06 inheritance

1
TCSS 143, Autumn 2004
Lecture Notes
Inheritance

2
Inheritance: Definition
 inheritance: a parent-child relationship
between classes
 allows sharing of the behavior of the parent
class into its child classes
 one of the major benefits of object-oriented
programming (OOP) is this code sharing between
classes through inheritance
 child class can add new behavior or override
existing behavior from parent

3
Inheritance terms
 superclass, base class, parent class:
terms to describe the parent in the relationship,
which shares its functionality
 subclass, derived class, child class: terms
to describe the child in the relationship, which
accepts functionality from its parent
 extend, inherit, derive: become a subclass
of another class

4
Inheritance in Java
 in Java, you specify another class as your
parent by using the keyword extends
 public class CheckingAccount
extends BankAccount {
 the objects of your class will now receive all of the state
(fields) and behavior (methods) of the parent class
 constructors and static methods/fields are not inherited
 by default, a class's parent is Object
 Java forces a class to have exactly one parent
("single inheritance")
 other languages (C++) allow multiple inheritance

5
Inheritance Example
class BankAccount {
private double myBal;
public BankAccount() { myBal = 0; }
public double getBalance() { return myBal; }
}
class CheckingAccount extends BankAccount {
private double myInterest;
public CheckingAccount(double interest) { }
public double getInterest() { return myInterest; }
public void applyInterest() { }
}
 CheckingAccount objects have myBal and myInterest fields,
and getBalance(), getInterest(), and applyInterest() methods

6
Multiple layers of inheritance
 it is possible to extend a class that itself is a child
class; inheritance chains like this can be arbitrarily
deep
public class TransactionFeeCheckingAccount
extends CheckingAccount {
private static final double FEE = 2.00;
public void chargeFee() {
withdraw(FEE);
}
}

7
Inheritance Hierarchies
 Deeper layered chain of classes, many children
extending many layers of parents

8
"Has-a" Relationships
 "Has-a" relationship: when one object contains
another as a field
public class BankAccountManager {
private List myAccounts;
// ...
}
 a BankAccountManager object "has-a" List inside
it, and therefore can use it

9
"Is-a" relationships
 "Is-a" relationships represent sets of abilities;
implemented through interfaces and inheritance
public class CheckingAccount
// ...
}
 a CheckingAccount object "is-a" BankAccount
 therefore, it can do anything an BankAccount can do
 it can be substituted wherever a BankAccount is needed
 a variable of type BankAccount may refer to a
CheckingAccount object

10
Using the account classes
 CheckingAccount inherits BankAccount's methods
CheckingAccount c = new CheckingAccount(0.10);
System.out.println(c.getBalance());
c.applyInterest();
 a BankAccount variable can refer to a
CheckingAccount object
BankAccount b2 = new CheckingAccount(0.06);
System.out.println(b2.getBalance());
 an Object variable can point to either account type
Object o = new BankAccount();
Object o2 = new CheckingAccount(0.09);

11
Some code that won't compile
 CheckingAccount variable can't refer to BankAccount
(not every BankAccount "is-a" CheckingAccount)
CheckingAccount c = new BankAccount();
 cannot call a CheckingAccount method on a variable of type
BankAccount (can only use BankAccount behavior)
BankAccount b = new CheckingAccount(0.10);
b.applyInterest();
 cannot use any account behavior on an Object variable
Object o = new CheckingAccount(0.06);
System.out.println(o.getBalance());
o.applyInterest();

12
Mixing inheritance, interfaces
 It is legal for a class to extend a parent and to implement any number of
interfaces
public class BankAccount {
// ...
}
public class NumberedAccount
extends BankAccount implements Comparable {
private int myNumber;
public int compareTo(Object o) {
return myNumber - ((BankAccount)o).myNumber;
}
}

13
Overriding behavior
 Child class can replace the behavior of its parent's
methods by redefining them
 you have already done this ... where?
private double myBalance; // ....
public String toString() {
return getID() + " $" + getBalance();
} }
public class FeeAccount extends BankAccount {
private static final double FEE = 2.00;
public String toString() { // overriding
return getID() + " $" + getBalance()
+ " (Fee: $" + FEE + ")";
} }

14
Overriding behavior example
BankAccount b = new BankAccount("Ed", 9.0);
FeeAccount f = new FeeAccount("Jen", 9.0);
System.out.println(b);
System.out.println(f);
 Output:
Ed $9.0
Jen $9.0 (Fee: $2.0)

15
UML class diagrams
 an industry-standard way to
draw pictures of your
classes and their
relationships to each other
 classes are boxes that list
the fields, methods, and
constructors of the type
 classes that have
inheritance relationships, or
are tightly related to each
other, are connected by
arrows

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Class Diagram: Single Class
 attributes (fields) are written as
accessModifier name : type
where accessModifier is one of
 - for private
 + for public
 # for protected
 example: - mySize: int
 methods are written as
accessModifier name(arg1 : type1,
arg2 : type2, ...) : returnType
 omit the : returnType if returnType is void
 example: + withdraw(amount : double) : boolean

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Class diagram: inheritance
 inheritance relationships
 hierarchies drawn top-down with arrows from child to parent
 if parent is an interface, write its class name in << >> and
draw white dashed arrows
 if parent is a class, use black arrows

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Class diagram: associations
 associational relationships
1. multiplicity (how many)
2. name (what relationship the objects have)
3. navigability (who has relationship with whom)

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Access modifiers
 public: visible to all other classes
public class BankAccount
 private: visible only to the current class, its
methods, and every instance (object) of its class
 a child class cannot refer to its parent's private members!
private String myID;
 protected (this one's new to us): visible to the
current class, and all of its child classes
protected int myWidth;
 package (default access; no modifier): visible to all
classes in the current "package" (seen later)
int myHeight;

21
Access modifier problem (pt.1)
public class Parent {
private int field1;
protected int field2;
public int field3;
private void method1() {}
public void method2() {}
protected void setField1(int value) {
field1 = value;
}
}

22
Access modifier problem (pt.2)
public class Child extends Parent {
public int field4;
public Child() { // Which are legal?
field4 = 0; // _________
field1++; // _________
field2++; // _________
field3++; // _________
method1(); // _________
method2(); // _________
setField1(field4); // _________
}
}

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Some code that won't compile
public class Point2D {
private int x, y;
public Point2D(int x, int y) {
this.x = x; this.y = y;
}
}
public class Point3D extends Point2D {
private int z;
public Point3D(int x, int y, int z) {
this.x = x; this.y = y; // can't do this!
this.z = z;
}
}

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super keyword
 used to refer to superclass (parent) of current class
 can be used to refer to parent class's methods,
variables, constructors to call them
 needed when there is a name conflict with current class
 useful when overriding and you want to keep the old
behavior but add new behavior to it
 syntax:
super(args); // call parent’s constructor
super.fieldName // access parent’s field
super.methodName(args); // or method

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super example
private double myBalance;
public void withdraw(double amount) {
myBalance -= amount;
} }
public class FeeAccount
public void withdraw(double amount) {
super.withdraw(amount);
if (getBalance() < 100.00)
withdraw(2.00); // charge $2 fee
} }
 didn't need to say super.getBalance() because the FeeAccount
subclass doesn't override that method (it is unambiguous which version of
the method we are talking about)

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super and constructors
 if the superclass has a constructor that requires any arguments (not ()),
you must put a constructor in the subclass and have it call the super-
constructor (call to super-constructor must be the first statement)
public class Point2D {
private int x, y;
public Point2D(int x, int y) {
this.x = x; this.y = y;
}
}
public class Point3D extends Point2D {
private int z;
public Point3D(int x, int y, int z) {
super(x, y); // calls Point2D constructor
this.z = z;
}
}

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The instanceof keyword
 Performs run-time type check on the object referred to
by a reference variable
 Usage: object-reference instanceof type
(result is a boolean expression)
 if type is a class, evaluates to true if the variable refers to an
object of type or any subclass of it.
 if type is an interface, evaluates to true if the variable refers
to an object that implements that interface.
 if object-reference is null, the result is false.
 Example:
Object o = myList.get(2);
if (o instanceof BankAccount)
((BankAccount)o).deposit(10.0);

28
Some instanceof problems
Object o = new BankAccount(...);
BankAccount c = new CheckingAccount(...);
BankAccount n = new NumberedAccount(...);
CheckingAccount c2 = null;
T/F ???
o instanceof Object _______
o instanceof BankAccount _______
o instanceof CheckingAccount _______
c instanceof BankAccount _______
c instanceof CheckingAccount _______
n instanceof CheckingAccount _______
n instanceof Comparable _______
n instanceof NumberedAccount _______
c2 instanceof Object _______

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Which method gets called?
BankAccount b = new FeeAccount("Ed", 9.00);
b.withdraw(5.00);
System.out.println(b.getBalance());
 Will it call the withdraw method in
BankAccount, leaving Ed with $4?
 Will it call the withdraw method in
FeeAccount, leaving Ed with $2 (after his $2
fee)?

30
A hint about the right behavior
ArrayList list = new ArrayList();
list.add(new BankAccount("Ed", $9.0));
for (int i = 0; i < list.size(); i++) {
Object o = list.get(i);
System.out.println(o);
}
 Does it print "Object@FED87C" or
"Ed $9.00"?

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The answer: dynamic binding
 The version of withdraw from FeeAccount
will be called
 The version of an object's method that gets
executed is always determined by that object's
type, not by the type of the variable
 The variable should only be looked at to
determine whether the code would compile;
after that, all behavior is determined by the
object itself

32
Static and Dynamic Binding
 static binding: methods and types that are
hard-wired at compile time
 static methods
 referring to instance variables
 the types of the reference variables you declare
 dynamic binding: methods and types that are
determined and checked as the program is
running
 non-static (a.k.a virtual) methods that are called
 types of objects that your variables refer to

33
Polymorphism
 inheritance, like interfaces, provides a way to
achieve polymorphism in Java
 polymorphism: the ability to use identical
syntax on different data types, causing possibly
different underlying behavior to execute
 example: If we have a variable of type
BankAccount and call withdraw on it, it might
execute the version that charges a fee, or the
version from the checking account that tallies
interest, or the regular version, depending on the
type of the actual object.

34
Type-casting and objects
 You cannot call a method on a reference unless the
reference's type has that method
Object o = new BankAccount("Ed", 9.00);
o.withdraw(5.00); // doesn't compile
 You can cast a reference to any subtype of its current
type, and this will compile successfully
((BankAccount)o).withdraw(5.00);

35
Down-casting and runtime
 It is illegal to cast a reference variable into an
unrelated type (example: casting a String
variable into a BankAccount)
 It is legal to cast a reference to the wrong
subtype; this will compile but crash when the
program runs
 Will crash even if the type you cast it to has the
method in question
((String)o).toUpperCase(); // crashes
((FeeAccount)o).withdraw(5.00); // crashes

36
A dynamic binding problem
class A {
public void method1() { System.out.println(“A1”); }
public void method3() { System.out.println(“A3”); }
}
class B extends A {
public void method2() { System.out.println(“B2”); }
public void method3() { System.out.println(“B3”); }
}
A var1 = new B();
Object var2 = new A();
var1.method1();
var1.method2();
var2.method1(); OUTPUT???
((A)var2).method3();

37
A problem with interfaces
public interface Shape2D {
int getX();
int getY();
double getArea();
double getPerimeter();
}
 Every shape will implement getX and getY the
same, but each shape probably implements
getArea and getPerimeter differently

38
A bad solution
public class Shape implements Shape2D {
private int myX, myY;
public Shape(int x, int y) {
myX = x; myY = y;
}
public int getX() { return myX; }
public int getY() { return myY; }
// subclasses should override these, please
public double getArea() { return 0; }
public double getPerimeter() { return 0; }
}
 BAD: the Shape class can be instantiated
 BAD: a subclass might forget to override the methods

39
Abstract classes
 abstract class: a hybrid between an interface and a
class
 used to define a generic parent type that can contain method
declarations (like an interface) and/or method bodies (like a
class)
 like interfaces, abstract classes that cannot be instantiated
(cannot use new to create any objects of their type)
What goes in an abstract class?
 implement common state and behavior that will be
inherited by subclasses (parent class role)
 declare generic behaviors that subclasses must
implement (interface role)

40
Abstract class syntax
 put abstract keyword on class header and on
any generic (abstract) methods
 A class can be declared abstract even though it
has no abstract methods
 Any class with abstract methods must be declared
abstract, or it will not compile
 Variables of abstract types may be declared,
but objects of abstract types cannot be
constructed

41
Abstract class example
public abstract class Shape implements Shape2D {
private int myX, myY;
public Shape(int x, int y) {
myX = x; myY = y;
}
public int getX() { return myX; }
public int getY() { return myY; }
public abstract double getArea();
public abstract double getPerimeter();
}
 Shape class cannot be instantiated
 all classes that extend Shape must implement getArea and
getPerimeter or else must also be declared abstract

42
Extending an abstract class
public class Rectangle extends Shape {
private int myWidth, myHeight;
public Rectangle(int x, int y, int w, int h) {
super(x, y);
myWidth = w; myHeight = h;
}
public double getArea() {
return myWidth * myHeight;
}
public double getPerimeter() {
return 2*myWidth + 2*myHeight;
}
}
// ... example usage ...
Shape rect = new Rectangle(1, 2, 10, 5);

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Interface / abstract class chart

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Questions, Practice problem
 What are the differences between interfaces
and abstract classes? Why is it useful that both
exist in the Java language? When should one
use an abstract class and when an interface?
 Modify our old IntArrayList and
IntLinkedList so that they have a common
abstract base class AbstractIntList that
contains the common functionality between
them.

45
Inner classes
 inner class: a class defined inside of another
class
 can be created as static or non-static
 we will focus on standard non-static inner classes
 usefulness:
 inner classes can be "hidden" from other classes
(encapsulation at class level)
 useful when implementing data structures (e.g.
linked lists) or for GUI programming

46
Inner class behavior
 provide syntactic nesting of classes,
plus an association to the particular
object of the outer "enclosing" class that
created them
 each object I of the inner class is implicitly bound to
the object O (of the outer class) that created it
 can refer to it as OuterClassName.this if necessary
 an object of an inner class can access the fields and
methods of the outer object that created it (including
private fields and methods)

47
Inner class example
public class MyArrayList {
private Object[] items;
public Iterator iterator() {
return new MyIterator();
}
private class MyIterator implements Iterator {
int index = 0;
public boolean hasNext() {
return currentItem < size();
}
public Object next() {
if (!hasNext())
throw new NoSuchElementException();
else return items[index++];
}
}
}

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References
 Koffman/Wolfgang Ch. 3, pp. 125-149, 155-
159; Appendix B.1, pp. 738-744
 The Java Tutorial: Implementing Nested Classes.
http://java.sun.com/docs/books/tutorial/java/
javaOO/nested.html
 The Java Tutorial: Inheritance.
http://java.sun.com/docs/books/tutorial/java/
concepts/inheritance.html

06 inheritance

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