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CMPE212 – Section 001 - Reminders
• Assignment 3 on Encapsulation due Friday.
• You know everything you need to know for
Assignment 4, as well.
• Quiz 2 this week in the lab.
Winter 2017
CMPE212 - Prof. McLeod
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Today
• Generic wildcards.
• Generic Factory Demo.
• Next topic (we won’t have time today): Lambda
Functions.
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Generic Methods and Java 7
• Constructors can also be generic. Even in a nongeneric class.
• And you can use type inference as demonstrated in
TestUtility.java:
midStrings = Utility.getMidpoint(strings);
• Same as:
midStrings = Utility.<String>getMidpoint(strings);
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CMPE212 - Prof. McLeod
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Generics & Backwards Compatibility
• Generic code can make for very versatile
programs that can be applied to a range of
objects – saving you a great deal of boring coding
work.
• But, generic code will not work easily with preJava 5.0 code.
• And, of course, the Java 7 type inference stunts
will not work with pre-Java 7.0 code.
• Generics are a bit like Templates in C++ and C#,
but are implemented in a different way.
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CMPE212 - Prof. McLeod
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Generics in Modern Java
• This discussion assumes newer versions of Java
(>= 1.7)
• Generics allow you to write powerful, compact
code that can work with many object types.
Particularly useful with hierarchies.
• Type inference, bounding, wildcards and the use
of the Class<T> object adds power and
convenience.
• Let’s experiment!
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First Demo
• TestWildcards.java
• Uses the Person hierarchy from last week.
• Plays with wildcards, bounded generic methods,
and how to create an array of generic classes.
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Summary: Wildcards
• If Child is a subclass of Parent then this works:
Parent obj = new Child();
• But this will not work:
ArrayList<Parent> alObj = ArrayList<Child>();
• This will work:
ArrayList<?> alObj = ArrayList<Child>();
• A “wildcard” is the ? within the < >.
• A wildcard can provide a super type generic class
for other generic classes.
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Summary: Wildcards, Cont.
• You can bind a wildcard to classes that extend
other classes using the extends keyword. For
example:
ArrayList<? extends Person> db
• db can be assigned to an object of type
ArrayList<Person>, ArrayList<Student> or
ArrayList<Professor>.
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Summary: Wildcards, Cont.
• However, a generic typed with a wildcard is not
mutable.
• You can get around this using generic methods
without the wildcard. But:
– This is more restrictive and potentially “brittle”.
– Avoid casting using the generic type. This may lead to
“Unchecked Casts” which reduces the type safety of
the generic method. (Hint: use
@SuppressWarnings("unchecked"))
– Use of Class<T> may be a way around this problem.
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Summary: Wildcards, Cont.
• ArrayList<?>[] is the only possible type that
allows the creation of an array of generic classes.
• But, why would you want to?
• Easier to create an ArrayList of ArrayLists, for
example.
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CMPE212 - Prof. McLeod
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Aside – Lower Bound for Wildcard
• In
? extends T
the extends keyword represents an upper bound
for the type – T or any sub-type of T.
• You can also use super as in
? super T
which provides a lower bound for the type – T or
any super-type of T.
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Second Demo
• GenericFactoryDemo.java
• Can a generic method instantiate an array of type
T?
• Can a generic method instantiate a generic class
typed to T?
• Can a generic method instantiate objects of type
T? If so, how?
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Summary: Generic Factories
• Factory methods generate objects, or more
specifically, collections of objects. How can you
make these methods non- type-specific?
• You cannot create an array of type T in a generic
method unless you can tolerate an unchecked
cast to T[].
• You cannot instantiate objects of type T in a
generic method directly.
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The Class<T> Object
• You can pass a type into a method using a
Class<T> object.
• The Object class contains a method .getClass()
that returns a Class<T> object. Or you can just
use the .class literal on a type. For example:
String aString = "Hello!";
System.out.println(aString.getClass());
System.out.println(String.class);
• Shows:
class java.lang.String
class java.lang.String
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The Class<T> Object, Cont.
• See the API for lots of methods belonging to this
Object. Useful for determining all kinds of
information about a type or an instance.
• Also has a method called .newInstance() that
returns an instance of type T.
• This method can only invoke the default
constructor of the type T. So, you must use
mutators to set attributes. But now you have an
instance to work with!
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From Before: Anonymous Class Example
public class AnonymousClassDemo {
public static void main(String[] args) {
MessageSender ms = new MessageSender() {
public void sendGreeting (String name) {
System.out.println("Hello " + name + "!");
} // end sendGreeting
}; // end ms, NOTE ";"!
ms.sendGreeting(“Steve");
} // end main
} // end AnonymousClassDemo
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Anonymous Class Example - Cont.
• MessageSender is an interface, not an Object:
public interface MessageSender {
void sendGreeting (String name);
} // end MessageSender interface
• Java 8 now has a very tidy solution to using
messy anonymous classes – Lambda Functions:
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The Lambda Version
public class LambdaDemo {
public static void main(String[] args) {
MessageSender ms =
name -> System.out.println("Hello " + name);
ms.sendGreeting("Steve");
} // end main
} // end LambdaDemo
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A Lambda Function
• Kind of like an “anonymous method”.
• Syntax:
– Parameters for the method first. No
parameters, use { }, one parameter by itself –
no brackets required, more than one use (a,
b, etc.). No types required.
– Then the fancy -> “arrow”.
– Then the method body. Put more than one
statement inside { }.
• Can even define an inner interface:
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The Lambda Version, Cont.
public class LambdaDemoAgain {
interface MessageSender {
void sendGreeting(String aName);
} // end MessageSender interface
public static void main(String[] args) {
MessageSender ms =
name -> System.out.println("Hello " + name);
ms.sendGreeting("Steve");
} // end main
} // end LambdaDemoAgain
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Lambda Functions, Cont.
• Note how the abstract method in the interface
determines the structure of the lambda function –
the parameter and parameter type, the method
name and the lack of a return statement.
• These functions could be useful! (Especially
when attaching events to GUI components.)
• Only certain interface structures can be used.
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