CSC 142 Computer Science II

Zhen Jiang West Chester University

zjiang@wcupa.edu

Table of Contents Introduction to field (attribute) Public and private modifiers Accessor and mutator methods Constructor Static method Instance method Overloading Static variables Summary

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Fields field: A variable inside an object that represents part

of its internal state. Each object will have its own copy of the data fields we

declare.

Declaring a field, general syntax:public <type> <name>;orpublic <type> <name> = <value>;(with initialization)

Example:public class Student { public String name; // each student object has a public double gpa; // name and gpa data field}

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public class Point3D { public int x; public int y;

public int z;}

Every object of type Point3D contains three integers.

Point3D objects (so far) do not contain any behavior.

Class declarations are saved in a file of the same name: Point3D.java

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Public and private modifiers

P332 Type specified as public

Any other class can directly access that object by name

Classes generally specified as public Instance variables usually not public

Instead specify as private

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Dog.java Change field to private http://www.cs.wcupa.edu/~zjiang/

Dog2trial.pdf

DogDemo.java http://www.cs.wcupa.edu/~zjiang/

DogDemo.pdf

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Demonstration of need for private variables

Statement such asbalto.name = "Balto";

is illegal since name is private

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Accessor and Mutator Methods

P337-338

Must also provide methods to change the values of the private instance variable Typically named setSomeValue Referred to as a mutator method

When instance variables are private must provide methods to access values stored there Typically named getSomeValue Referred to as an accessor method

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Consider the example class with accessor and mutator methods

http://www.cs.wcupa.edu/~zjiang/Dog2.pdf

Note the mutator method setName , setBreed , setAge

Note accessor methods getName, getBreed, getAge

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Using the accessor and mutator methods

http://www.cs.wcupa.edu/~zjiang/DogDemo2.pdf

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1 public class DogDemo2 2 { 3 public static void main(String[] args) 4 { 5 Dog2 balto = new Dog2(); 6 balto.setName("Balto"); 7 balto.setAge(8); 8 balto.setBreed("Siberian Husky"); 9 balto.writeOutput();10 11 Dog2 scooby = new Dog2();12 scooby.setName("Scooby");13 scooby.setAge(42);14 scooby.setBreed("Great Dane");15 System.out.println(scooby.getName()+" is a "+scooby.getBreed()+".");16 System.out.print("He is " + scooby.getAge() + " years old, or ");17 int humanYears = scooby.getAgeInHumanYears();18 System.out.println(humanYears + " in human years.");19 }20 }21

Tedious

We want something more like:

// better!Dog3 balto = new Dog3(“Balto”, “Siberian Husky”, 8);

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Constructor constructor: A special method that initializes

the state of new objects as they are created.

Constructor syntax, p352:public <class name> (<parameter(s)>) { <statement(s)>;}

How does this differ from previous methods? Method name No return!

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public class Dog3 { private String name; private String breed; private int age;

public Dog3(String strName, String strBreed, int

intAge) { name = strName; breed = strBreed; age = intAge; }}

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http://www.cs.wcupa.edu/~zjiang/Dog3.pdf http://www.cs.wcupa.edu/~zjiang/

DogDemo3.pdf

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Calling method from constructor: a constructor can call methods within its method body, in order to avoid unnecessarily repeating some code.

To avoid the problem with inheritance, which we will cover later, we need to make such a method private!

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Static Methods Some methods may have no relation

to any type of object, p501 One method each class

Example Compute max of two integers Convert character from upper- to lower case

Static method declared in a class Can be invoked without using an object Instead use the class name

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Adding Method main to a Class Method main used so far in its own class Often useful to include method main within

class definition To create objects of other classes To be run as a start of program For any ordinary class, method main is

ignored

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public class Point3D { public int x; public int y;

public int z;}

Every object of type Point3D contains three integers.

Point3D objects (so far) do not contain any behavior.

Class declarations are saved in a file of the same name: Point3D.java

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How would we translate several points?p1.x += 11;p1.y += 6;p1.z + 2;

p2.x += 2;p2.y += 4;p2.z += -2;

p3.x += 1;p3.y += 7;p3.z += 17;

What is unsettling about this code?04/20/23

23

Write a static method in the client code to translate points.

// Shifts the location of the given point.public static void translate(

Point p, int dx, int dy, int dz) { p.x += dx; p.y += dy; p.z += dz;}

Example:// move p2 translate(p2, 2, 4, 6);

Question: Why doesn't the method need to return the modified point?04/20/23

24

Every client code (application class) that declares Point3D and wants to translate points would have to write their own static translate method!

Also, the call syntax doesn't match the way we're used to interacting with objects:

translate(p2, 2, 4);

We want something more like:

p2.translate(2, 4);04/20/23

25

Instance methods

instance method: A method inside an object that operates on that object, p347

Declaring an object's method, general syntax:public <type> <name> (<parameter(s)>) {

<statement(s)>;

}

How does this differ from previous methods?

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An object's instance methods can refer to its fields.

public void translate(int dx, int dy, int dz) {

x += dx;

y += dy;

z += dz;

}

How does the translate method know which x, y, z to modify?04/20/23

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Each instance method call happens on a particular object.

Example: p1.translate(11, 6, 2);

The code for an instance method has an implied knowledge of what object it is operating on.

implicit parameter: The object on which an instance method is called.

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Think of each Point3D object as having its own copy of the translate method, which operates on that object's state:

Point3D p1 = new Point3D(7, 2, -2);Point3D p2 = new Point3D(4, 3, 2);

p1:

p2:

public void translate(int dx, int dy, int dz){ ...

}

y:x: 7 2 z: -2

public void translate(int dx, int dy, int dz){ ...

}

y:x: 4 3 z: 2

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29

What happens when the following calls are made?p1.translate(11, 6, 3);

p2.translate(1, 7, -10);

p1:

p2:

public void translate(int dx, int dy, int dz){ ...

}

y:x: 7 2 z: -2

public void translate(int dx, int dy, int dz){ ...

}

y:x: 4 3 z: 2

8 1

5 10 -8

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public class Point3D { public int x; public int y;

public Point3D(int initialX, int initialY, int initialZ) {

x = initialX; y = initialY; z = initialZ; }

// Changes the location of this Point3D object. public void translate(int dx, int dy, int dz){ x += dx; y += dy; z += dz; }}

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public class PointMain3 { public static void main(String[] args) { // create two Point3D objects Point3D p1 = new Point3D(5, 2, -2);

Point3D p2 = new Point3D(4, 3, 2);

// print each point System.out.println("p1 is (" + p1.x + ", " + p1.y + ")"); System.out.println("p2 is (" + p2.x + ", " + p2.y + ")");

// move p2 and then print it again p2.translate(2, 4, -2); System.out.println("p2 is (" + p2.x + ", " + p2.y + ")"); }}

Output:p1 is (5, 2, -2)p2 is (4, 3, 2)p2 is (6, 7, 0)

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Instead ofSystem.out.println("p1 is (" + p1.x + ", " + p1.y + ")");

It would be nice to have something more like:System.out.println("p1 is " + p1);

What does this line currently do? Does it even compile?It will print: p1 is Point@9e8c34

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When an object is printed or concatenated with a String, Java calls the object's toString method, p509-516.

System.out.println("p1 is " + p1);

is equivalent to:

System.out.println("p1 is " + p1.toString());

Note: Every class has a toString method.04/20/23

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The default toString behavior is to return the class's name followed by gibberish (as far as you are concerned).

You can replace the default behavior by defining a toString method in your class.

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The toString method, general syntax:public String toString() { <statement(s) that return a String>;}

NB: The method must have this exact name and signature (i.e., public String toString()).

Example:// Returns a String representing this Point.public String toString() { return "(" + x + ", " + y + “, “ + z + ")";}

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The Math Class Provides many standard mathematical

methods See in examples

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Recall that arguments of primitive type treated differently from those of a class type May need to treat primitive value as

an object Java provides wrapper classes for

each primitive type, p562 Methods provided to act on values

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Contain useful predefined constants and methods, for instance: Integer.MAX_VALUE Double.parseDouble(“199.88”) Double.toString(199.88)

Have no default constructor Programmer must specify an initializing value

when creating new object

Have no set methods04/20/23 39

Overloading When two or more methods have

same name within the same class Java distinguishes the methods by

number and types of parameters If it cannot match a call with a definition,

it attempts to do type conversions A method's name and number and

type of parameters is called the signature

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Overloading and automatic type conversion can conflict

Remember the compiler attempts to overload before it does type conversion

Use descriptive method names, avoid overloading

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P376 http://www.cs.wcupa.edu/~zjiang/Overload.pdf

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You must not overload a method where the only difference is the type of value returned

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Static Variables Are shared by all objects of a class

Variables declared static final are considered constants – value cannot be changed

e.g., public static final double FEET_PER_YARD = 3;

Variables declared static (without final) can be changed, p498 Only one instance of the variable exists It can be accessed by all instances of the class e.g., public static int numberOfInvocations;

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Static variables also called class variables Contrast with instance variables

Do not confuse class variables with variables of a class type

Both static variables and instance variables are sometimes called fields or data members

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Public and private modifier, p332 Accessor and mutator method, p337 Constructor, p352 Calling private method from constructor Static method, p501 Instance method, p347 Adding method main to a class Math class Wrapper class, p562 Overloading, p376 Static Variables, p498

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Summary