1. Inheritance and Encapsulation
CSIS 3701: Advanced Object Oriented Programming

2. Inheritance and Encapsulation
Key question:
What components of superclass do subclass methods have access to?
Can subclass methods access private superclass member variables?
Subclass
Superclass
Superclass variables
Superclass methods
Additional variables
Additional methods
Superclass variables
Superclass methods
inherits

3. Inheritance and Encapsulation
Subclass methods have no access to private components of superclass
public class SecondClock extends Clock { …
public String toString() {
String result = "";
if (hour < 10) result += "0";
result += hour + ":";
if (minute < 10) result += "0";
result += minute + ":";
if (second < 10) result += "0";
result += second;
return result; }
Compiler error
hour and minute are private to the Clock class

4. Inheritance and Encapsulation
Why does a subclass not have access to its own variables?
Key idea: developers of superclass and subclass may not be the same
Clock int hour int minute
SecondClock Code that directly refers to hour and minute
Fred develops Clock
Does not know that Barney has extended it
Barney extends Clock

5. Inheritance and Encapsulation
If developer of superclass changes internal representation, subclass will no longer work!
Clock String hour String minute
SecondClock Code that directly refers to hour and minute as integers
Fred changes Clock
Does not know that it will affect Barney’s class
SecondClock no longer works

6. Public Access as Solution
Solutions:
Only access superclass variables using superclass public methods
public String toString() {
String result = "";
if (getHour() < 10) result += "0";
result += getHour() + ":";
if (getMinute() < 10) result += "0";
result += getMinute() + ":"; …
Problem: What if no such public methods exist in superclass?

7. NameList Example Example: FlexName class extends NameList
Override add for list full case:
Increment maximum
Rebuild names array to be one element longer
Problem: these are private to NameList
No public methods to allow access, either
public class FlexName extends NameList { … public void add(String name) { if (isFull()) { maximum++; …

8. Protected Encapsulation
Declare component as protected encapsulation
May be accessed by subclasses but not other classes
public class NameList { protected int maximum; protected int count; protected String[] names; …
public class FlexName extends NameList { … public void add(String name) { if (isFull()) { maximum++; …
Since FlexName extends NameList, this is now legal

9. Protected Encapsulation
Problem: Developer of superclass may want to change internal representation in future
No longer possible without affecting derived classes
Better approach: Provide protected methods
Other classes still have no access
If internal representation changes, can just change method so behavior is still the same

10. Protected Methods public class NameList { private int maximum;
… protected int getMaximum() {return maximum;}
protected void setMaximum(int m) {maximum = m;}
Other classes cannot use these
public class FlexName extends NameList { … public void add(String name) { if (isFull()) { setMaximum(getMaximum() + 1); …