Java Type System and Object Model Horstmann ch.7.1-7.3.1, 7.7 - - PowerPoint PPT Presentation

Java Type System and Object Model

Horstmann ch.7.1-7.3.1, 7.7

Types

• Non-primitive types
• Primitive types
• Enum types
• Object
• Generics

Nonprimitive types

type

• bject

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

• Variable of non-primitive type

contains reference to object of

same type

Vehicle b;

b = new Vehicle();

• or of a subtype

Vehicle 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 non-primitive and T2 is Object – or T1 is null type and T2 is non-primitive – or T1 is array type and T2 is Cloneable or Serializable – or T1 is T3[], T2 is T4[] and T3 T4

Subtype Example

Class Hierarchy

Subtype relations:

Integer ≤ Number ≤ Serializable ≤ Object Integer ≤ Comparable<Integer> ≤ Object int[] ≤ Serializable ≤ Object

QUIZ

• 1. Yes 2. No

Comparable<String> is a subtype of Object? Comparable<String> is a subtype of String? String is a subtype of Comparable<String>? boolean is a subtype of Object? boolean[][] is a subtype of Object?

Subtype relation

Type check: compile time

• Static versus dynamic type

Vehicle v; v = new Automobile(); Object obj;

• bj = v;
• Compiler looks at static type only

Automobile bmw bmw = v;

• Use type cast

bmw = (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

public void test(Employee e) { e.setBonus(10); ((Manager)e).setBonus(10); } … test(new Driver()); …

QUIZ

No error Compiler error Exception

• n 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 Which – if any – errors arise? Static / dynamic type A B

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

QUIZ

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 What is written ? Type inquiry

Types

• Non-primitive types
• Primitive types
• Enum types
• Object
• Generics

Primitive types

Type 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;

5 n 3.0 a 4 k false good

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: ArrayList

ArrayList<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();

QUIZ

int n1 = 5/2; double n2 = 5/2; double n3 = 5/(double)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 Primitive types What is written ?

Types

• Non-primitive 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 instances

public enum Size { SMALL, MEDIUM, LARGE }; – is equivalent to 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(); } – Private constructor! – Implements Comparable ( SMALL < MEDIUM < LARGE) – Defines toString() (returns “SMALL”, “MEDIUM”, “LARGE”)

• Typical use:

Size imageSize = Size.MEDIUM; if (imageSize == Size.SMALL) . . .

Types

• Non-primitive types
• Primitive types
• Enum types
• Object
• Generics

Object: The Cosmic Superclass

• All classes extend Object
• Selected methods:

String toString() boolean equals(Object obj) Object clone() Class getClass() int hashCode()

Types

• Non-primitive types
• Primitive types
• Enum types
• Object
• Generics

Generic Types

public class NumberPair { int a; int b; public NumberPair(int x, int y) { a = x; b = y; } public String toString() { return ”[”+a+”,”+b+”]”; } } public class TextPair { String a; String b; public TextPair(String x, String y) { a = x; b = y; } public String toString() { return ”[”+a+”,”+b+”]”; } } public class Pair<T> { T a; T b; public Pair(T x, T y) { a = x; b = y; } public String toString() { return ”[”+a+”,”+b+”]”; } } NumberPair p = new NumberPair(3,5); TextPair s = new TextPair(”a”,”b”); Pair<Integer> p = new Pair<Integer>(3,5); Pair<String> s = new Pair<String>(”a”,”b”); Type variable T is declared

• 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> list1 = new ArrayList<Integer>(); ArrayList<int> list2 = new ArrayList<int>(); ArrayList<Object> list3 = new ArrayList<Integer>();

Generic Types

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

public int countNullNumber( ArrayList<Integer> l) { int count = 0; for (Integer n : l) if (n==null) count++; return count; } public int countNullText( ArrayList<String> l) { int count = 0; for (String s : l) if (s==null) count++; return count; } public <T> int countNull( ArrayList<T> l) { int count = 0; for (T t : l) if (t==null) count++; return count; } ArrayList<Integer> l1 = … ArrayList<String> l2 = … countNullNumber(l1) + countNullText(l2) countNull(l1) + countNull(l2) Type variable T is declared

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 F Make restriction: F is subtype of E

QUIZ

/* @param basket: some kind of container holding elements * @param value: a specific element (value != null) * @return true: if value occurs in basket * false: otherwise */ public <E, F extends Iterable<E>> boolean search(F basket, E value) { for (E elem : basket) if (value.equals(elem)) return true; return false; }

Is method correct? 1. No, compiler error 2. No, it compiles, but it is not correct 3. Yes, it compiles and it is correct 4. I don’t know Generic type

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)

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

• 1. <E>
• 2. <E extends Comparable<E>>
• 3. <E super Comparable<E>>
• 4. <E extends Comparable<? extends E>>
• 5. <E extends Comparable<? super E>>
• 6. <E super Comparable<? extends E>>
• 7. <E super Comparable<? super E>>
• 8. I don’t know

QUIZ

public static void sort(List<E> a) { ... } Generic type/wildcard