David Streader & Peter Andreae

Computer Science

Victoria University of Wellington

Copyright: David Streader, Victoria University of Wellington

Objects Real and JavaCOMP 112 2015T1 Week 3

COMP 112 lect 8: 2

© D.Streader

Menu

• Explanation• Assignment 1

• Challenging! Some tricky computation & Learning Java • Assignment 2:

• FloorCleaner, Tracer, Dodgem Cars

• Questions about what you are not clear on?

• Objects

COMP 112 lect 8: 3

© D.Streader

Objects

The world contains many objects for example:• Student objects (Physical objects)• Car objects (Physical objects)• Course objects (Abstract objects)

1. The students are of a different type or class to the courses.

2. Many objects can be of the same type/class.

3. Different classes of object can do different things

4. Different objects can be in different states orhave different properties.

COMP 112 lect 8: 4

© D.Streader

Java Objects

• A Class defines a type of object• What kind of data the objects contain• What actions the objects can do / can be done on the objects

• A Class is a collection of:• Private fields to hold data

• numbers, Strings, booleans, …or other objects• Public methods

• Each object has its own fields that define its state/information

• The methods defined for a class can be applied to an object of that class and can access/change the state of the object.

COMP 112 lect 8: 5

© D.Streader

Student Record System

If implementing in Java, might have:1. A Student class

• a Student object to represent each student.• Fields for information about each student• Methods to act on students• getEmail() ….

2. A Course class • a Course object to represent each course• Fields for information about each course• Methods to act on courses:

• printCourseList()• addStudent(Student s) ….

COMP 112 lect 8: 6

© D.Streader

Objects in Java

• File MyObj.java should define the class MyObj• MyObj class defines

• Fields for MyObj objects (hold data) • Constructor(s) to specify how to create a new MyObj object.• Name of constructor = name of Class

• Methods called on MyObj objects

• May have static fields: belong to class as whole, not a field for each object

• May have main method, executed when Foo run from command line

COMP 112 lect 8: 7

© D.Streader

Student Class

public class Student {// fieldsprivate static long nextID = 0; // static field – belongs to

class

private long ID; private String name;private String email = “unknown”;

// constructor public Student(String name){

this.ID = nextID;nextID++;this.name = name;

}// methods …….public void setEmail(String e) {

this.email = e;}public String getEmail(String e) {

return this.email;}

Student-85

ID:

name:

email:

COMP 112 lect 8: 8

© D.Streader

More on Student Class

• Can have multiple constructors• Same name,• Distinguished by the parameters:

// constructorspublic Student(String name){

this.ID = nextID;nextID++;this.name = name;

}

public Student(String nm, long id, String e ){ID = id;name = nm;email = e;

}

Student s1 = new Student(“Jamie Booker”);Student s2 = new Student(“Lindsay Meyer”, 300123001, “lm9@gmail.com”);

COMP 112 lect 8: 9

© D.Streader

Car class

public class Car{// fieldsprivate String color;

public Car(String col){this.color = col;

}public void draw(double x, double y) {

if (color.equals(“red”)) { UI.setColor(Color.red);}else if (color.equals(“blue”)) { UI.setColor(Color.blue);}UI.fillRect(x, y, 30, 15);

}

public boolean equals(Object other) {….

}}

COMP 112 lect 8: 10

© D.Streader

Primitives and Objects

int x = 1;

x: 1

x names a piece of memory in which the value 1 is stored

Car z = new Car(“red”);

z: Car-21 Car-21col: “red”

z names a piece of memory in which a reference to a car object is stored

COMP 112 lect 8: 11

© D.Streader

Assignment of object values

Car c1 = new Car(“red”); Car c2 = new Car(“red”);Car c3 = new Car(“blue”);Car c4 = c3;

c1: Car-21Car-21

col: “red”

c2: Car-24 Car-24

col: “red”c3: Car-25

Car-25

col: “blue”c4: Car-

COMP 112 lect 8: 12

© D.Streader

Comparing object values

Car c1 = new Car(“red”); c1 == c2 ?Car c2 = new Car(“red”); c1.equals(c2) ?Car c3 = new Car(“blue”); c3 == c4 ? Car c4 = c3; c3.equals(c4) ?

c1: Car-21Car-21

col: “red”

c2: Car-24 Car-24

col: “red”c3: Car-25

Car-25

col: “blue”c4: Car-

COMP 112 lect 8: 13

© D.Streader

Passing and modifying values

int age = 35;this.change1(age);UI.println(“age: ” + age);

public void change1(int v){v = v + 10;

}

Student s = new Student(“Roy”);this.change2(s);UI.println(“email: ” + s.getEmail());

public void change2(Student st){st.setEmail(“jobs@ecs.vuw.ac.nz”);

}

COMP 112 lect 8: 15

© D.Streader

Car class: equals method (1)

public class Car{

private String color;

public Car(String col){this.color = col;

}public void draw(double x, double y) {

if (color.equals(“red”)) { UI.setColor(Color.red);}else if (color.equals(“blue”)) { UI.setColor(Color.blue);}UI.fillRect(x, y, 30, 15);

}

public boolean equals(Car other) {

return (this.color.equals(other.color));}

}

What if other objectisn’t a car?

COMP 112 lect 8: 16

© D.Streader

Equals

• You can call equals on any Object• The default operation of equals is to do ==

public boolean equals(Object other){ in Object class return (this == other);

}

• Many Object types have their own equals:• String will check that all the characters in the strings are the

same:

public boolean equals(String other){ in String class

if (this.length() != other.length()) { return false; }for (int i = 0; i < this.length(); i++){

if (this.charAt(i) != other.charAt(i)) { return false; }}return true;

}

COMP 112 lect 8: 17

© D.Streader

Car class: equals method (1)

In your class: need to define equals, if it shouldn’t mean ==

public class Car{private Color color;private double x, y;

:

public boolean equals(Car other) {

return (this.color.equals(other.color));}

public boolean equals(Object other) { better!

if (this == other) return true;

if (! (other instanceof Car) ) return false;

Car otherCar = (Car) other;

return (this.color.equals(otherCar.color));}

}