11. Collections of Objects

Transcription

1 11. Collections of Objects 1

2 Arrays There are three issues that distinguish arrays from other types of containers: efficiency, type, and the ability to hold primitives. When you re solving a more general problem, arrays can be too restrictive. Regardless of what type of array you re working with, the array identifier is actually a reference to a true object that s created on the heap. The read-only length member tells you how many elements can be stored in that array object. The [] syntax is the only other access to the array object. 2

3 Arrays public class ArraySize { public static void main (String[] args) { // Arrays of objects: Weeble[] a; // Local uninitialized variable Weeble[] b = new Weeble[5]; // Null references Weeble[] c = new Weeble[4]; for(int i = 0; i < c.length; i++) if(c[i] == null) // Can test for null reference c[i] = new Weeble(); // Aggregate initialization: Weeble[] d = { new Weeble(), new Weeble(), new Weeble() ; // Dynamic aggregate initialization: a = new Weeble[] { new Weeble(), new Weeble() ; System.out.println ("a.length=" + a.length); System.out.println("b.length = " + b.length); // The references inside the array are // automatically initialized to null: for(int i = 0; i < b.length; i++) System.out.println("b[" + i + "]=" + b[i]); System.out.println("c.length = " + c.length); System.out.println("d.length = " + d.length); a = d; System.out.println("a.length = " + a.length); // Arrays of primitives: int[] e; // Null reference int[] f = new int[5]; int[] g = new int[4]; for(int i = 0; i < g.length; i++) g[i] = i*i; int[] h = { 11, 47, 93 ; // Compile error: variable e not initialized: //!System.out.println("e.length=" + e.length); System.out.println("f.length = " + f.length); // The primitives inside the array are // automatically initialized to zero: for(int i = 0; i < f.length; i++) System.out.println("f[" + i + "]=" + f[i]); System.out.println("g.length = " + g.length); System.out.println("h.length = " + h.length); e = h; System.out.println("e.length = " + e.length); e = new int[] { 1, 2 ; System.out.println("e.length = " + e.length); class Weeble { 3

4 Returning an array Suppose you re writing a method and you don t just want to return just one thing, but a whole bunch of things. Languages like C and C++ make this difficult because you can t just return an array, only a pointer to an array. Java takes a similar approach, but you just return an array. Unlike C++, with Java you never worry about responsibility for that array. 4

5 public class IceCream { private static Random rand = new Random(); public static final String[] flavors = { "Chocolate", "Strawberry", "Vanilla Fudge Swirl", "Mint Chip", "Mocha Almond Fudge", "Rum Raisin", "Praline Cream", "Mud Pie" ; public static String[] flavorset(int n) { String[] results = new String[n]; boolean[] picked = new boolean[flavors.length]; for(int i = 0; i < n; i++) { int t; do { t = rand.nextint(flavors.length); while(picked[t]); results[i] = flavors[t]; picked[t] = true; return results; public static void main(string[] args) { for(int i = 0; i < 20; i++) { System.out.println( "flavorset(" + i + ") = "); String[] fl = flavorset (flavors.length); for(int j = 0; j < fl.length; j++) System.out.println("\t" + fl[j]); 5

6 Containers The Java 2 container library takes the issue of holding your objects and divides it into two distinct concepts: Collection: a group of individual elements Map: a group of key-value object pairs 6

7 Collection functionality boolean add(object) boolean addall(collection) Ensures that the container holds the argument. Returns false if it doesn t add the argument. Adds all the elements in the argument. Returns true if any elements were added. void clear( ) Removes all the elements in the container. ( Optional. ) boolean contains(object) boolean containsall(collection) boolean isempty( ) Iterator iterator( ) boolean remove(object) true if the container holds the argument. true if the container holds all the elements in the argument. true if the container has no elements. Returns an Iterator that you can use to move through the elements in the container. If the argument is in the container, one instance of that element is removed. Returns true if a removal occurred. 7

8 Collection functionality boolean removeall(collection) boolean retainall(collection) int size( ) Object[] toarray( ) Object[] toarray(object[] a) boolean remove(object) Removes all the elements that are contained in the argument. Returns true if any removals occurred. Retains only elements that are contained in the argument (an intersection from set theory). Returns true if any changes occurred. Returns the number of elements in the container. Returns an array containing all the elements in the container. Returns an array containing all the elements in the container, whose type is that of the array a rather than plain Object. If the argument is in the container, one instance of that element is removed. Returns true if a removal occurred. 8

9 ArrayList, LinkedList, Vector ArrayList Resizable-array implementation of the List interface See the following website for the APIs LinkedList Linked list implementation of the List interface See the following website for the APIs Vector The Vector class implements a growable array of objects. Like an array, it contains components that can be accessed using an integer index. The size of a Vector can grow or shrink as needed to accommodate adding and removing items after the Vector has been created. See the following website for the APIs 9

10 Example: ArrayList class AAA { public void f () { class BBB { public void g () { public class ArrayListExam { public static void main(string[] args) { ArrayList aaalist = new ArrayList (); ArrayList bbblist = new ArrayList (10); for (int i = 0; i < 20; i++) { AAA aobj = new AAA (); aaalist.add (aobj); for (int i = 0; i < 20; i++) { BBB bobj = new BBB (); bbblist.add (bobj); for (int i = 0; i < 10; i++) { AAA aobj = (AAA) aaalist.get (i); BBB bobj = (BBB) bbblist.get (i); aobj.f (); bobj.g(); 10

11 Making a queue from a LinkedList A queue is a first-in, first-out (FIFO) container. You put things in at one end and pull them out at the other. So the order in which you put them in will be the same order that they come out. import java.util.*; public class Queue { private LinkedList list = new LinkedList(); public void enqueue (Object v) { list.addfirst (v); public Object dequeue () { return list.removelast(); public boolean isempty() { return list.isempty(); public class QueueTest { public static void main (String args[]) { Queue queue = new Queue (); for (int i = 0; i < 10; i++) queue.enqueue (new Integer(i)); while (!queue.isempty()) System.out.println (queue.dequeue()); 11

12 Making a stack from a LinkedList A stack is sometimes referred to as a last-in, first-out (LIFO) container. Whatever you push on the stack last is the first item you can pop out. import java.util.*; public class StackTest { public static void main (String args[]) { public class StackL { StackL stack = new StackL (); private LinkedList list = new LinkedList(); public void push (Object v) { for (int i = 0; i < 10; i++) list.addfirst(v); stack.push (new Integer(i)); public Object top() { while (!stack.isempty()) return list.getfirst(); System.out.println (stack.pop()); public Object pop() { return list.removefirst(); public boolean isempty() { return list.isempty(); 12

13 Choosing between Lists The most convincing way to see the differences between the implem entations of List is with a performance test. Type Get Iteration Insert Remove array ArrayList LinkedList Vector As expected, arrays are faster than any container for random-access lookups and iteration. Random accesses (get( )) are cheap for ArrayLists and expensive for LinkedLists. Oddly, iteration is faster for a LinkedList than an ArrayList, which is a bit counterintuitive. On the other hand, insertions and removals from the middle of a list are dramatically cheaper for a LinkedList than for an ArrayList especially removals. Of course, if you are working with a fixed-sized group of elements, use an array. 13