9.29. List - Summary

In this chapter you learned about List and ArrayList. A list holds objects in order, and you can add objects to a list or remove objects from a list. You learned how to declare lists, create them, add objects, set the object at an index, and get the object at an index. The first element in a list is at index 0.

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.

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.

ArrayList is a Java class that implements the list interface using an array. An interface is a special kind of class that only has public abstract methods. An abstract method is one that only has a header and no body (no code). Other classes implement an interface by providing the code for the interface methods. You can think of an interface as specifying a contract and implementing classes agree to abide by the contract.

9.29.1. 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).
  • Interface - A special type of class that only has public and abstract methods. It is used to specify what a subclass needs to be able to do, not how it does it. One example is the List interface.
  • List - A list can hold many objects of the same type in order. It can grow or shrink as needed. You can add and remove items at any index.
  • List 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.
  • List Declaration - To declare a list use List<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.
  • List Creation - To create list use new ArrayList<Type>, where Type is the class name for the type of objects you want to store in the list. There are other classes that implement the List interface, but you only need to know the ArrayList class for the exam.
  • List Get - To get an object at an index from a list use listName.get(index).
  • List 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].
  • List 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.
  • List Set - To set the value at an index in a list use listName.set(index,obj).
  • List 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.

9.29.2. Practice

    Drag the item from the left and drop it on its corresponding answer on the right. Click the "Check Me" button to see if you are correct. Review the summaries above.
  • The index of the last element
  • size() - 1
  • The number of elements in the list
  • size()
  • The index of the first element
  • 0
  • The index of the second element
  • 1
    Drag the description from the left and drop it on the correct code on the right. Click the "Check Me" button to see if you are correct. Review the summaries above.
  • Declare an integer list named numList
  • List<Integer> numList = null;
  • Declare and create a list of strings named list1
  • List<String> list1 = new ArrayList<String>();
  • Declare and create a list of integers named list1
  • List<Integer> list1 = new ArrayList<Integer>();
  • Get the first object in a list named list1
  • list1.get(0);
  • Get the last object in a list named list1
  • list1.get(list1.size() - 1);
Next Section - 9.30. Code Practice with ArrayLists