4.9 ArrayList Traversals

1. Exam Points

• Traverse ArrayLists using loops (for, for-each, while).
• Use ArrayList methods while traversing ArrayLists.
• Use a simplified trace table to track changes of an ArrayList.

2. Knowledge Points

(1) ArrayList Traversals

• You may traverse an ArrayList using:
  • for loops (use indexes)
  • while loops (use indexes)
  • for-each loops (enhanced for loops) (no use of indexes)
• Attempting to access an index value outside of its range will result in an IndexOutOfBoundsException.
• 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);
  }
  

(2) Note

• Changing the size of an ArrayList while traversing it using an enhanced for loop can result in a ConcurrentModificationException.
• Therefore, you should not add or remove elements when using an enhanced for loop to traverse an ArrayList.

3. Exercises

• Practice
• Solution