Transcript Objects First with Java

More about inheritance
Exploring polymorphism
5.0
Main concepts to be covered
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method polymorphism
static and dynamic type
overriding
dynamic method lookup
protected access
Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling
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The inheritance hierarchy
display() method is in superclass Post
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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 - 4 people like this.
No comments.
Leonardo da Vinci
40 seconds ago No comments.
2 people like this.
Alexander Graham Bell
12 minutes ago - 4 people like this.
No comments.
What we want
(original w/o
inheritance)
What we have
(results w/
inheritance)
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The problem
• The display method in Post only prints
the common fields
• Inheritance is a one-way street:
– A subclass inherits the superclass fields
– Superclass knows nothing about subclass’s fields
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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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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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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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Static and dynamic type
What is the type of c1?
Car c1 = new Car();
Car (type)
What is the type of v1?
Vehicle v1 = new Car();
Vehicle (static type)
Car (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.
}
Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling
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Overriding: the solution
display method
in both superand subclasses
Satisfies both
static and
dynamic type
checking
NOTE: display method in Post would never execute!
Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling
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Overriding
• Superclass and subclass define
methods with the same signature
• Each has access to the fields of its
own class
• Superclass satisfies static type check
• Subclass method is called at runtime
– it overrides the superclass version
• What becomes of the superclass
version?
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Distinct static and
dynamic types
The variable post has a:
• static type of Post
• dynamic type of PhotoPost
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Method lookup
No inheritance or polymorphism.
The obvious method is selected.
display is executed from PhotoPost
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Method lookup
Inheritance (without overriding) where the inheritance
hierarchy is ascended, searching for a match.
display is NOT found in PhotoPost,
but it is found and executed from Post
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Method lookup
Polymorphism and overriding exists,
where search starts with dynamic type (ascending up)
and the first version found is used.
display is found & executed in PhotoPost
Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling
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Method lookup summary
• The variable is accessed
• The object stored in the variable is found
• The (dynamic) 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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Super call in methods
• Overridden methods are hidden, but we
may still want to be able to call them
• An overridden method can be called
from the method that overrides it by
using super (i.e. super.method())
• Compare with the use of super in
constructors (i.e. super())
– method name is explicitly stated (i.e. super.method())
– super.method may be used anywhere, and
not just the first statement (as with constructors)
– no automatic super call (as in constructors) is
generated, nor is a super.method call required
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Calling an overridden method
public void display()
{
super.display();
System.out.println(" [" + filename
+ "]");
System.out.println(" " + caption);
}
Calls the superclass display method and then
continues with printing its own fields.
Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling
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Method polymorphism
• We have been discussing polymorphic
method dispatch
• A polymorphic variable can store
objects of varying types
• Method calls are also polymorphic
– The actual method called depends on
the dynamic object type
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The Object class’s methods
• Methods in the Object library class
are inherited by ALL classes
• Any of those found in Object may
be overridden (i.e. toString)
• The implicitly-defined toString
method is commonly overridden:
– public String toString()
– returns a string representation of the
object
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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";
}
else {
return text + " " + comments.size() +
" comment(s). Click here to view.\n";
}
}
Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling
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Overriding toString
• Explicit print methods can often be
omitted from a class:
System.out.println(post.toString());
• Calls to println and print with
just an object automatically result in
toString being called:
System.out.println(post);
Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling
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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();
The overloaded append method takes a
parameter of any type and effectively converts it
to its string representation before appending it
to the original StringBuilder object.
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Object equality
• What does it mean for two objects to
be ‘the same’?
– Reference equality
(only 1 object)
– Content equality
(2 objects with the same internal states)
• Compare use of == with equals() in
with strings in the String class:
– == compares reference equality
– equals() compares content equality
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Overriding equals
Object class method actually tests for reference equality!
public boolean equals(Object obj)
{
return this == obj; // reference to itself
}
To test for content equality, we must override the method!
public boolean equals(Object obj)
{
if(this == obj) {
return true;
// reference to itself
}
if(!(obj instanceof ThisType)) {
return false; // different object types
}
ThisType other = (ThisType) obj;
// cast into right variable type
return field.equals(other.field) && …
// compare all field contents of this and other
}
// for content equality
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Overriding equals
in Student
public boolean equals(Object obj)
{
if(this == obj) {
return true;
// reference equality
}
if(!(obj instanceof Student)) {
return false;
// different object types
}
Student other = (Student) obj; // cast
return name.equals(other.name) &&
id.equals(other.id) &&
credits == other.credits;
}
// test content equality
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Overriding hashCode
in Student
/**
* Uses values of name, id & credits fields
* to calculate the distinct hashCode
* representation of a specific Student object
*/
public int hashCode()
{
int result = 17;
result = 37 * result + name.hashCode();
result = 37 * result + id.hashCode();
result = 37 * result + credits;
return result;
}
hashCode method should also be overridden
whenever the equals method is overridden.
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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
• But allows access to fields/methods within
a class itself AND from all its subclasses
– protected accessors, mutators & constructors
• We still recommend keeping fields private
– to maintain encapsulation
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Access levels
Method calls from Subclass to protected methods in its superclass
SomeClass are made similar to calls to its own internal methods.
Thus, NO object variable (with dot notation) is necessary.
getSomeClassField()
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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(post instanceof MessagePost) {
MessagePost msg =(MessagePost) post;
… access MessagePost methods via msg …
}
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Review
• The declared type of a variable is its static
type
– Compilers check static types
• The type of an object 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
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