Object-Oriented Design & Patterns

Cay S. Horstmann

Chapter 7

The Java Object Model

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 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 name.equals(other.name) 
           && salary == other.salary; 
     } 
     ... 
  } 
  

• Must cast the Object parameter to subclass
• Use == for primitive types, 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.equalsMethod

• 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 name.hashCode() 
           + new Double(salary).hashCode(); 
     } 
     ... 
  } 
  

Shallow and Deep Copy

• Assignment (copy = e) makes shallow copy
• Clone to make deep copy
• Employee cloned = (Employee)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 Object clone() 
     { 
        return 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 Object clone() 
   { 
      try 
      { 
         return super.clone(); 
      } 
      catch(CloneNotSupportedException e) 
      { 
         return null; // won't happen 
      }
   } 
   ... 
}

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
• Security? Anonymous classes? Programmer cluelessness?
• Example: Ellipse2D.Double
• How can we serialize Car?
  
• Suppress default serialization to avoid exception
• Mark with transient:
  private transient Ellipse2D frontTire;
• Supply private (!) methods
  private void writeObject(ObjectOutputStream out)
  private void readObject(ObjectInputStream in)
• In these methods
  • Call writeDefaultObject/readDefaultObject
  • Manually save other data
    
• Ch7/serial/Car.java

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/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<? extendsE>.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

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

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

Façade Pattern

Façade Pattern

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

Composing Beans