Object-Oriented Design & Patterns

Cay S. Horstmann

Chapter 7 The Java Object Model

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Chapter Topics

The Java Type System
Type Inquiry
The Object Class
Shallow and Deep Copy
Serialization
Reflection
The Java Beans Component Model

Types

Type: set of values and the operations that can be applied to the values
Strongly typed language: compiler and run-time system check that no operation can execute that violates type system rules

Compile-time check

Employee e = new Employee();
e.clear(); // ERROR

Run-time check:
```
e = null;
e.setSalary(20000); // ERROR
```

Java Types

Primitive types: int short long byte char float double boolean
Class types
Interface types
Array types
The null type
Note: void is not a type

Exercise: What kind of type?

int
Rectangle
Shape
String[]
double[][]

Java Values

value of primitive type
reference to object of class type
reference to array
null
Note: Can't have value of interface type

Exercise: What kind of value?

13
new Rectangle(5, 10, 20, 30);
"Hello"
new int[] { 2, 3, 5, 7, 11, 13 }
null

Subtype Relationship

S is a subtype of T if

S and T are the same type
S and T are both class types, and T is a direct or indirect superclass of S
S is a class type, T is an interface type, and S or one of its superclasses implements T
S and T are both interface types, and T is a direct or indirect superinterface of S
S and T are both array types, and the component type of S is a subtype of the component type of T
S is not a primitive type and T is the type Object
S is an array type and T is Cloneable or Serializable
S is the null type and T is not a primitive type

Subtype Examples

Container is a subtype of Component
JButton is a subtype of Component
FlowLayout is a subtype of LayoutManager
ListIterator is a subtype of Iterator
Rectangle[] is a subtype of Shape[]
int[] is a subtype of Object
int is not a subtype of long
long is not a subtype of int
int[] is not a subtype of Object[]

Subtype Examples

The ArrayStoreException

Rectangle[] is a subtype of Shape[]

Can assign Rectangle[] value to Shape[] variable:

Rectangle[] r = new Rectangle[10];
Shape[] s = r;

Both r and s are references to the same array
That array holds rectangles
The assignment s[0] = new Polygon(); compiles
Throws an ArrayStoreException at runtime
Each array remembers its component type

Array References

Wrapper Classes

Primitive types aren't classes
Use wrappers when objects are expected

Wrapper for each type:

Integer Short Long Byte
Character Float Double Boolean

Auto-boxing and auto-unboxing

ArrayList<Integer> numbers = new ArrayList<Integer>();
numbers.add(13); // calls new Integer(13)
int n = numbers.get(0); // calls intValue();

Enumerated Types

Finite set of values
Example:
```
enum Size { SMALL, MEDIUM, LARGE }
```
Typical use:
```
Size imageSize = Size.MEDIUM;
```
if (imageSize == Size.SMALL) . . .

Safer than integer constants

public static final int SMALL = 1;
 public static final int MEDIUM = 2;
 public static final int LARGE = 3;

Typesafe Enumerations

enum equivalent to class with fixed number of instances

public class Size
{
   private /* ! */ Size() { }
   public static final Size SMALL = new Size();
   public static final Size MEDIUM = new Size();
   public static final Size LARGE = new Size();
}

enum types are classes; can add methods, fields, constructors

Type Inquiry

Test whether e is a Shape:
```
if (e instanceof Shape) . . .
```
Common before casts:
```
Shape s = (Shape) e;
```
Don't know exact type of e
Could be any class implementing Shape
If e is null, test returns false (no exception)

The `Class` Class

getClass method gets class of any object
Returns object of type Class

Class object describes a type

Object e = new Rectangle();
Class c = e.getClass();
System.out.println(c.getName()); // prints java.awt.Rectangle

Class.forName method yields Class object:

Class c = Class.forName("java.awt.Rectangle");

.class suffix yields Class object:

Class c = Rectangle.class; // java.awt prefix not needed

Class is a misnomer: int.class, void.class,Shape.class

An `Employee` Object vs. the `Employee.class` Object

Type Inquiry

Test whether e is a Rectangle:

if (e.getClass() == Rectangle.class) . . .

Ok to use ==
A unique Class object for every class
Test fails for subclasses
Use instanceof to test for subtypes:
```
if (e instanceof Rectangle) . . .
```

Array Types

Can apply getClass to an array

Returned object describes an array type

double[] a = new double[10];
Class c = a.getClass();
if (c.isArray())
   System.out.println(c.getComponentType()); 
      // prints double

getName produces strange names for array types

[D for double[])
[[java.lang.String; for String[][]

Object: The Cosmic Superclass

All classes extend Object
Most useful methods:
- String toString()
- boolean equals(Object otherObject)
- Object clone()
- int hashCode()

The `toString` Method

Returns a string representation of the object
Useful for debugging
Example: Rectangle.toString returns something like
```
java.awt.Rectangle[x=5,y=10,width=20,height=30]
```
toString used by concatenation operator
aString + anObject means
```
aString + anObject.toString()
```

Object.toString prints class name and object address

System.out.println(System.out)

yields

java.io.PrintStream@d2460bf

Implementor of PrintStream didn't override toString:

Overriding the `toString` Method

Format all fields:

public class Employee 
{ 
   public String toString() 
   { 
      return getClass().getName() 
         + "[name=" + name 
         + ",salary=" + salary 
         + "]"; 
   } 
   ... 
}

Typical string:

Employee[name=Harry Hacker,salary=35000]

Overriding `toString` in Subclass

Format superclass first

public class Manager extends Employee 
{ 
   public String toString() 
   {  
      return super.toString() 
         + "[department=" + department + "]"; 
   } 
   ... 
}

Typical string

Manager[name=Dolly Dollar,salary=100000][department=Finance]

Note that superclass reports actual class name

The `equals` Method

equals tests for equal contents
== tests for equal location
Used in many standard library methods

Example:

ArrayList.indexOf

/** 
   Searches for the first occurrence of the given argument, 
   testing for equality using the equals method. 
   @param elem an object. 
   @return the index of the first occurrence 
   of the argument in this list; returns -1 if 
   the object is not found. 
*/ 
public int indexOf(Object elem) 
{ 
   if (elem == null) 
   { 
      for (int i = 0; i < size; i++) 
         if (elementData[i] == null) return i; 
   } 
   else 
   { 
      for (int i = 0; i < size; i++) 
         if (elem.equals(elementData[i])) return i; 
   } 
   return -1; 
}

Overriding the `equals` Method

Notion of equality depends on class

Common definition: compare all fields

public class Employee 
{ 
   public boolean equals(Object otherObject) 
      // not complete--see below 
   { 
      Employee other = (Employee)otherObject; 
      return Objects.equals(name, other.name) 
         && salary == other.salary; 
   } 
   ... 
}

Must cast the Object parameter to subclass
Use == for primitive types, Objects.equals (null-safe version of Object.equals) for object fields

Overriding `equals` in Subclass

Call equals on superclass

public class Manager 
{ 
   public boolean equals(Object otherObject) 
   { 
      Manager other = (Manager)otherObject; 
      return super.equals(other) 
         && department.equals(other.department); 
   } 
}

Not all `equals` Methods are Simple

Two sets are equal if they have the same elements in some order

public boolean equals(Object o) 
{ 
   if (o == this) return true; 
   if (!(o instanceof Set)) return false; 
   Collection c = (Collection) o; 
   if (c.size() != size()) return false; 
   return containsAll(c); 
}

The `Object.equals`Method

Object.equals tests for identity:

public class Object 
{ 
   public boolean equals(Object obj) 
   { 
      return this == obj; 
   } 
   ... 
}

Override equals if you don't want to inherit that behavior

Requirements for `equals` Method

reflexive: x.equals(x)
symmetric: x.equals(y) if and only if y.equals(x)
transitive: if x.equals(y) and y.equals(z), then x.equals(z)
x.equals(null) must return false

Fixing `Employee.equals`

Violates two rules
Add test for null: if (otherObject == null) return false
What happens if otherObject not an Employee
Should return false (because of symmetry)
Common error: use of instanceof if (!(otherObject instanceof Employee)) return false; // don't do this for non-final classes
Violates symmetry: Suppose e, m have same name, salary e.equals(m) is true (because m instanceof Employee) m.equals(e) is false (because e isn't an instance of Manager)
Remedy: Test for class equality if (getClass() != otherObject.getClass()) return false;

The Perfect `equals` Method

Start with these three tests:

public boolean equals(Object otherObject) 
{ 
   if (this == otherObject) return true; 
   if (otherObject == null) return false; 
   if (getClass() != otherObject.getClass()) return false; 
   ... 
}

First test is an optimization

Hashing

hashCode method used in HashMap, HashSet
Computes an int from an object

Example: hash code of String

int h = 0;
for (int i = 0; i < s.length(); i++)
   h = 31 * h + s.charAt(i);

Hash code of "eat" is 100184
Hash code of "tea" is 114704

Hashing

Must be compatible with equals: if x.equals(y), then x.hashCode() == y.hashCode()
Object.hashCode hashes memory address
NOT compatible with redefined equals

Remedy: Hash all fields and combine codes:

public class Employee 
{ 
   public int hashCode() 
   { 
      return Objects.hash(name, salary);
   } 
   ... 
}

Shallow and Deep Copy

Assignment (copy = e) makes shallow copy
Clone to make deep copy
Employee cloned = e.clone();

Cloning

Cloning

Object.clone makes new object and copies all fields
Cloning is subtle
Object.clone is protected

Subclass must redefine clone to be public

public class Employee 
{ 
   public Employee clone()
   { 
      return (Employee) super.clone(); // not complete 
   } 
   ... 
}

The `Cloneable` Interface

Object.clone is nervous about cloning
Will only clone objects that implement Cloneable interface
```
public interface Cloneable
{
}
```
Interface has no methods!
Tagging interface--used in test
```
if x implements Cloneable
```
Object.clone throws CloneNotSupportedException
A checked exception

The `clone` Method

public class Employee 
  implements Cloneable 
{ 
   public Employee clone() 
   { 
      try 
      { 
         return (Employee) super.clone(); 
      } 
      catch(CloneNotSupportedException e) 
      { 
         return null; // won't happen 
      }
   } 
   ... 
}

Or better:

public Employee clone() throws CloneNotSupportedException
{ 
   return (Employee) super.clone(); 
}

Shallow Cloning

clone makes a shallow copy
Instance fields aren't cloned

Deep Cloning

Why doesn't clone make a deep copy? Wouldn't work for cyclic data structures
Not a problem for immutable fields
You must clone mutable fields

public class Employee 
  implements Cloneable 
{ 
   public Object clone() 
   { 
      try 
      { 
         Employee cloned = (Employee)super.clone();
         cloned.hireDate = (Date)hiredate.clone();
         return cloned; 
      } 
      catch(CloneNotSupportedException e) 
      { 
         return null; // won't happen 
      }
   } 
   ... 
}

Deep Cloning

Cloning and Inheritance

Object.clone is paranoid
- clone is protected
- clone only clones Cloneable objects
- clone throws checked exception
You don't have that luxury
Manager.clone must be defined if Manager adds mutable fields
Rule of thumb: if you extend a class that defines clone, redefine clone
Lesson to learn: Tagging interfaces are inherited. Use them only to tag properties that inherit

Serialization

Save collection of objects to stream

Employee[] staff = new Employee[2]; 
staff.add(new Employee(...)); 
staff.add(new Employee(...));

Construct ObjectOutputStream:

ObjectOutputStream out 
   = new ObjectOutputStream( 
      new FileOutputStream("staff.dat"));

Save the array and close the stream
```
out.writeObject(staff); 
out.close(); 
```

Serialization

The array and all of its objects and their dependent objects are saved
Employee doesn't have to define any method
Needs to implement the Serializable interface
Another tagging interface with no methods

How Serialization Works

Each newly encountered object is saved
Each object gets a serial number in the stream
No object is saved twice
Reference to already encountered object saved as "reference to #"

How Serialization Works

Serialing Unserializable Classes

Some classes are not serializable
Example: BufferedImage
How can we serialize a class with a BufferedImage field?
Suppress default serialization to avoid exception
Mark with transient:
```
private transient BufferedImage image;
```
Supply private (!) methods private void writeObject(ObjectOutputStream out) private void readObject(ObjectInputStream in)
In these methods
- Call writeDefaultObject/readDefaultObject
- Manually save other data

Reflection

Ability of running program to find out about its objects and classes
Class object reveals
- superclass
- interfaces
- package
- names and types of fields
- names, parameter types, return types of methods
- parameter types of constructors

Reflection

Class getSuperclass()
Class[] getInterfaces()
Package getPackage()
Field[] getDeclaredFields()
Constructor[] getDeclaredConstructors()
Method[] getDeclaredMethods()

Enumerating Fields

Print the names of all static fields of the Math class:

Field[] fields = Math.class.getDeclaredFields(); 
for (Field f : fields) 
   if (Modifier.isStatic(f.getModifiers())) 
      System.out.println(f.getName());

Enumerating Constructors

Print the names and parameter types of all Rectangle constructors:

for (Constructor c : cons) 
{ 
   Class[] params = cc.getParameterTypes(); 
   System.out.print("Rectangle(");
   boolean first = true; 
   for (Class p : params) 
   { 
      if (first) first = false; else System.out.print(", "); 
      System.out.print(p.getName()); 
   } 
   System.out.println(")"); 
}

Yields

Rectangle() 
Rectangle(java.awt.Rectangle) 
Rectangle(int, int, int, int) 
Rectangle(int, int) 
Rectangle(java.awt.Point, java.awt.Dimension) 
Rectangle(java.awt.Point) 
Rectangle(java.awt.Dimension)

Getting A Single Method Descriptor

Supply method name
Supply array of parameter types

Example: Get Rectangle.contains(int, int):

Method m = Rectangle.class.getDeclaredMethod(
   "contains", int.class, int.class);

Example: Get default Rectangle constructor:

Constructor c = Rectangle.class.getDeclaredConstructor();

getDeclaredMethod, getDeclaredConstructor are varargs methods

Invoking a Method

Supply implicit parameter (null for static methods)
Supply array of explicit parameter values
Wrap primitive types
Unwrap primitive return value

Example: Call System.out.println("Hello, World") the hard way.

Method m = PrintStream.class.getDeclaredMethod(
   "println", String.class);
m.invoke(System.out, "Hello, World!");

invoke is a varargs method

Inspecting Objects

Can obtain object contents at runtime
Useful for generic debugging tools

Need to gain access to private fields

Class c = obj.getClass(); 
Field f = c.getDeclaredField(name); 
f.setAccessible(true);

Throws exception if security manager disallows access

Access field value:

Object value = f.get(obj); 
f.set(obj, value);

Use wrappers for primitive types

Inspecting Objects

Example: Peek inside string tokenizer
Ch7/../../code/ch07/reflect2/FieldTester.java

Output

int currentPosition=0 
int newPosition=-1 
int maxPosition=13 
java.lang.String str=Hello, World! 
java.lang.String delimiters=, 
boolean retDelims=false 
boolean delimsChanged=false 
char maxDelimChar=, 
--- 
int currentPosition=5 
. . .

Inspecting Array Elements

Use static methods of Array class

Object value = Array.get(a, i);
Array.set(a, i, value);

```
int n = Array.getLength(a);
```

Construct new array:

Object a = Array.newInstance(type, length);

Generic Types

A generic type has one or more type variables
Type variables are instantiated with class or interface types
Cannot use primitive types, e.g. no ArrayList<int>

When defining generic classes, use type variables in definition:

public class ArrayList<E>
{
   public E get(int i) { . . . }
   public E set(int i, E newValue) { . . . }
   . . .
   private E[] elementData;
}

NOTE: If S a subtyoe of T, ArrayList<S> is not a subtype of ArrayList<T>.

Generic Methods

Generic method = method with type parameter(s)

public class Utils
{
   public static <E> void fill(ArrayList<E> a, E value, int count)
   {
      for (int i = 0; i < count; i++)
         a.add(value);
   }  
}

A generic method in an ordinary (non-generic) class

Type parameters are inferred in call

ArrayList<String> ids = new ArrayList<String>();
Utils.fill(ids, "default", 10); // calls Utils.<String>fill

Type Bounds

Type variables can be constrained with type bounds
Constraints can make a method more useful

The following method is limited:

public static <E> void append(ArrayList<E> a, ArrayList<E> b, int count)
{
   for (int i = 0; i < count && i < b.size(); i++)
      a.add(b.get(i));
}

Cannot append an ArrayList<Rectangle> to an ArrayList<Shape>

Type Bounds

Overcome limitation with type bound:

public static <E, F extends E> void append(
   ArrayList<E> a, ArrayList<F> b, int count)
{
   for (int i = 0; i < count && i < b.size(); i++)
      a.add(b.get(i));
}

extends means "subtype", i.e. extends or implements
Can specify multiple bounds: E extends Cloneable & Serializable

Wildcards

Definition of append never uses type F. Can simplify with wildcard:

public static <E> void append(
   ArrayList<E> a, ArrayList<? extends E> b, int count)
{
   for (int i = 0; i < count && i < b.size(); i++)
      a.add(b.get(i));
}

Wildcards

Wildcards restrict methods that can be called:

ArrayList<? extends E>.set

method has the form

? extends E add(? extends E newElement)

You cannot call this method!
No value matches ? extends E because ? is unknown
Ok to call get: ? extends E get(int i)
Can assign return value to an element of type E

Wildcards

Wildcards can be bounded in opposite direction
? super F matches any supertype of F

public static <F> void append(
   ArrayList<? super F> a, ArrayList<F> b, int count)
{
   for (int i = 0; i < count && i < b.size(); i++)
      a.add(b.get(i));
}

Safe to call ArrayList<? super F>.add:
```
boolean add(? super F newElement)
```
Can pass any element of type F (but not a supertype!)

Wildcards

Typical example--start with

public static <E extends Comparable<E>> E getMax(ArrayList<E> a)
{
   E max = a.get(0);
   for (int i = 1; i < a.size(); i++)
      if (a.get(i).compareTo(max) > 0) max = a.get(i);
   return max;
}

E extends Comparable<E> so that we can call compareTo
Too restrictive--can't call with ArrayList<GregorianCalendar>
GregorianCalendar does not implement Comparable<GregorianCalendar>, only Comparable<Calendar>

Wildcards to the rescue:

public static <E extends Comparable<? super E>> E getMax(ArrayList<E> a)

Type Erasure

Virtual machine does not know about generic types
Type variables are erased--replaced by type bound or Object if unbounded

Ex. ArrayList<E> becomes

public class ArrayList
{
   public Object get(int i) { . . . }
   public Object set(int i, Object newValue) { . . . }
   . . .
   private Object[] elementData;
}

Ex. getMax becomes

public static Comparable getMax(ArrayList a)
   // E extends Comparable<? super E> erased to Comparable

Erasure necessary to interoperate with legacy (pre-JDK 5.0) code

Limitations of Generics

Cannot replace type variables with primitive types
Cannot construct new objects of generic type a.add(new E()); // Error--would erase to new Object()

Workaround: Use class literals

public static <E> void fillWithDefaults(ArrayList<E>,
   Class<? extends E> cl, int count)
   throws InstantiationException, IllegalAccessException
{
   for (int i = 0; i < count; i++)
      a.add(cl.newInstance());
}

Call as fillWithDefaults(a, Rectangle.class, count)

Limitations of Generics

Cannot form arrays of parameterized types Comparable<E>[] is illegal. Remedy: ArrayList<Comparable<E>>
Cannot reference type parameters in a static context (static fields, methods, inner classes)
Cannot throw or catch generic types
Cannot have type clashes after erasure. Ex. GregorianCalendar cannot implement Comparable<GregorianCalendar> since it already implements Comparable<Calendar>, and both erase to Comparable

Components

More functionality than a single class
Reuse and customize in multiple contexts
"Plug components together" to form applications
Successful model: Visual Basic controls
- calendar
- graph
- database
- link to robot or instrument
Componens composed into program inside builder environment

A Builder Environment

Java Beans

Java component model
Bean has
- methods (just like classes)
- properties
- events

Java Beans

A Calendar Bean

A Property Sheet

Edit properties with property sheet

Façade Class

Bean usually composed of multiple classes
One class nominated as facade class
Clients use only facade class methods

Façade Pattern

Context

A subsystem consists of multiple classes, making it complicated for clients to use
Implementor may want to change subsystem classes
Want to give a coherent entry point

Solution

Define a facade class that exposes all capabilities of the subsystem as methods
The facade methods delegate requests to the subsystem classes
The subsystem classes do not know about the facade class

Façade Pattern

Façade Pattern

Name in Design Pattern	Actual Name (Beans)
Client	Builder tool
Facade	Main bean class with which the tool interacts
SubsystemClass	Class used to implement bean functionality

Bean Properties

Property = value that you can get and/or set
Most properties are get-and-set
Can also have get-only and set-only
Property not the same as instance field
Setter can set fields, then call repaint
Getter can query database

Property Syntax

Not Java :-(
C#, JavaScript, Visual Basic
b.propertyName = value calls setter
variable = b.propertyName calls getter

Java Naming Conventions

property = pair of methods

public X getPropertyName()
public void setPropertyName(X newValue)

Replace propertyName with actual name (e.g. getColor/setColor)
Exception for boolean properties:
```
public boolean isPropertyName()
```
Decapitalization hokus-pokus:
```
getColor -> color
getURL -> URL
```

Editing Beans in a Builder Tool

Use wizard to make empty frame

Editing Beans in a Builder Tool

Add button to frame
Edit button with property sheet

Packaging a Bean

Compile bean classes Ch7/carbean/CarBean.java
Create manifest file Ch7/carbean/CarBean.mf
Run JAR tool:
```
jar cvfm CarBean.jar CarBean.mf *.class
```
Import JAR file into builder environment

Composing Beans

Make new frame
Add car bean, slider to frame
Edit stateChanged event of slider
Add handler code
```
carBean1.setX(jSlider1.getValue());
```
Compile and run
Move slider: the car moves

Object-Oriented Design & Patterns

Cay S. Horstmann

Chapter 7

The Java Object Model

Chapter Topics

Types

Java Types

Exercise: What kind of type?

Java Values

Exercise: What kind of value?

Subtype Relationship

Subtype Examples

Subtype Examples

The ArrayStoreException

Array References

Wrapper Classes

Enumerated Types

Typesafe Enumerations

Type Inquiry

The Class Class

An Employee Object vs. the Employee.class Object

Type Inquiry

Array Types

Object: The Cosmic Superclass

The toString Method

Overriding the toString Method

Overriding toString in Subclass

The equals Method

Overriding the equals Method

Overriding equals in Subclass

Not all equals Methods are Simple

The Object.equalsMethod

Requirements for equals Method

Fixing Employee.equals

The Perfect equals Method

Hashing

Hashing

Shallow and Deep Copy

Cloning

Cloning

The Cloneable Interface

The clone Method

Shallow Cloning

Deep Cloning

Deep Cloning

Cloning and Inheritance

Serialization

Serialization

How Serialization Works

How Serialization Works

Serialing Unserializable Classes

Reflection

Reflection

Enumerating Fields

Enumerating Constructors

Getting A Single Method Descriptor

Invoking a Method

Inspecting Objects

Inspecting Objects

Inspecting Array Elements

Generic Types

Generic Methods

Type Bounds

Type Bounds

Wildcards

Wildcards

Wildcards

Wildcards

Type Erasure

Limitations of Generics

Limitations of Generics

Components

A Builder Environment

Java Beans

Java Beans

A Calendar Bean

A Property Sheet

Façade Class

Façade Pattern

Façade Pattern

The `Class` Class

An `Employee` Object vs. the `Employee.class` Object

The `toString` Method

Overriding the `toString` Method

Overriding `toString` in Subclass

The `equals` Method

Overriding the `equals` Method

Overriding `equals` in Subclass

Not all `equals` Methods are Simple

The `Object.equals`Method

Requirements for `equals` Method

Fixing `Employee.equals`

The Perfect `equals` Method

The `Cloneable` Interface

The `clone` Method