4.8 ArrayList Methods

1. Exam Points

• ArrayList methods: get(), size(), add(), remove(), set()
• For an ArrayList, indexes are 0 to size -1 .
• For the add(int index, E obj) method, 0 <= index <= size.
• Create an ArrayList: ArrayList<E> list=new ArrayList<E>();
• When you try to remove elements in a loop, the size and i will change.
• When you try to remove elements in an ArrayList, remove reversely.
• Remember to compare String values using the equals() method.

2. Knowledge Points

(1) ArrayList vs. Arrays

• An array is a collection with a fixed length (number of elements), an array can store both primitive values or object references.
• Example:
  int[] nums = new int[10]; Dog[] dogs = new Dog[20];
• An ArrayList is mutable in size and contains only object references.
• Example:
  ArrayList list1 = new ArrayList(); ArrayList list2 = new ArrayList(); ArrayList list3 = new ArrayList(); ArrayList list4 = new ArrayList();

(2) Create ArrayList Objects

• The ArrayList constructor ArrayList() constructs an empty list.

• Java allows the generic type ArrayList<E>, where the type parameter E specifies the type of the elements.

• Example:
  

  // For an ArrayList of type Integer, E is Integer
  ArrayList<Integer> list1 = new ArrayList<Integer>(); 
  list1.add(10);
  Integer x = list1.get(0);
  
  // For an ArrayList of type Dog, E is Dog
  ArrayList<Dog> list2 = new ArrayList<Dog>(); 
  list2.add(new Dog("Lucky"));
  Dog dog = list2.get(0);
  
  

• ArrayList<E> is preferred over ArrayList, E represents a data type.

• Example:
  // Without specifying E, an ArrayList can contain objects of any type. ArrayList list1 = new ArrayList(); list1.add(10); list1.add(true); list1.add("Annabelle");

  // With specifying E, an ArrayList can contain only objects of type E.
  ArrayList<Integer> list2 = new ArrayList<Integer>(); 
  list2.add(10);
  list2.add(20);
  

(3) ArrayList Methods

• The indexes for an ArrayList are 0 to size - 1.
• ArrayList Methods:
  • int size(): returns the number of elements in the list.
  • boolean add(E obj): appends obj to end of list.
  • void add(int index, E obj): inserts obj at position index (0 <= index <= size).
  • E get(int index): get the element at position index in the list.
  • E set(int index, E obj): replaces the element at position index with obj; returns the former element at position index.
  • E remove(int index): removes element from position index; returns the removed element.
• Example:
  

  ArrayList<Integer> list1 = new ArrayList<Integer>();
  list1.add(10);
  list1.add(20);
  
  ArrayList<String> list2 = new ArrayList<String>();
  list2.add("Amy");
  list2.add("Anna");
  list2.add("David");
  
  // Note: The indexes for an ArrayList start at 0 and end at size - 1.
  
  // 1. int size()
  int size = list1.size();
  
  // 2. boolean add(E obj): add one element to the end of the list
  list1.add(30);
  list2.add("Judy");
  
  // 3. void add(int index, E obj)
  list1.add(0, 666);
  
  // 4. E get(int index)
  int x = list1.get(0);
  String y = list2.get(1); // 1 means the second element
  
  // 5. E set(int index, E obj): replace the element at a specified position
  list2.set(1, "Annabelle");
  
  // 6. E remove(int index)
  list2.remove(0);
  
  

(4) Traverse ArrayLists

• You can traverse an ArrayList using for, for-each, while loops.
• Example: Traverse ArrayLists using for loops
  

  
  ArrayList<Integer> list1 = new ArrayList<Integer>();
  list1.add(10);
  list1.add(20);
  list1.add(30);
  list1.add(40);
  
  ArrayList<String> list2 = new ArrayList<String>();
  list2.add("Amy");
  list2.add("Anna");
  list2.add("David");
  
  ArrayList<Dog> dogList = new ArrayList<Dog>();
  dogList.add(new Dog("haha"));
  dogList.add(new Dog("lele"));
  dogList.add(new Dog("doudou"));
  
  // for loop: use indexes
  for (int i = 0; i < list1.size(); i++) {
      Integer x = list1.get(i);
      System.out.print(x + " ");
  }
  
  System.out.println();
  
  for (int i = 0; i < list2.size(); i++) {
      String x = list2.get(i);
      System.out.print(x + " ");
  }
  
  System.out.println();
  
  for (int i = 0; i < dogList.size(); i++) {
      Dog d = dogList.get(i);
      System.out.println(d.getName());
  }
  

• Example: Traverse ArrayLists using for-each loops
  

  // for-each loop: no need to use indexes
  
  for (Integer x : list1) { // x means the current element
      System.out.print(x + " ");
  }
  
  System.out.println();
  
  for (String name : list2) { // x means the current element
      System.out.print(name + " ");
  }
  
  for(Dog x:dogList) {
      System.out.println(x.getName());
  }
  

(5) ArrayLists as Parameters

• An parameter can be of ArrayList type.
• A returned value from a method can be of ArrayList type.
• If you pass an ArrayList object as an argument to a parameter of a method, changes will reflect on the ArrayList.
• Example:
  

  static void replace(ArrayList<Integer> list, int index, int value) {
      list.set(index, value);
  }
  
  public static void main(String[] args) {
  
      ArrayList<Integer> list1 = new ArrayList<Integer>();
      list1.add(1);
      list1.add(2);
      
      // if you pass an ArrayList object as an argument to a parameter 
      // of a method, changes will reflect on the ArrayList.
      
      System.out.println(list1);
      replace(list1, 0, 666);
      System.out.println(list1);
  }
  

(6) Remove Elements in Loops

• If you try to remove elements in a loop, remove the elements reversely.
• Example:
  

  
  ArrayList<Integer> list = new ArrayList<Integer>();
  list.add(-10);
  list.add(20);
  list.add(30);
  list.add(-40);
  
  // Example: remove negative elements in an ArrayList
  for(int i=list.size()-1;i>=0;i--) {
      if (list.get(i) < 0) {
          list.remove(i);
      }
  }
  

3. Exercises

• Practice
• Solution