ArrayLists 22-Feb-19

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.) AP Computer Science

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];

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 AP Computer Science

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"}; AP Computer Science

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); AP Computer Science

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

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 AP Computer Science

About Lists (in general) 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 AP Computer Science

Methods ArrayList() //constructor void add(int index, Object x) 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()                  AP Computer Science

How the methods work add: Arrays in Java 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) AP Computer Science

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

How the methods work set Arrays in Java replaces value of Object parameter at the given index size is not changed AP Computer Science

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

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<?>();

Adding elements to a ArrayList boolean add(Object obj) Appends the object obj to the end of this ArrayList With generics, the obj must 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(int index, 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

Removing elements boolean remove(Object obj) void remove(int index) 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

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

Searching a ArrayList boolean contains(Object element) Tests if element is 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 element in this ArrayList Returns -1 if element was not found in this ArrayList int lastIndexOf(Object element) Returns the index of the last occurrence of element in this ArrayList

Getting information boolean isEmpty() int size() Object[ ] toArray() 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

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

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