11. Reference Organization Packages. What is reference organization? It is a process that enables us to group classes and interfaces into isolated namespaces.

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);

}

}