11. reference organization packages. what is reference organization? it is a process that enables us...
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11. Reference Organization
Packages
What is reference organization?
• It is a process that enables us to group classes and interfaces into isolated namespaces.
What is an isolated namespace?
• A name is an identifier.
• A namespace is place where identifiers are grouped.
• An isolated namespace is an isolated identifier group.
Why isolate identifier groups?
• To enable the reuse of identifiers w/o causing confusion.
• For example:– import java.util.List;– import java.awt.List;– ....– List l = new List(); – // this is an error, how do I fix it?
Ambiguous class name conflict...
• java.awt.List l = new java.awt.List();
• java.util.List ll = new java.util.List();
Why not just use unique identifiers?
• Because we can’t control all the programmers who write libraries that we want to use.
What is a package?
• Isolated grouping of reference identifiers (classes and interfaces)
How do I create a package?
reserved word “package” appears as the first java statement in a file.
It appears only once.
ex:
package com.docjava.futil;
public class Foo {}
Packages are ALWAYS public!
• private package foo; // syntax error
• public package foo; // syntax error
• protected package foo; //syntax error
• package foo; // OK.
Visibilty
• Public – world access
• Protected – sub-class access
• Default – package access
• Private – only within the class.
ReadOnly Variables
public class Customer {
private String name = “J. Shmoe”;
public String getName() {
return name;
}
}
Why do I need public?
• Publish a constant:– public static final double PI =
3.141592653589626
• Publish a method – public static double sin(double x){…}
publish a class
- allows others to use your API!
Why do I need protected?
• You cannot have a protected class.
• You can have protected variables and methods.
• Only class authors can use protected items.
• Sub-Class authors know what they are doing!??!? Trust them!?!?!
Why do I need private?
• Don't trust the subclass author!
• Some things can be read but not written.
• Some things can be written but not read.
• I can control access to state variables.
Read-only variable
public class Taxes {private int interestRate = 3;
public getInterestRate() {// accessor
return interestRate;
}
public void greenSpanManeuver() {
interestRate = interestRate + 1;
}
}
Write-only Variable
public class Taxes {private int interestRate = 3;
public void setInterestRate(int r) {// mutator
interestRate = r;
}
public void greenSpanManeuver() {
interestRate = interestRate + 1;
}
}
Why do I need private methods?
• you don't want to publish the method!
• Perhaps the spec will change.
• public stuff must be stable
• public stuff must be supported
• public stuff must be documented
Be careful what you make public
• public interface car {– public int getSpeed();– public int getOdometer();– public int getGas();– public void setAcceleration(int a);– public void setSteerWheelAngle(int degrees);
• }
Default is easy!
• do nothing, it is default!
• Why not make everything default?– No one can use your stuff!– Your stuff is unpublished!– complexity in an OOD is a function of class
interdependence. – You must encapsulate complexity!
How do I share protected elements?
package dl;
public class ShareMe {
protected int i = 10;
}
Using the protected member
package sop;
class SomeClass extends dl.ShareMe {
SomeClass() {
System.out.println(i);
}
}