Week 2 1.Notes From “Just Java” by Linden 2.An example Java class - The BigInteger class 3.Homework 1 Pricing a fixed-rate bond.

Week 2

1. Notes From “Just Java” by Linden

2. An example Java class - The BigInteger class

3. Homework 1 Pricing a fixed-rate bond

1. Notes From “Just Java”

Compiling and Executing

javac SomeClass.java produces syntax errors or SomeClass.class

java SomeClass runs the main routine found in SomeClass.class

OOPAbstraction

Ignore the details Java provides a very large library of Classes You can’t get by without ignoring details

Encapsulation

Information hiding Data and operations are bundled together Java provides the Class mechanism

Composition

The “has a” relationship

Chapter 2 OOP

Inheritance

A Car “is a” kind of Vehicle So, properties of a vehicle should be inherited by Car Java provides the “extends” key word

Polymorphism

What happens when we say a = b + c; It depends on the types of the operands Java provides both static and dynamic polymorphism

Primitive Types & Class Types

int a; what picture do we draw?

a

a = 3; what picture do we draw?

a 3

Primitive Types & Class Types

BigInteger b; what picture do we draw? b

b = new BigInteger(“1234567897272727272”);

b

Creating a Class and an Object

class Fruit { public int grams; public int calsPerGram;}

public class TestFruit { public static void main(String a[]) {

Fruit f = new Fruit(); f.grams = 30; }}

Some issues: -initialization -encapsulation -data without methods -like a struct in c

Adding a constructorclass Fruit { public int grams; public int calsPerGram; Fruit(int grams, int c) { this.grams = grams; calsPerGram = c; }}

public class TestFruit { public static void main(String a[]) { Fruit f = new Fruit(100,5); f.grams = 30; }}

Adding Some Methodsclass Fruit { private int grams; private int calsPerGram; Fruit(int grams, int c) { this.grams = grams; calsPerGram = c; } int getGrams() { return grams; } int getCalsPerGram() { return calsPerGram; } void setGrams(int g) { grams = g; } void setCalsPerGram(int cpg) { calsPerGram = cpg; } }

public class TestFruit {

public static void main(String a[]) {

Fruit melon = new Fruit(4,5); Fruit banana = new Fruit(2,6);

banana.setGrams(87);

System.out.println(banana.getGrams());

}}

A Glimpse at Inheritance

class Fruit { private int grams; private int calsPerGram; Fruit(int grams, int c) { this.grams = grams; calsPerGram = c; } int getGrams() { return grams; } int getCalsPerGram() { return calsPerGram; } void setGrams(int g) { grams = g; } void setCalsPerGram(int cpg) { calsPerGram = cpg; } }

class Citrus extends Fruit { private double acidity; Citrus(double acid, int grams, int calsPerGram) { super(grams,calsPerGram) ; acidity = acid; } double getAcidity() { return acidity; } void setAcidity(double a) { acidity = a; }}

public class TestFruit { public static void main(String a[]) { Citrus lemon = new Citrus(5.0,4,5); System.out.println(lemon.getAcidity()); System.out.println(lemon.getGrams()); }}

Per-Instance & Per-Class Members

The keyword static makes something exists on a per-class basis.

The keyword is used for

data methods blocks classes

Static dataclass Employee { String name; long salary; short employee_number; static int total_employees;}public class EmployeeTest {

public static void main(String arg[]) { Employee a = new Employee(); Employee b = new Employee(); }}

We have twocopies ofname, salary,employee_number.

We have one copyof total_employees.

total_employeesis in an object of class Class thatis used to createEmployee objects.

Static methods

public class MainIsStatic {

int a; void foo() { a = 3; } public static void main(String arg[]) {

MainIsStatic x = new MainIsStatic(); MainIsStatic y = new MainIsStatic(); x.foo(); }}

Conceptually,there are twofoo()’s. Therereally are twoa’s and there isonly one main().

x’s a is 3 and y’s a is 0.

Passing Primitive Parameters

void foo(int a) { // changes to a are local to foo()}

You can’t pass a pointer to an int (like c).You can’t pass a reference to int (like c++).You can just pass an int.

Passing Object Parameters

void foo(Object a) { // changes to the Object referenced by a // are known outside of foo().}

We can’t pass the object itself (like c++)We can’t pass a struct by value (like c)We can only pass a pointer to an Object.

2. The BigInteger Class

Object-Oriented Programming and Java’s BigInteger Class

• Abstraction The focus is on “what” not “how”.• Encapsulation Information hiding is used to promote modularity and the use of abstractions.• Polymorphism We may want to treat an object not as an instance of its specific type but as an instance of its base type.• Inheritance OOP promotes code reuse via the “is-a” relationship.• Composition OOP promotes code reuse via the “has-a” relationship.

Using Java’s BigInteger Classimport java.math.*;public class TestBigInts { public static void main(String args[] ) {

BigInteger x = new BigInteger("1234567890123" + "45678901234567890"); BigInteger y = new BigInteger("93939393929292" + "9191919191919192");

BigInteger z = x.multiply(y); System.out.println(z);

}}

AbstractionEncapsulation

The toString() method

public class BigInteger {

public String toString() {

// Returns the decimal String representation of this // BigInteger. // Overrides: toString() in class Object }}

A BigInteger “is a”Object.

The println() methodcalls toString().

Polymorphism

Another Exampleimport java.math.*;import java.util.*;

public class TestBigInts {

public static void main(String args[] ) {

BigInteger m = new BigInteger(4,10, new Random(2456)); BigInteger n = new BigInteger(4,10, new Random(94));

if(m.compareTo(n) < 0) System.out.println(m + " < " + n ); else System.out.println(m + " >= " + n ); }

}

bitLength - bitLength ofthe returned BigInteger.

certainty - a measure of the uncertainty that the caller is willing to tolerate. Prob(prime) = 1-(2-10).

A Random objectassists with the computation.

Consider compareTo()if(m.compareTo(n) < 0) System.out.println(m + " < " + n ); else System.out.println(m + " >= " + n );

public class BigIntegerextends Numberimplements Comparable

Comparable is an interface that BigIntegerimplements.

The BigInteger classtherefore has a methodcalled compareTo().

Any method foo(Comparable x) canbe called with a BigInteger.

Inheritance and Polymorphism

Another exampleimport java.math.*;public class TestBigInts {

public static void main(String args[] ) { BigInteger m = new BigInteger("34"); foo(m); } public static void foo(Number n) { long x = n.longValue(); x++; System.out.println("x = " + x); }}

BigInteger extendsthe abstract Number class.

No problem

Compositionimport java.math.*;

class BigFraction {

private BigInteger numerator; private BigInteger denominator;

public BigFraction(BigInteger num, BigInteger denom) {

numerator = num; denominator = denom; } public BigFraction(String num, String denom) {

this(new BigInteger(num), new BigInteger(denom)); }

A BigFraction“has-a” BigIntegernumerator and a BigInteger denominator.

Composition (cont.)public String toString() { return numerator + "/" + denominator; }}

public class TestBigInts {

public static void main(String args[]) {

BigFraction x = new BigFraction("1","2");

System.out.println(x); }}