8.8. ArrayList Summary

In this chapter you learned about ArrayLists which are dynamic re-sizable arrays. You learned how to declare and create ArrayLists, add and remove objects from them, set the object at an index, and get the object at an index.

List are like arrays in that you can store many objects of the same type in a list, just as you can in an array. Lists are different from arrays in that they can grow or shrink as needed. You can also add an element anywhere in a list and remove an element from any index. The first element in a list is at index 0 just like arrays.

Lists also differ from arrays in that you can have an array of any of the primitive types: int, double, or boolean, but you can only put objects in a list. You can use the wrapper classes Integer, Double, and Boolean to wrap a primitive value in an object so that you can put it in a list. Java will also do this automatically for you if you try to add a primitive value to a list or set a primitive variable to an item of a list. This is called autoboxing and unboxing.

8.8.1. Concept Summary

  • Autoboxing - Automatically wrapping a primitive type in a wrapper class object. For instance if you try to add an int value to a list, it will automatically be converted to an Integer object.

  • Abstract Method - A method that only has a declaration and no method body (no code inside the method).

  • ArrayList - An ArrayList can hold many objects of the same type. It can grow or shrink as needed. You can add and remove items at any index.

  • Add - You can add an object to the end of a list using listName.add(obj). You can add an object at an index of a list using add(index,obj). This will first move any objects at that index or higher to the right one position to make room for the new object.

  • Declaration - To declare an ArrayList use ArrayList<Type> name, where Type is the class name for the type of objects in the list. If you leave off the <Type> it will default to Object.

  • Creation - To create an ArrayList use new ArrayList<Type>() or new ArrayList<>(). In the latter case compiler will infer the type parameter from the declared type of the variable you are assigning to. There are other classes that implement the List interface, but you only need to know the ArrayList class for the exam.

  • Get - To get an object at an index from a list use listName.get(index).

  • Index - You can access and set values in a list using an index. The first element in a list called list1 is at index 0 list1.get(0). The last element in a list is at the length minus one - list1[list1.size() - 1].

  • Remove - To remove the object at an index use ListName.remove(index). This will move all object past that index to the left one index.

  • Set - To set the value at an index in a list use listName.set(index,obj).

  • Size - Use listName.size() to get the number of objects in the list.

  • Wrapper Class - Classes used to create objects that hold primitive type values like Integer for int, Double for double and Boolean for boolean.

  • Unboxing - Automatically converting a wrapper object like an Integer into a primitive type such as an int.

8.8.2. Vocabulary Practice

For more practice, see this Quizlet.

8.8.3. Common Mistakes

  • forgetting that set replaces the item at the index

  • forgetting that remove at an index moves all items that were to the right of that index left one index

  • forgetting that add at an index moves everything that was at the index and greater to the right one index

  • incrementing an index when looping through a list even though you removed an item from the list

  • using nameList[0] instead of nameList.get(0).

  • using nameList.length instead of nameList.size() to get the number of elements in a list

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