Lecture Notes – Classes and Objects (Ch 7-8)

Yonglei Tao

Arrays Hold sequences of values of primitive types or

objects When an array is created, its values are

initialized zero, false, or null

int [] scores;scores = new int [10];for (int i = 0; i < scores.length; i++)

scores[i] = i * i; for (int n: scores) // a “for each” loop

System.out.println (n);

Arrays (Cont.)String[] names = {“Ed”, “Bob”, “Cindy”};System.out.println( names[0] );if ( names[0].equals(“Ed”)) …

String[] list = new String[5];list[0] = new String(“Tom”);…

A two-dimensional array

int rows = 10, cols = 20;int [][] table = new int[rows][cols];

Partially Filled Arrays - Loadingint numValues = 0;Scanner in = new Scanner(System.in);while (in.hasNextDouble()) { if (numValues < values.length) { values[numValues] = in.nextDouble(); numValues ++; }}

for (int i = 0; i < numValues; i++) { System.out.println(values[i]);}

Class ArrayList A sequence of objects

Can grow and shrink as needed A generic class with a type parameter

ArrayList<String> classlist = new ArrayList<String> ();classlist.add ( “Ed” );classlist.add ( “Bob” );classlist.add ( “Cindy” );

ArrayList (Cont.)

for (int i = 0; i < classlist.size(); i++ ) {String name = classlist.get(i);System.out.println (name); // Ed, Bob, Cindy

}

for ( String name: classlist )System.out.println (name);

classlist.set (1, “Tim”); // Ed, Tim, Cindyclasslist.add (1, “Ron”); // Ed, Ron, Tim, Cindyclasslist.remove (2); // Ed, Ron, Cindy

Wrapper Classes

Auto Boxing and Unboxing

int n = 25;Integer num = n; // same as num = new Integer (n);Double rate = 5.25;double d = rate;rate = rate + i;Boolean isDone = new Boolean(false);

ArrayList<Double> values = new ArrayList<Double>(); values.add(29.95); double x = values.get(0); // inefficient

Arrays vs. ArrayLists When do you use which?

Element type Number of elements Basic operations

Locate insert (at the front, in the middle, at the end) delete (the first, a middle one, the last)

Common Array Algorithm - Search

int pos = 0; boolean found = false; while (pos < values.size() && !found) { if (values.get(pos) > 100)

{ found = true; } else

{ pos++; } } if (found)

{ System.out.println("Position: " + pos); } else

{ System.out.println("Not found"); }

Common Array Algorithm - Insert• ArrayList: use method add

• Unordered array:

if (numValues < values.length) { values[numValues] = newElement; numValues++; }

Common Array Algorithm - Insert Ordered array:

if (numValues < values.length) { for (int i = numValues ; i > pos; i--) values[i] = values[i - 1];

values[pos] = newElement; numValues ++; }

Common Array Algorithm – Array Copy

double[] values = new double[6];

. . . // Fill array

double[] prices = values; ?

double[] prices = Arrays.copyOf(values, values.length);

values = Arrays.copyOf(values, 2 * values.length);

Classes Each has a responsibility Each provides services for clients Objects are instances of a class

Members Instance variables private Instance methods private Instance methods public - interface

Method Call A call

obj.doIt (num, name);

Method definition

public void doIt (int n, String s) {n += 4;System.out.println (“Start with “ +

s.charAt(0));}

Every thing is passed by value What if having s = “Mr. “+ s; in method doIt()?

“tom”

6

6

Overloaded Methods // Methods of class MyClass

public void doIt (int n) { … }public void doIt (char c) { … } public void doIt (int n, String s) { … }

// Method callsMyClass obj = new MyClass();obj.doIt(5);obj.doIt(‘a’);obj.doIt(“xyz”);

Constructorspublic class A {

private int num;private String name;

public A () { this(10,

“tom”);}public A (int n) {

num = n; }public A (int n,

String s) {num = n;

name = s;}…

}

// use of constructors

A ref1 = new A();

A ref2 = new A(5);

A ref3 = new A(10, “tom”);

Static Members of a Class Instance variable

One copy per object Static variable

One copy per class Static Method

Operating on class-wide data

Examples Integer.MAX_VALUE, Integer.MIN_VALUE Math.pow(x, y), Math.sqrt(x)

Scope of Local Variables

public class RectangleTester {

public static double area(Rectangle rect) { double r = rect.getWidth() * rect.getHeight(); return r; }

public static void main(String[] args) { Rectangle r = new Rectangle(5, 10, 20, 30);

double a = area(r); System.out.println(r); }

}

Scope of Local Variables

if (x >= 0) { double r = Math.sqrt(x); ... } // Scope of r ends here else { Rectangle r = new Rectangle(5, 10, 20, 30); // OK - it is legal to declare another r here ... }

Overlapping Scope

public class Coin {... public double getExchangeValue(double exchangeRate) {

double value; ... return value;

}

private String name; private double value; }

Which one to refer to if using value in this method? How to refer to instance variable value in this

method?

Built-in Reference this Referring to an object from within

public A (int num, String name) {

this.num = num;

this.name = name;

} Referring to the current object

A x = new A(10, “bob”);

A y = new A(5, “ed”);

x.setNum(1);

Packages in Java

Package Purpose Sample Class

java.lang Language support Math

java.util Utilities Random

java.io Input and output PrintStream

java.awt Abstract Windowing Toolkit Color

java.applet Applets Applet

java.net Networking Socket

java.sql Database Access ResultSet

javax.swing Swing user interface JButton

omg.w3c.dom Document Object Model for XML documents

Document

Packages - Organizing related classes• To put classes in a package

package cis500.proj1;

public class MyClass { ... }

• To use class with importing

import cis500.proj1.MyClass;

• Or import cis500.proj1.*;