Chapter 21 Generics
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Chapter 21 Generics 1
description
Chapter 21 Generics. Generics - Overview. Generic Methods specify a set of related methods Generic classes specify a set of related types Software reuse with compile-time type safety. What is Generics?. Generics is the capability to parameterize types. - PowerPoint PPT Presentation
Transcript of Chapter 21 Generics
Chapter 21 Generics
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Generics - Overview
•Generic Methods specify a set of related methods
•Generic classes specify a set of related types
•Software reuse with compile-time type safety
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What is Generics?
• Generics is the capability to parameterize types.
• Can define class or method with generic types
• Compiler substitutes with concrete types. • Example:
▫ a generic stack class that stores elements of a generic type.
▫ Can create stack of strings and stack of numbers.
▫ String and Number are concrete types.
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Generic Methods
•Method overloading – can have several identical methods with same number of arguments, but of different types (printArray for Integer, Double, Character)
•Can replace with one generic method
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Generic Methods
•Syntax:Public static <E> void print (E[] list)
Type parameter section in angle brackets before return type of method
Can be comma separated list.
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Generic Methods6
public static <E> void print(E[] list) {
for (int i = 0; i < list.length; i++)
System.out.print(list[i] + " ");
System.out.println();
}
public static void print(Object[] list) {
for (int i = 0; i < list.length; i++)
System.out.print(list[i] + " ");
System.out.println();
}
Generic Methods
Generic methods can be defined in ordinary classes.
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public class ArrayAlg{
public static <T> T getMiddle(T[] a) {
return a[a.length/2];
}
}
Pre-JDK 1.5
•Generics introduced in JDK 1.5•Prior versions of Java used inheritance•Parameters can be of type Object
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Why Generics?
•What was wrong with using Object?▫Casting is necessary when return value is
of general typeAND▫No error checking - the type of object can
keep changing.
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Why Generics?
• Key benefit: generics enable errors to be detected at compile time rather than at runtime.
• With generic class or method, can specify types of objects that the class or method may work with.
• Attempt to use class or method with incompatible object -> compilation error.
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Generic Type11
package java.lang; public interface Comparable { public int compareTo(Object o)
}
package java.lang; public interface Comaprable<T> { public int compareTo(T o)
}
(a) Prior to JDK 1.5 (b) JDK 1.5
Generic Instantiation Runtime error
Compile error
Comparable c = new Date(); System.out.println(c.compareTo("red"));
(a) Prior to JDK 1.5
Comparable<Date> c = new Date(); System.out.println(c.compareTo("red"));
(b) JDK 1.5
Improves reliability
Why Do You Get a Warning?
public class ShowUncheckedWarning { public static void main(String[] args) { java.util.ArrayList list = new java.util.ArrayList(); list.add("Java Programming"); }}
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JDK 1.5 generics. (Types may not be compatible.)
Fix the Warning
public class ShowUncheckedWarning { public static void main(String[] args) { java.util.ArrayList<String> list = new java.util.ArrayList<String>(); list.add("Java Programming"); }}
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No compile warning on this line.
`
Generic ArrayList in JDK 1.514
java.util.ArrayList
+ArrayList()
+add(o: Object) : void
+add(index: int, o: Object) : void
+clear(): void
+contains(o: Object): boolean
+get(index: int) : Object
+indexOf(o: Object) : int
+isEmpty(): boolean
+lastIndexOf(o: Object) : int
+remove(o: Object): boolean
+size(): int
+remove(index: int) : boolean
+set(index: int, o: Object) : Object
java.util.ArrayList<E>
+ArrayList()
+add(o: E) : void
+add(index: int, o: E) : void
+clear(): void
+contains(o: Object): boolean
+get(index: int) : E
+indexOf(o: Object) : int
+isEmpty(): boolean
+lastIndexOf(o: Object) : int
+remove(o: Object): boolean
+size(): int
+remove(index: int) : boolean
+set(index: int, o: E) : E
(a) ArrayList before JDK 1.5 (b) ArrayList in JDK 1.5
No Casting NeededArrayList<Double> list = new ArrayList<Double>();list.add(5.5); // 5.5 is automatically converted to new Double(5.5)list.add(3.0); // 3.0 is automatically converted to new Double(3.0)Double doubleObject = list.get(0); // No casting is neededdouble d = list.get(1); // Automatically converted to double
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Declaring Generic Classes and Interfaces
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GenericStackGenericStack
GenericStack<E> -list: java.util.ArrayList<E>
+GenericStack()
+getSize(): int
+peek(): E
+pop(): E
+push(o: E): E
+isEmpty(): boolean
Creates an empty stack.
Returns the number of elements in this stack.
Returns the top element in this stack.
Returns and removes the top element in this stack.
Adds a new element to the top of this stack.
Returns true if the stack is empty.
An array list to store elements.
Bounded Generic Type
Syntax:<E extends BoundingType>
•Use keyword extends.•Both E and BoundingType can be a class
or interface.•Separate several bounds with & •(comma separates type variables)
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Bounded Generic Type18
public static void main(String[] args ) {
Rectangle rectangle = new Rectangle(2, 2);
Circle9 circle = new Circle9(2);
System.out.println("Same area? " + equalArea(rectangle, circle));
}
public static <E extends GeometricObject> boolean
equalArea(E object1, E object2) {
return object1.getArea() == object2.getArea();
}
C++
•C++ templates are like Java generics.
•C++ creates multiple copies of code for different possible template instantiations.
•Java uses a single, general copy of code
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Erasure
Java uses type erasure to create one version of code with a raw type.
•Type variables are erased and replaced by their bounding types
•What if no bounding types? Object•Cast - method calls that return raw type.
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Raw Type and Backward Compatibility
// raw typeArrayList list = new ArrayList();
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This is roughly equivalent to ArrayList<Object> list = new ArrayList<Object>();
Raw Type is Unsafe
Max.max("Welcome", 23);
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// Max.java: Find a maximum object
public class Max {
/** Return the maximum between two objects */
public static Comparable max(Comparable o1, Comparable o2) {
if (o1.compareTo(o2) > 0)
return o1;
else
return o2;
}
}
Runtime Error:
Make it Safe
Max.max("Welcome", 23);
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// Max1.java: Find a maximum object
public class Max1 {
/** Return the maximum between two objects */
public static <E extends Comparable<E>> E max(E o1, E o2) {
if (o1.compareTo(o2) > 0)
return o1;
else
return o2;
}
}
Compile-time Error:
Limitations•Primitive types cannot be used for type
parameter - use wrapper classes.
•Type inquiries (instanceof) yield only the raw types.
•Arrays cannot be of generic types. (Use ArrayList to collect generic types.)
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Limitations (cont.)
•Cannot instantiate generic types. Error: new E().
•Type variables cannot be used in a static context - which type should they be?!
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Inheritance
•No relationship between Pair<S> and Pair<T>, no matter how S and T are related
Why not? •Code won’t always work•(You can insert Double to ArrayList of
Number, not to ArrayList of Integer)
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Wildcards
Why wildcards are necessary? See this example.
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WildCardDemo1WildCardDemo1
? unbounded wildcard
? extends T bounded wildcard
? super T lower bound wildcard
WildCardDemo2WildCardDemo2 WildCardDemo3WildCardDemo3
Wildcards
? unbounded wildcard
? extends T bounded wildcard T or any of its subclasses
? super T lower bound wildcard T or any of its
superclasses
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Generic Types and Wildcard Types
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Object
?
? super E
E’s superclass
E
? extends E
E’s subclass
Object
A<?>
A<? super B>
A<B’s subclass>
A<? extends B>
A<B>
A<B’s superclass>
Wildcards
Advantage:•Wildcards provide flexibility when passing
parameterized types to a method.
Disadvantage:•Cannot be used as type name throughout
method body.
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Avoiding Unsafe Raw Types Use
new ArrayList<ConcreteType>()
Instead of
new ArrayList();
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TestArrayListNewTestArrayListNew RunRun
Erasure and Restrictions on Generics
Implementation of generics: type erasure.
The compiler uses the generic type information to compile the code, but erases it afterwards.
Generic information is not available at run time.
Backward-compatibility with legacy code that uses raw types.
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Compile Time Checking
For example, the compiler checks whether generics is used correctly for the following code in (a) and translates it into the equivalent code in (b) for runtime use. The code in (b) uses the raw type.
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ArrayList<String> list = new ArrayList<String>();
list.add("Oklahoma");
String state = list.get(0);
(a) (b)
ArrayList list = new ArrayList();
list.add("Oklahoma");
String state = (String)(list.get(0));
Important Facts
It is important to note that a generic class is shared by all its instances regardless of its actual generic type.
GenericStack<String> stack1 = new GenericStack<String>();
GenericStack<Integer> stack2 = new GenericStack<Integer>();
Although GenericStack<String> and GenericStack<Integer> are two types, there is only one class GenericStack loaded into the JVM.
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Designing Generic Matrix Classes
•Objective: This example gives a generic class for matrix arithmetic. This class implements matrix addition and multiplication common for all types of matrices.
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GenericMatrixGenericMatrix
UML Diagram
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GenericMatrix<E>
#add(element1:E, element2: E): E #multiply(element1: E, element2: E): E #zero(): E +addMatrix(matrix1: E[][], matrix2: E[][]): E[][] +multiplyMatrix(matrix1: E[][], matrix2: E[][]): E[][] +printResult(m1: Number[][], m2: Number[][], m3: Number[][], op: char): void
IntegerMatrix
RationalMatrix
