1. 10 More about inheritance
Objektorienterad programmering d2, förel. 10
10 More about inheritance
Polymorphism
BK Chap. 11
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2. Main concepts to be covered
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Main concepts to be covered
method polymorphism
static and dynamic type
overriding
dynamic method lookup
protected access
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3. The inheritance hierarchy
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The inheritance hierarchy
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4. Objektorienterad programmering d2, förel. 10
Conflicting output
Leonardo da Vinci
Had a great idea this morning.
But now I forgot what it was. Something to do with flying ...
40 seconds ago - 2 people like this.
No comments.
Alexander Graham Bell
[experiment.jpg]
I think I might call this thing 'telephone'.
12 minutes ago people like this.
What we want
Leonardo da Vinci
40 seconds ago people like this.
No comments.
Alexander Graham Bell
12 minutes ago people like this.
What we have
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5. Objektorienterad programmering d2, förel. 10
The problem
The display method in Post only prints the common fields.
Inheritance is a one-way street:
A subclass inherits the superclass fields.
The superclass knows nothing about its subclass’s fields.
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6. Attempting to solve the problem
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Attempting to solve the problem
Place display where it has access to the information it needs.
Each subclass has its own version.
But Post’s fields are private.
NewsFeed cannot find a display method in Post.
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7. Static type and dynamic type
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Static type and dynamic type
A more complex type hierarchy requires further concepts to describe it.
Some new terminology:
static type
dynamic type
method dispatch/lookup
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8. Static and dynamic type
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Static and dynamic type
What is the type of c1?
Car c1 = new Car();
What is the type of v1?
Vehicle v1 = new Car();
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9. Static and dynamic type
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Static and dynamic type
The declared type of a variable is its static type.
The type of the object a variable refers to is its dynamic type.
The compiler’s job is to check for static-type violations. for ( Post post : posts ) { post.display(); // Compile-time error. }
… print blir osynlig om vi flyttar den till subklasserna, post har ju typen Post.
Då klagar givetvis kompilatorn.
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10. The instanceof operator
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The instanceof operator
Used to determine the dynamic type.
Recovers ‘lost’ type information.
Usually precedes assignment with a cast to the dynamic type: if ( v1 instanceof Car ) { Car c2 = (Car) v1; … access Car methods via c2… }
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11. Overriding: the solution
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Overriding: the solution
display method in both super- and subclasses.
Satisfies both static and dynamic type checking.
Överskuggning, omdefinition
- är inte samma sak som överlagring
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12. Why static type checking?
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Why static type checking?
Has the static type void display()?
Object
Compiler
It has a void display()
… whatever it does
Static type
Dynamic type
The compiler only sees the reference variable
- not the object
Överskuggning, omdefinition
- är inte samma sak som överlagring
JVM
(runtime)
JVM relies on the compiler’s type checking
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13. Objektorienterad programmering d2, förel. 10
Overriding
Superclass and subclass define methods with the same signature.
Each has access to the fields of its class.
Superclass method satisfies static type checking.
Subclass method is called at runtime – it overrides the superclass version.
What becomes of the superclass version?
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14. Objektorienterad programmering d2, förel. 10
Method lookup 1
No inheritance.
The obvious method is selected.
Vid run-time!
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15. Objektorienterad programmering d2, förel. 10
Method lookup 2
Inheritance but no overriding. The inheritance hierarchy is ascended, searching for a match.
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16. Distinct static and dynamic types
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Distinct static and dynamic types
Post v1;
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17. Objektorienterad programmering d2, förel. 10
Method lookup 3
Overriding and polymorphism.
The ‘first’ version found is used.
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18. Objektorienterad programmering d2, förel. 10
Method lookup summary
The variable is accessed.
The object stored in the variable is found.
The class of the object is found.
The class is searched for a method match.
If no match is found, the superclass is searched.
This is repeated until a match is found, or the class hierarchy is exhausted.
Overriding methods take precedence.
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19. Objektorienterad programmering d2, förel. 10
Super call in methods
Overridden methods are hidden ...
... but we often still want to be able to call them.
An overridden method can be called from the method that overrides it.
super.method(...)
Compare with the use of super in constructors.
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20. Calling an overridden method
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Calling an overridden method
public class PhotoPost extends Post { ...
public void display() {
super.display();
System.out.println(" [" +
filename +
"]");
System.out.println(" " + caption); }
...
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21. Objektorienterad programmering d2, förel. 10
Method polymorphism
We have been discussing polymorphic method dispatch.
A polymorphic variable can store objects of varying types.
Method calls are polymorphic.
The actual method called depends on the dynamic object type.
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22. The Object class’s methods
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The Object class’s methods
Methods in Object are inherited by all classes.
Any of these may be overridden.
The toString method is commonly overridden:
public String toString()
Returns a string representation of the object.
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23. Overriding toString in Post
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Overriding toString in Post
public String toString() {
String text = username + "\n" +
timeString(timestamp);
if(likes > 0) {
text += " - " + likes + " people like this.\n"; }
else {
text += "\n";
if(comments.isEmpty()) {
return text + " No comments.\n";
return text + " " + comments.size() +
" comment(s). Click here to view.\n";
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24. Objektorienterad programmering d2, förel. 10
StringBuilder
Consider using StringBuilder as an alternative to concatenation: StringBuilder builder = new StringBuilder(); builder.append(username); builder.append('\n'); builder.append(timeString(timestamp)); ... return builder.toString();
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25. Objektorienterad programmering d2, förel. 10
Overriding toString
Explicit print methods can often be omitted from a class:
System.out.println(post.toString());
Calls to println with just an object automatically result in toString being called:
System.out.println(post);
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26. Overriding toString in MessagePost
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Overriding toString in MessagePost
public class MessagePost extends Post {
...
public String toString()
return super.toString() + "\n" +
" message: " + message+ "\n"; }
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27. Objektorienterad programmering d2, förel. 10
Protected access
Private access in the superclass may be too restrictive for a subclass.
The closer inheritance relationship is supported by protected access.
Protected access is more restricted than public access.
We still recommend keeping fields private.
Define protected accessors and mutators.
Klasser kan vara public, protected eller paketprivata
(men ej privata)
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28. Visibility of class members
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Visibility of class members
Modifier
In class
In package
In subclass
World
public
Yes
protected No
no modifier
(package private)
Yes inside package
No ow
private
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29. Objektorienterad programmering d2, förel. 10
Access levels
SomeClass
SubClass1
Client1
private
somePackage
someOtherPackage
SubClass2
no modifier
public
protected
Client2
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30. Objektorienterad programmering d2, förel. 10
Access levels
Example
A class member (method, constructor, variable) which is declared in SomeClass with access level
private is only visible in SomeClass
no modifier (package private) is visible in SomeClass, SubClass1, Client1
protected is visible in SomeClass, SubClass1, Client1, SubClass2
public is visible everywhere
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31. Access levels and overriding
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Access levels and overriding
An overriding method in a sub class must be at least as visible as the overridden method in its base class.
Visibility in base class
Allowed visibility in sub class
public
protected
protected public
package private
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32. Overriding and Covariant Return Types
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Overriding and Covariant Return Types
In Java, the return type of an overriding method is allowed to be a subtype of the return type of the overridden method.
This is called covariance.
The benefit is type safety
Less need for unsafe type casts when calling overridden methods.
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33. Overriding and Covariant Return Types
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Overriding and Covariant Return Types
public class A {} public class B extends A {} public class Base {
public A f() { return new A(); }
public Base g() { return new Base(); }
} public class Sub extends Base {
public A f() { return new B(); }
public B f() { return new B(); }
// In particular, covariance can be
// applied to the class itself:
public Base g() { return new Base(); } public Base g() { return new Sub(); }
public Sub g() { return new Sub(); } }
Any of these
is a correct overriding of f
Any of these
is a correct overriding of g
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34. Overriding and Covariant Return Types
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Overriding and Covariant Return Types
Example (no covariance)
public class SomeClass implements Cloneable {
public Object clone() {
return super.clone(); }
} ...
SomeClass x = new SomeClass();
...
SomeClass y = (SomeClass)x.clone();
clone overrides Object.clone
Without covariance we need an unsafe type cast here
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35. Overriding and Covariant Return Types
Objektorienterad programmering d2, förel. 10
Overriding and Covariant Return Types
Example (using covariance)
public class SomeClass implements Cloneable {
public SomeClass clone() {
return (SomeClass)super.clone(); }
} ...
SomeClass x = new SomeClass();
...
SomeClass y = x.clone();
Overriding Object.clone using sub type as return type
No need for type cast
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36. The @Override annotation
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The @Override annotation
A simple typing misstake can easily spoil overriding!
Example
public class Base { public void f() { ... }
public void f(int x) { ... } public void g(float x) { ... }
} public class Sub extends Base {
public void f(int x) { ... } public void g(int x) { ... } }
Ok, f overrides Base.f(int)
Sub.g(int) does not override Base.g(float)
- rather, it is a new method!
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37. The @Override annotation
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The @Override annotation
annotation indicates to the compiler that a method is intended to override some method in a super class.
If a method is annotated but does not override a super class method, a compilation error results.
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38. The @Override annotation
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The @Override annotation
public class Base { public void f() { ... }
public void f(int x) { ... } public void g(float x) { ... }
} public class Sub extends Base {
@Override
public void f(int x) { ... } @Override public void g(int x) { ... } }
Ok, f overrides Base.f(int)
The compiler signals an error
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39. Objektorienterad programmering d2, förel. 10
Review
The declared type of a variable is its static type.
Compilers check static types.
The type of the object a variable refers to is its dynamic type.
Dynamic types are used at runtime.
Methods may be overridden in a subclass.
Method lookup starts with the dynamic type.
Protected access supports inheritance.
Overriding methods may have covariant return types.
annotation makes overriding safer.
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