Head First Java Chapter 16 Notes

Head First Java Chapter 16

This is the Sixteenth Chapter learning of Head First Java by Kathy Sierra and Bert Bates,
This markdown file is my notes for the Sixteenth chapter, not sure if any copyright issues are there.

Collections with Generics


  • ArrayList does not have a sort method.
  • ArrayList is not the only Collection
  • TreeSet : Keeps the elements sorted and prevents duplicates.
  • HashMap : Store and Access elements as name/value pairs.
  • LinkedList : Gives better performance when you insert or delete elements from middle.
  • HashSet : Prevents duplicates in the collection, and given an element, can find that element quickly.
  • LinkedHashMap : It remembers the order in which elements were inserted in addition to HashMap.
  • To get the sort functionality from ArrayList we can change it to a TreeSet
  • BUT TreeSet might be more expensive. Every insert into a TreeSet requires hugh time to figure out where it should reside.


static <T extends Comparable<? super T>> void     sort(List<T> list)
  • Sorts the specified list into ascending order, according to the natural ordering of its elements.
  • The sort method declaration looks little strange. Because the Collection framework make heavy use of generics.


  • Generics means more type-safety.
  • With generics, you can create type-safe collections where more problems are caught at compile-time instead of runtime.
  • Without generics, the compiler would happily let you put a Pumpkin into an ArrayList that was supposed to hold only Cat Objects.
  • Three things are of importance while dealing with generics.
  • Create instances of generified classes (like ArrayList)
new ArrayList<Song>();
  • Declaring and assigning variables of generic types.
List<Song> songList = new ArrayList<Song>();
  • Declaring (and invoking) methods that take generic types.
void foo(List<Song> list);

Generic Classes

public class ArrayList<E>
extends AbstractList<E>
implements List<E>, RandomAccess, Cloneable, Serializable
  • Think of ‘E’ as a stand-in for “the type of element you want this collection to hold and return.”

Generic Methods.

  • A generic method means that the method declaration uses a type parameter in its signature. It is used in this manner.
  • Using the type parameter defined in the class declaration.
public class ArrayList<E> extends AbstractList<E>{
public boolean add(E o);
  • Using a type parameter that was NOT defined in the class definition.
public <T extends Animal> void takeThing(ArrayList<T> list);

  • Difference between the below Code sample.
public <T extends Animal> void takeThing(ArrayList<T> list);
  • This one where <T extends Animal> is part of the method declaration, means that any ArrayList declared of a type that is Animal, or one of Animal’s sub type is legal.
public void takeThing(ArrayList<Animal> list);
  • This method argument means that only an ArrayList<Animal> is legal and not sub type.
  • If you declare a method to take ArrayList<Animal> it can only take an ArrayList<Animal>, not ArrayList<Dog>.
  • Array types are checked again at runtime, but collection type checks happens only when you compile.

Genrics Implement and Extends

  • In generics, the keywords “extends” really means “is-a” and works for BOTH classes and interfaces.

Collection API.

  • There are three main interfaces, List,Set, and Map.
  1. LIST : Where sequence matters.
  2. SET : Where uniqueness matters.
  3. MAP : Where finding something by key matters.
  4. Map interface doesn’t actually extend the Collection interface.

Object Equality

  • There are two type of Equality.
  1. Reference Equality

* Two reference, one object on the heap.
* If you call hashCode() method on both reference, you’ll get same result.
2. Object Equality.
* Two reference, two objects on the heap, but the objects are considered meaningfully equivalent.
* We must override the hashCode() and equals() methods.


  • A HashSet checks the hasCodes, if they are different the object are assumed to be different.


  • A TreeSet is similar to HashSet in that it prevents duplicates, and it also keeps the list sorted.
  • To use a TreeSet, one of these things must be true.
  • The element in the list must be of a type that implements Comparable.
  • You use the TreeSet’s overloaded constructor that takes a Comparator.


  • Each element in a Map is actually TWO Objects – a key and a value. You can have duplicate values, but NOT Duplicate keys.

Generic WildCards

  • There is a way a method argument that can accept an ArrayList of any Animal sub type.
public void takeAnimal(ArrayList <? extends Animal> animals){
for (Animal animal : animals) {


public <T extends Animal>void takeAnimal(ArrayList <T> animals){
for (Animal animal : animals) {
  • When you use a wild card in you method argument, the compiler will STOP you from doing anything that could hurt the list referenced by the method parameters.
  • You can still invoke methods on the elements in the list, but you cannot add elements to the list.
  • In other words, you can do things with the list elements, but you cannot put new things in the list.

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