What does this output? Is there a better way to say the same thing? What does this do? Is there a better way to say the same thing?

Transcription

1 public class Person { protected String name; public Person (String name) { public String getname() { return name; public class Student extends Person {... public String getname() { return super.getname(); What does this output? Is there a better way to say the same thing? Student s = new Student ( Julie ); System.out.println (s.getname()); 1 public class Person { protected String name; public Person (String name) { public String getname() { return name; public class Student extends Person { public Student (String name) { super(name); What does this do? Is there a better way to say the same thing? 2

2 public class Person { protected String name; public Person (String name) { public class Student extends Person { private int age; public Student (String name, int age) { super(name); this.age = age; Is there a better way to say the same thing? public class Alum extends Person { private int age; public Alum (String name, int age) { super(name); this.age = age; 3 public class Person { protected String name; public Person (String name) { public class Student extends Person { private int age; public Student (String name, int age) { super(name); this.age = age; public class Person { protected String name; protected int age; public Person (String name, int age) { this.age = age; public class Student extends Person { public Student (String name, int age) super(name, age); public class Alum extends Person { private int age; public Alum (String name, int age) { super(name); this.age = age; public class Alum extends Person { public Alum (String name, int age) { super(name, age); 4

3 public class AsteroidGameShape { public class Asteroid extends AsteroidGameShape { private static final int SPEED = 2; public void move() { top = top + SPEED; public class Ship extends AsteroidGameShape { private int speed = 0; public void move() { top = top + speed; private void setspeed (int newspeed) { speed = newspeed; Is there a better way to say the same thing? 5 public class AsteroidGameShape { public class Asteroid extends AsteroidGameShape { private static final int SPEED = 2; public void move() { top = top + SPEED; public class Ship extends AsteroidGameShape { private int speed = 0; public void move() { top = top + speed; private void setspeed (int newspeed) { speed = newspeed; public class AsteroidGameShape { protected int speed; public AsteroidGameShape (int speed) { this.speed = speed; public void move() { top = top + SPEED; public class Asteroid extends AsteroidGameShape { private static final int SPEED = 2; public Asteroid() { super (SPEED); public class Ship extends AsteroidGameShape { public Ship() { super(0); private void setspeed (int newspeed) { speed = newspeed; 6

4 Arrays What can we do with arrays? Manage collections of information Put a value into an array a[i] = 1; Get a value out of an array value = a[i]; Ask the array what its size is size = a.length; 7 Fixed in size Array Limitation 1 How do we get around this? Make the array as big as we think it ever needs to be Add another data member to the class that remembers how many entries are in the array Example from Asteroids extra credit: int bullets[1000]; int numbullets; 8

5 Array Limitation 2 Adding an element at a specific position (as opposed to replacing an element) may require moving other elements public void insertinfront(int value) throws ArrayFullException { if (numelements == a.length) { throw new ArrayFullException(); for (int i = numelements; i > 0; i--) { a[i] = a[i-1]; a[0] = value; numelements++; 9 Array Limitation 3 Removing an element at a specific position may require moving elements to avoid gaps public int removefromfront() throws ArrayEmptyException { if (numelements == 0) { throw new ArrayEmptyException(); int returnvalue = a[0]; for (int i = 0; i < numelements-1; i++) { a[i] = a[i+1]; numelements--; return returnvalue; 10

6 Wouldn t it be nice if... Collections could change size as necessary Inserting into a specific position was as easy as assigning to an array element Removing an element was easy ArrayLists to the rescue!! 11 Interface java.util.list Array-like operations: public void set (int index, Object o) public Object get (int index) public int size() Insertion and removal: public void add (Object o) public void add (int index, Object o) public Object remove (int index) public boolean remove (Object o) Other useful operations: public boolean contains (Object o) public int indexof (Object o) 12

7 Arrays vs Lists Can change size Arrays No Lists Yes Random access Yes Yes Easy insertion & removal Elements can be any type No Yes Yes No 13 <<interface>> List <<abstract>> AbstractList ArrayList LinkedList 14

8 ArrayList Implements all methods from List interface Random access like arrays Easy insertion & removal Search operations Can grow as necessary Has a capacity in addition to a current size public ArrayList() public ArrayList (int initialcapacity) public void ensurecapacity (int mincapacity) public void trimtosize() 15 ArrayList Methods Kelsey Kira.add( Mindan ); Kelsey Kira Mindan.add( Carol, 0); Carol Kelsey Kira Mindan.set( Cade, 0) returns Carol Cade Kelsey Kira Mindan 16

9 ArrayList Methods Cade Kelsey Kira Mindan.contains( Kira ) returns true.indexof( Kelsey ) returns 1.get(2) returns Kira 17 ArrayList Methods Cade Kelsey Kira Mindan.remove( Kelsey ) returns true Cade Kira Mindan.remove( Tricia ) returns false.remove(0) returns Cade Kira Mindan.remove(2) throws IndexOutOfBoundsException.add(5, Sarah ) throws IndexOutOfBoundsException 18

10 Declaring and Constructing an ArrayList // Construct an ArrayList that can hold Strings ArrayList<String> links = new ArrayList<String>(); 19 Javadoc for Generics Class ArrayList<E> boolean add (E e) E get (int index) E remove (int index) E set (int index, E element) There is no type named E Means whatever type parameter is used in the declaration ArrayList<String> links; -> add and set must be passed Strings -> get, remove and set will return Strings 20