Exploring Java Arrays and ArrayLists

1. ArrayLists

2. Arrays in Java • Java has built in arrays as well as more complicated classes to automate many array tasks (the ArrayList class) • arrays hold elements of the same type • primitive data types or classes • space for array must be dynamically allocated with new operator. (Size is any integer expression. Due to dynamic allocation does not have to be a constant.)

3. public void arrayExamples() { int[] intList = new int[10]; for(int i = 0; i < intList.length; i++) { assert 0 >= i && i < intList.length; intList[i] = i * i * i; } intList[3] = intList[4] * intList[3]; }

4. Array Details • all arrays must be dynamically allocated • arrays have a public, final field called length • built in size field, no separate variable needed • don't confuse length (capacity) with elements in use • elements start with an index of zero, last index is length - 1

5. Array Initialization • Array variables are object variables • They hold the memory address of an array object • The array must be dynamically allocated • All values in the array are initialized (0, 0.0, char 0, false, or null) • Arrays of primitives and Strings may be initialized with an initializer list: int[] intList = {2, 3, 5, 7, 11, 13}; double[] dList = {12.12, 0.12, 45.3}; String[] sList = {"Olivia", "Kelly", "Isabelle"};

6. Arrays of objects • A native array of objects is actually a native array of object variables • all object variables in Java are really what? • Pointers! public void objectArrayExamples() { Rectangle[] rectList = new Rectangle[10]; // How many Rectangle objects exist? rectList[5].setSize(5,10); //uh oh! for(int i = 0; i < rectList.length; i++) { rectList[i] = new Rectangle(); } rectList[3].setSize(100,200); }

7. ArrayLists and arrays • A ArrayList is like an array of Objects • Differences between arrays and ArrayLists: • Arrays have special convenience syntax; ArrayLists don’t • An array is a fixed size, but a ArrayList expands as you add things to it • This means you don’t need to know the size beforehand • Arrays can hold primitives or objects, but ArrayLists can only hold objects • However, autoboxing can make it appear that an ArrayList can hold primitives

8. Arrays in Java The ArrayList Class • A class that is part of the Java Standard Library and a class that is part of the AP subset • a kind of automated array • not all methods are part of the ap subset

9. Arrays in Java About Lists (in general) • A list is an ordered collection or a sequence. • ArrayList implements the List interface • The user of this interface will have control over where in the list each element is inserted. • The user can access elements by their integer index (position in the list), and search for elements in the list. • Items can be added, removed, and accessed from the list

10. Arrays in Java Methods • ArrayList() //constructor • void add(int index, Object x) • boolean add(Object x) • Object set(int index, Object x)               • Object remove(int index) • int size () • Object get(int index)   • Iterator iterator()                 

11. Arrays in Java How the methods work • add: • boolean add(Object x) – inserts the Object x at the end of the list (size increases by 1), returns true • void add(int index, Object x) – inserts the Object x at the given index position (elements will be shifted to make room and size increases by 1)

12. Arrays in Java How the methods work • get: • returns the Object at the specified index • should cast when using value returned • throws IndexOutOfBoundsException if index<0 or index>=size

13. Arrays in Java How the methods work • set • replaces value of Object parameter at the given index • size is not changed

14. Arrays in Java How the methods work • remove • removes the element at the specified index • throws IndexOutOfBoundsException if index<0 or index>=size • size will be decreased by 1 • returns Object removed

15. Creating a ArrayList the new way • Specify, in angle brackets after the name, the type of object that the class will hold • Examples: • ArrayList<String> vec1 = new ArrayList<String>(); • ArrayList<String> vec2 = new ArrayList<String>(10); • To get the old behavior, but without the warning messages, use the <?> wildcard • Example: ArrayList<?> vec1 = new ArrayList<?>();

16. Adding elements to a ArrayList • boolean add(Object obj) • Appends the object objto the end of this ArrayList • With generics, the objmust be of the correct type, or you get a compile-time (syntax) error • Always returns true • This is for consistency with other, similar classes • void add(intindex, Object element) • Inserts the element at position index in this ArrayList • The index must be greater than or equal to zero and less than or equal to the number of elements in the ArrayList • With generics, the objmust be of the correct type, or you get a compile-time (syntax) error

17. Removing elements • boolean remove(Object obj) • Removes the first occurrence of obj from this ArrayList • Returns true if an element was removed • Uses equals to test if it has found the correct element • void remove(int index) • Removes the element at position index from this ArrayList • void clear() • Removes all elements

18. Accessing with and without generics • Object get(int index) • Returns the component at position index • Using get the old way: • ArrayList myList = new ArrayList();myList.add("Some string");String s = (String)myList.get(0); • Using get the new way: • ArrayList<String> myList = new ArrayList<String>();myList.add("Some string");String s = myList.get(0); • Notice that casting is no longer necessary when we retrieve an element from a “genericized” ArrayList

19. Searching a ArrayList • boolean contains(Object element) • Tests if elementis a component of this ArrayList • Uses equals to test if it has found the correct element • int indexOf(Object element) • Returns the index of the first occurrence of elementin this ArrayList • Uses equals to test if it has found the correct element • Returns -1 if elementwas not found in this ArrayList • int lastIndexOf(Object element) • Returns the index of the last occurrence of elementin this ArrayList • Uses equals to test if it has found the correct element • Returns -1 if elementwas not found in this ArrayList

20. Getting information • boolean isEmpty() • Returns true if this ArrayList has no elements • int size() • Returns the number of elements currently in this ArrayList • Object[ ] toArray() • Returns an array containing all the elements of this ArrayList in the correct order

21. More about equals • There are many different notions of equality • Example: two sets are equal if they contain the same elements; order of elements is irrelevant • Java defines public boolean equals(Object) in the Object class, but • equals is defined to be the same as == • It’s often a good idea to override equals for your own objects • If you do this, note that the argument should be a general Object • The String class (and some others) override equals

22. Conclusion • A ArrayList is like an array of Objects • The advantage of a ArrayList is that you don’t need to know beforehand how big to make it • The disadvantage of a ArrayList is that you can’t use the special syntax for arrays • You should never use an array that you hope is “big enough”—use a ArrayList instead