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Chapter 10

Using arrays to create collections ArrayList class. Chapter 10. Collections. There are many examples of programs that need many objects of a particular type A class roster needs many Student objects A CD object needs many songs A photo album needs many photos We can use arrays for this

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Chapter 10

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  1. Using arrays to create collections ArrayList class Chapter 10

  2. Collections • There are many examples of programs that need many objects of a particular type • A class roster needs many Student objects • A CD object needs many songs • A photo album needs many photos • We can use arrays for this • How do we deal with the fact that we don't know ahead of time how many objects there will be? • Some kinds of collections change size frequently

  3. What does a Collection class look like? public class ItemCollection { private Item [] collection; private int size, capacity; public ItemCollection() {…} public void add( Item item) {…} public Item get( int index) {…} …}

  4. Dynamic Arrays • Create the array with some standard size Item [] collection = new Item[capacity]; • Use a counter variable to keep track of the number of elements which are not null; • Each time you add an item, put it in the next available element and increment the counter. collection[size] = newItem; size++;

  5. Dynamic Arrays • Before adding a new item, check to see if there is room • If the array is full, make a bigger array and copy the elements from the original if (size == capacity) Item [] newArray = new Item[2*capacity]; for (int i=0; i< size; i++) newArray[i] = collection[i]; collection = newArray; capacity *= 2; } // add element as above

  6. JCF Lists • JCF includes two classes that support methods to maintain a collection of objects as a linear list L = (l0, l1, l2, . . . , lN) • We can add to, remove from, and retrieve objects in a given list. • A list does not have a set limit to the number of objects we can add to it.

  7. boolean add(Objecto) Adds an object o to the list void clear( ) Clears this list, i.e., make the list empty Object get(int idx) Returns the element at position idx boolean remove(intidx) Removes the element at position idx int size( ) Returns the number of elements in the list List Methods • Here are five of the 25 list methods:

  8. Using Lists • To use a list in a program, we must create an instance of one of the List classes. • ArrayList • LinkedList • The ArrayList class uses an array to manage data. • The LinkedList class uses a technique called linked-node representation.

  9. import java.util.*; ArrayList<Person> friends; Person person; friends = new ArrayList<Person>( ); person = new Person("jane", 10, 'F'); friends.add( person ); person = new Person("jack", 6, 'M'); friends.add( person ); Person p = friends.get( 1 ); Sample List Usage • Here's an example of manipulating a list:

  10. How to process the elements of a list? • Use a for loop for (int i=0; i<list.size(); i++) process( list.get(i)); • Use for in for (BaseType element : list) process( element); • Use an iterator

  11. Using an Iterator • The iterator method of ArrayList returns an object you can use to loop through all the elements of the list Iterator<Person> iter = friends.iterator(); while (iter.hasNext()) System.out.println( iter.next()); • The next() method returns each element in turn Person p = iter.next(); • Use hasNext() to determine when all the elements of the list have been processed

  12. Interfaces • Sometimes we have a number of classes that should have a common set of methods. • All lists should have methods like add, get, size, … • Interfaces provide a way to specify a set of methods that a group of classes should have in common. • A Java interface defines only behavior • It includes headers for public methods (no method bodies) • It does not include any data members except public constants • No instances of a Java interface can be created

  13. Interfaces in Java • The Java libraries include many interfaces • There is a List interface that includes all the methods needed by lists • Both ArrayList and LinkedList implement List • They have the same methods List<Person> list; list = new ArrayList<Person>; list = new LinkedList<Person>; • The code works independent of which way you instantiated list • This is an example of polymorphism

  14. Java Interfaces • Template for an interface public interface <interfaceName> { public <returnType> <methodName>( <paramList>); public final <constType> constName = <constValue>; }

  15. Using an interface • Use the implements keyword header of a class that needs to have all the methods of the interface • Implement (provide code for) each method in the interface public class <implementingClassName> implements <interfaceName> { public <returnType> <methodName>( <paramList>){ // code for method } // add any other methods and data that is // appropriate for the class }

  16. Using an interface type • You can use an interface type as the type for a reference (object) variable <interfaceName> <identifier>; • You have to instantiate the object using a class that implements the interface <identifier> = new <implementingClassName>(); • Now you can only call the methods that appear in the interface.

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