1. CSC240 Computer Science III
Zhen Jiang
Dept. of Computer Science
West Chester University
West Chester, PA 19383
11/12/2018

2. Table of Contents Introduction to field (attribute)
Public and private modifiers
Accessor and mutator methods
Constructor
Static method
Instance method
Overloading
Static variables
Packages and importing
Summary
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3. 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>; or
public <type> <name> = <value>; (with initialization)‏
Example:
public class Dog {
public String name; // each dog object has a
public int age; // name and age data field }
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4. Public and private modifiers
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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5. Dog.java DogDemo.java Change field to private
DogDemo.java
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6. Error!
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7. Demonstration of need for private variables
Statement such as balto.name = "Balto"; is illegal since name is private
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8. Accessor and Mutator Methods
P344
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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9. Consider the example class with accessor and mutator methods
Note the mutator method
setName , setBreed , setAge
Note accessor methods
getName, getBreed, getAge
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10. Using the accessor and mutator methods
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11. 1 public class DogDemo2 2 { public static void main(String[] args) { Dog2 balto = new Dog2(); balto.setName("Balto"); balto.setAge(8); balto.setBreed("Siberian Husky"); balto.writeOutput(); Dog2 scooby = new Dog2(); scooby.setName("Scooby"); scooby.setAge(42); scooby.setBreed("Great Dane"); System.out.println(scooby.getName()+" is a "+scooby.getBreed()+"."); System.out.print("He is " + scooby.getAge() + " years old, or "); int humanYears = scooby.getAgeInHumanYears(); System.out.println(humanYears + " in human years."); } 20 } 21
Tedious
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12. // better! We want something more like:
Dog3 balto = new Dog3(“Balto”, “Siberian Husky”, 8);
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13. 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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14. 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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15.
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17. 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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18. Static Methods
Some methods may have no relation to any type of object, p
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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19. 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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20. Every object of type Point3D contains three integers.
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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21. 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?
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22. 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?
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23. We want something more like: p2.translate(2, 4);
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);
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24. Instance methods
instance method: A method inside an object that operates on that object.
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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25. 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?
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26. 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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27. 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);
public void translate(int dx, int dy, int dz){
... } y: x: 7 2 z: -2
p1:
public void translate(int dx, int dy, int dz){
... } y: x: 4 3 z: 2
p2:
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28. What happens when the following calls are made?
p1.translate(11, 6, 3);
p2.translate(1, 7, -10);
public void translate(int dx, int dy, int dz){
... } x: 18 7 y: 8 2 z: 1 -2
p1:
public void translate(int dx, int dy, int dz){
... } x: 5 4 y: 10 3 z: 2 -8
p2:
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29. 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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30. 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); }
Output:
p1 is (5, 2, -2)‏
p2 is (4, 3, 2)‏
p2 is (6, 7, 0)‏
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31. It would be nice to have something more like:
Instead of
System.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
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32. Note: Every class has a toString method.
When an object is printed or concatenated with a String, Java calls the object's toString method.
System.out.println("p1 is " + p1);
is equivalent to:
System.out.println("p1 is " + p1.toString());
Note: Every class has a toString method.
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33. 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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34. 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.
return "(" + x + ", " + y + “, “ + z + ")";
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35. Provides many standard mathematical methods
The Math Class
Provides many standard mathematical methods
See in examples
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37. Java provides wrapper classes for each primitive type,
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,
Methods provided to act on values
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38. 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 methods
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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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40. 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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41. P376
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42. You must not overload a method where the only difference is the type of value returned
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43. 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
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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44. 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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45. Packages and Importing
A package is a collection of classes grouped together into a folder,
Name of folder is name of package
Each class
Placed in a separate file
Has this line at the beginning of the file package Package_Name;
Classes use packages by use of import statement
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46. Search for package begins in class path base directory
Package name tells compiler path name for directory containing classes of package
Search for package begins in class path base directory
Package name uses dots in place of / or \
Name of package uses relative path name starting from any directory in class path
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47. Figure 6.5 A package name
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48. Packages help in dealing with name clashes
When two classes have same name
Different programmers may give same name to two classes
Ambiguity resolved by using the package name
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49. Summary Public and private modifier, Accessor and mutator method,
Constructor,
Calling private method from constructor,
Static method,
Adding method main to a class,
Math class,
Wrapper class,
Overloading,
Static Variables,
Packages.
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