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.
• Do not add or remove elements inside a for-each loop, it is not allowed.
• If you try to remove elements inside a loop, use a for loop, and remove reversely.
• If you try to remove elements meeting a certain condition from the first to the last in a for loop, then it only works when qualified elements are not adjacent, for two adjacent elements, only the first one will be deleted.
• Note: pay attention how i changes: i++, i--, i+=2

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());
  }
  

(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.
• (Warning) If you try to remove elements that meet a certain condition from the first element to the last inside a for loop, then your algorithm will only work when all the qualified elements are not adjacent, and for two adjacent elements, only one of them will be deleted.
• Pay attention how i is changed: i++, i--, i+=2

3. Exercises

• Practice
• Solution