Class 3

Lists

In Java, we need to declare the size of an array before we can use it. Once the size of an array is declared, it’s not possible to change it. What if we don’t know how many elements will be in our array?

To handle this issue, we can use the ArrayList data structure. Unlike arrays, an ArrayLists can automatically adjust its capacity when we add or remove elements from it.

An ArrayList is an ordered collection (also known as a sequence). The user of this collection has precise control over where in the list each element is inserted. The user can access elements by their integer index (position in the list), and search for elements in the list.

ArrayList contains the following important methods:

class ArrayList<E>{
  // Adds the element to the end of the list increasing its size by 1.
  boolean add(E element){/*...*/}
  // Adds the element to position #index of the list increasing its size by 1.
  boolean add(int index, E element){/*...*/}
  // Replaces the element at position #index, does not increase the size.
  boolean set(int index, E element){/*...*/}
  // Returns the element at #index
  E get(int index){/*...*/}
  // Removes the element at #index and return that element.
  E remove(int index){/*...*/}
  // Removes the first occurrence of the specified element from this list, 
  // if it is present (optional operation).
  boolean remove(E element){/*...*/}
  // Returns the current size of the list
  int size(){/*...*/}
}

Question; What other methods can an ArrayList have?

Declare & Initialise ArrayList

// create Integer type arraylist
ArrayList<Integer> arrayList = new ArrayList<>();

// create String type arraylist
ArrayList<String> arrayList = new ArrayList<>();

Note: We can not create array lists of primitive data types like int, float, char, etc. Instead, we have to use their corresponding Wrapper class (Integer, Float, Boolean, etc).

Methods of ArrayList

Add elements

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

// Add elements
animals.add("Dog");
animals.add("Cat");
animals.add("Horse");
System.out.println("ArrayList: " + animals);

Add Using index number

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

// Add elements
animals.add(0,"Dog");
animals.add(1,"Cat");
animals.add(2,"Horse");
System.out.println("ArrayList: " + animals);

Access ArrayList elements

// Get the element from the array list
String str = animals.get(0);
System.out.print("Element at index 0: " + str);

Access ArrayList elements in foreach loop

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

for (String animal: animals) {
    System.out.println(animal);
}

Change ArrayList Elements

animals.set(2, "Zebra");
System.out.println("Modified ArrayList: " + animals);

Remove ArrayList Elements

String str = animals.remove(2);
System.out.println("Final ArrayList: " + animals);
System. out.println("Removed Element: " + str);

Java ArrayList To Array

// Create a new array of String type
String[] arr = new String[animals.size()];

// Convert ArrayList into an array
animals.toArray(arr);

Java Array to ArrayList

// Create an array of String type
String[] arr = {"Dog", "Cat", "Horse"};

// Create an ArrayList from an array
ArrayList<String> animals = new ArrayList<>(Arrays.asList(arr));

Question; What other kind of Lists do we have?

Question; How do you find out whether the given List is empty or not?

Exercises and Assignments

Please download the assignment from GitHub Classroom.

Materials

Assignment Solutions