Java Type System and Object Model Horstmann ch.7.1-7.5, 7.7.

Java Type System and Object Model

Horstmann ch.7.1-7.5, 7.7

QUIZstatic/dynamic type

Which – if any – errors arise?

Employee e = null;

a) e.clear();

b) e.setSalary(1000);

No error Compiler error

Exception on runtime

1. a),b)

2. a) b)

3. a) b)

4. b) a)

5. a),b)

6. a) b)

7. b) a)

8. b) a)

9. a),b)

10. I don’t know

Types

• Non-primitive types• Primitive types

• Enum types

• Object

• Equals

• Clone

• Generics

Nonprimitive typestype object

class types Rectangle

String

new Rectangle(2,4,8,8)

"dProg2"

interface types Shape

Comparable

array types int[][]

String[]

new int[3][7]

{"dIntProg","dProg2"}

• no objects of interface type!

type value

null type null

Non-primitive type: variables

• Variables of non-primitive type contains reference to object of same typeVehicle b;

b = new Vehicle();

• or of a subtypeVehicle c;

c = New Automobile();

Vehicle d;d = new HorseCart();

Subtype Relationship

• T1 is subtype of T2, T1 T2 – if T1 is the same type as T2– or T1 implements T2– or T1 extends T2– or there is T3 such that T1 T3 and T3 T2– or T1 is T3[], T2 is T4[] and T3 T4– or T1 is array type and T2 is Cloneable or Serializable

– or T1 is non-primitive and T2 is Object– or T1 is null type and T2 is non-primitive

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[]• Iterator is a subtype of Object

QUIZ

1 Comparable is a subtype of Object?2 Comparable is a subtype of String?3 String is a subtype of Comparable?4 int[] is a subtype of Object?5 int[] is a subtype of Object[]?6 Serializable[] is a subtype of Object[]?7 int[][] is a subtype of Serializable[]?

Yes/No

Type check: compile time

• Static versus dynamic typeVehicle v;

v = new Automobile();

Object obj;

obj = v;

• Compiler looks at static type onlyAutomobile bmw

bmw = v;

• Use type castbmw = (Automobile) v;

• Compiler is happy!

compile time error

Static type of v is Vehicle

Dynamic type of v is Automobile

Static type of (Automobile)v is Automobile

Type check: runtime• You may fool the compiler:

Vehicle v = new HorseCart();Automobile bmw = (Automobile) v;

• Compiler is happy• But at runtime

Static type of (Automobile)v is Automobile

Exception in thread "main“ java.lang.ClassCastException: HorseCart cannot be cast to Automobile

Dynamic type of v is HorseCart

QUIZstatic/dynamic type

Which – if any – errors arise?

Employee e = new Driver();

a) e.setBonus(10);

b) ((Manager)e).setBonus(10);

No error Compiler error

Exception on runtime

1. a),b)

2. a) b)

3. a) b)

4. b) a)

5. a),b)

6. a) b)

7. b) a)

8. b) a)

9. a),b)

10. I don’t know

Type Inquiry

• Test subtype:if (bmw instanceof Vehicle) . . .

• Common before casts:Vehicle fourWheeler = (Vehicle) bmw;

• Test exact type:if (bmw.getClass() == Vehicle.class)

true if dynamic type of bmw is subtype of Vehicle

true if dynamic type of bmw is Vehicle and not a subtype

QUIZsubtype

What is written?

Employee e = new Manager();

System.out.println(

(e instanceof Comparable) +” ”+

(e instanceof Manager) +” ”+

e.getClass().getName() );

1. true true Employee

2. true true Manager

3. true false Employee

4. true false Manager

5. false true Employee

6. false true Manager

7. false false Employee

8. false false Manager

9. I don’t know

Types

• Non-primitive types

• Primitive types• Enum types

• Object

• Equals

• Clone

• Generics

Primitive typesType value

int

boolean

char

double

. . .

-13

true

'k'

4.567E-3

. . .

Primitive type: variables• Variable of primitive type contains value of same type

int k = 4;boolean good = false;

• Widening – In assignment, a widening conversion

(char → int → double) is automatic

double a = 3;

• Narrowing– Conversion in the other direction (narrowing) requires a cast

int n = (int) 5.7; 5n

3.0a

4k

falsegood

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

• Example: ArrayListArrayList<Integer> numbers = new ArrayList<Integer>();

Autoboxing Shorthand for

numbers.add(13); numbers.add(new Integer(13));

int n = numbers.get(0); int n = (numbers.get(0)).intValue();

QUIZprimitive types

What is written?

int n1 = 5/2;

double n2 = 5/2;

double n3 = (double)5/2;

System.out.println( n1 +” ”+ n2 +” ”+ n3 );

1. 2 2 2

2. 2.0 2.0 2.0

3. 2.5 2.5 2.5

4. 2 2.5 2.5

5. 2 2.0 2.5

6. 2 2 2.5

7. 2.0 2.0 2.5

8. 2.0 2.5 2.5

9. I don’t know

Types


• Primitive types

• Enum types• Object

• Equals

• Clone

• Generics

Enumerated Types• Finite set of values

– Example: { SMALL, MEDIUM, LARGE }

• Modelled by class with fixed number of instancespublic enum Size { SMALL, MEDIUM, LARGE };

– is equivalent topublic 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();

} – Private constructor!

• Typical use:Size imageSize = Size.MEDIUM;if (imageSize == Size.SMALL) . . .

Types


• Primitive types

• Enum types

• Object• Equals

• Clone

• Generics

Object: The Cosmic Superclass

• All classes extend Object • Selected methods:

String toString()

boolean equals(Object obj)

Object clone()

Class getClass()

int hashCode()

Types


• Primitive types

• Enum types

• Object

• Equals• Clone

• Generics

equals(…)

a == b is false

a.equals(b)

false, if equals inherited from Object

maybe true, if equals is overridden

Overriding equals(…)

public Class Vehicle {

private int noOfWheels;

private String name;

…

public boolean equals(Object obj) {

?

}

}

Overriding equals(…)




…


if (obj instanceof Vehicle) {

Vehicle other = (Vehicle) obj;

return (noOfWheels == other.noOfWheels) &&

name.equals(other.name);

} else return false;

}

equals(…) and inheritance

public Class Automobile extends Vehicle {

private PowerSource power;

…


?

}

}


public Class Automobile extends Vehicle {


…


if (obj instanceof Automobile) {

Automobile other = (Automobile) obj;

return super.equals(other) &&

power.equals(other.power);

} else return false;

}


• Asymmetry:

– X.equals(Y) returns true

– Y.equals(X) returns false

Requirements for equals Method

equals must be equivalence relation

• 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

perfect equals(…)




…

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

Vehicle obj = (Vehicle) obj;

return (noOfWheels == other.noOfWheels) &&

name.equals(other.name);

}

equals(…) and hashing

• hashCode method used in HashMap, HashSet • Computes an int from an object • Example: hash code of Stringint 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

equals(…) and 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 Vehicle { public int hashCode() { return 11 * name.hashCode() + 13 * new Double(noOfWheels).hashCode(); } ...}

QUIZclone/equals

What output should we expect?

MyClass x = new MyClass();

MyClass y = x.clone();

System.out.println(

x.equals(y) +” ”+ (x == y)

);

1. true true

2. true false

3. false true

4. false false

5. I don’t know

Types


• Primitive types

• Enum types

• Object

• Equals

• Clone• Generics

Shallow copy

• Assignment x = y;

Deep copy

• cloning x = y.clone();

Shallow cloning

• clone inherited from Object makes shallow cloning x = y.clone();

Deep cloning

• Override clone to make deep cloning x = y.clone();

Overriding clone()




…

public Vehicle clone() {

Vehicle twin = (Vehicle) super.clone();

twin.power = power.clone();

return twin;

}

}

Shallow cloning

Deep cloning

Actually, the compiler won’t accept this

Overriding clone()Public class Object {

protected Object clone() { … }

}

public Class Vehicle implements Cloneable {

public Vehicle clone() {

try {

Vehicle twin = (Vehicle) super.clone();

twin.power = power.clone();

return twin;

catch (CloneNotSupportedException e) { return null; }

}

}

Why so complicated ?

Never happens

Overriding clone()

• Deep cloning ?

QUIZdeep/shallow copy

How much does viggos account hold at end - in case of cloning being

Customer viggo = new Customer(

2412451111,

new Account(1000)

);

Customer twin = viggo.clone();

twin.getAccount().withdraw(200);

Shallow? Deep?

1. 1000 1000

2. 1000 800

3. 800 1000

4. 800 800

5. I don’t know

Types


• Primitive types

• Enum types

• Object

• Equals

• Clone

• Generics

Generic Types• A generic type has one or more type variables

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

• Type variables are instantiated with non-primitive types

ArrayList<Integer> = new ArrayList<Integer>();

ArrayList<int> = new ArrayList<int>();

ArrayList<Object> = new ArrayList<Integer>();

Type variable E is declared

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

int is primitive

Generic Methods• A generic method in an ordinary (non-generic) class

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); } }

• Type parameters are inferred in call

ArrayList<String> ids = new ArrayList<String>();

Utils.fill(ids, "default", 10);

Type variable E is declared

calls Utils.<String>fill

Type Bounds• The following method is limited:

public static <E> void append(ArrayList<E> a, ArrayList<E> b) {

for (E elem : b) a.add(elem);}

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

• Overcome limitation with type bound:

public static <E, F extends E> void append( ArrayList<E> a, ArrayList<F> b) { for (E elem : b) a.add(elem);

}

Declare 2 type variables E and FMake restriction: F is subtype of E

QUIZgeneric type

Is method correct?

1. No, compiler error

2. No, it compiles, but it is not correct

3. Yes, it compiles and it returns true precisely when value occurs in basket

4. I don’t know

public <E, F extends Iterable<E>>

boolean search(F basket, E value) {

for (E elem : basket)

if (value.equals(elem)) return true;

return false;

}

Wildcards• Overcome limitation with type bound:

public static <E, F extends E> void append( ArrayList<E> a, ArrayList<F> b) { for (E elem : b) a.add(elem);

}

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

public static <E> void append( ArrayList<E> a, ArrayList<? extends E> b) {

for (E elem : b) a.add(elem);}

Wildcards• Typical example--start with

public static <E extends Comparable<E>> E getMax(ArrayList<E> a) { E max = a.get(0); for (E elem : a) if (elem.compareTo(max) > 0) max = elem; return max; } • E extends Comparable<E> so that we can call compareTo • Too restrictive--can't call with ArrayList<Manager> • Manager does not implement Comparable<Manager>, only

Comparable<Employee> • Wildcards to the rescue:

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

QUIZgeneric

type/wildcard

How would you expect the Collections.sort(…) method to be defined?

1. public static <E> void sort(List<E> a) { ... }

2. public static <E extends Comparable<E>>

void sort(List<E> a) { ... }

3. public static <E extends Comparable<? super E>>

void sort(List<E> a) { ... }

4. public static <E extends Comparable<E>>

void sort(List<? extends E> a) { ... }

5. public static <E>

void sort(List<? extends Comparable<? super E>> a) { ... }

6. I don’t know

Java Type System and Object Model Horstmann ch.7.1-7.5, 7.7.

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