Why Use Generics? Generic types Generic methods Bounded type parameters Generics, Inheritance, and Subtypes Wildcard types

Transcription

1 Why Use Generics? Generic types Generic methods Bounded type parameters Generics, Inheritance, and Subtypes Wildcard types

2 generics enable types (classes and interfaces) to be parameters when defining classes, interfaces and methods. Much like the more familiar formal parameters used in method declarations, type parameters provide a way for you to reuse the same code with different inputs. The difference is that the inputs to formal parameters are values, while the inputs to type parameters are types.

3 Stronger type checks at compile time. Elimination of casts List list = new ArrayList(); list.add("hello"); String s = (String) list.get(0); When re-written to use generics, List<String> list = new ArrayList<String>(); list.add("hello"); String s = list.get(0); // no cast Enabling programmers to implement generic algorithms.

4 A generic type is a generic class or interface that is parameterized over types. public class Box { private Object object; public void set(object object) { this.object = object; public Object get() { return object; public class Box<T> { // T stands for "Type" private T t; public void set(t t) { this.t = t; public T get() { return t; Invoking and Instantiating a Generic Type Box<Integer> integerbox = new Box<Integer>(); Java SE 7 and later Box<Integer> integerbox = new Box<>(); //Integer type argument

5 Raw type : is the name of a generic class or interface without any type arguments. public class Box<T> { public void set(t t) { /*... */ //... Box<Integer> intbox = new Box<>(); Box rawbox = new Box(); rawbox.set(3); -- Box is the raw type of the generic type Box<T>. Compile: Note: C:\Users\Test\src\test\Box.java uses unchecked or unsafe operations. Note: Recompile with -Xlint:unchecked for details.

6 Generic methods are methods that introduce their own type parameters. This is similar to declaring a generic type, but the type parameter's scope is limited to the method where it is declared.

7 public class Pair<K, V> { private K key; private V value; // Generic constructor public Pair(K key, V value) { this.key = key; this.value = value; // Generic methods public void setkey(k key) { this.key = key; public void setvalue(v value) { this.value = value; public K getkey() { return key; public V getvalue() { return value; public class Util { // Generic static method public static <K, V> boolean compare(pair<k, V> p1, Pair<K, V> p2) { return p1.getkey().equals(p2.getkey()) && p1.getvalue().equals(p2.getvalue()); Pair<Integer, String> p1 = new Pair<>(1, "apple"); Pair<Integer, String> p2 = new Pair<>(2, "pear"); boolean same = Util.compare(p1, p2);

8 To restrict the types that can be used as type arguments in a parameterized type public class Box<T extends Number> { // T stands for "Type" private T t; public void set(t t) { this.t = t; public T get() { return t; public class Test{ public static void main(string[] args) { Box <Integer> box1 = new Box<>(); Box <Double> box2 = new Box<>(); Box <String> box3 = new Box<>();

9 public class Box<T > { // T stands for "Type" private T t; public void set(t t) { this.t = t; public T get() { return t; Box<Object> box1 = new Box<Number>();//no Box<Object> box2 = new Box<Integer>(); //no Box<Number> box3 = new Box<Number>();//ok Box<Integer> box4 = new Box<Integer>();//ok Box<Number> box = new Box<Number>(); box.add(new Integer(10)); // OK box.add(new Double(10.1)); // OK

10 1. public class Box<T> { 2. private T t; 3. private List<T> list = new ArrayList<>();; 4. public void set(t t) { this.t = t; 5. public T get() { return t; 6. public void add(t elem){ 7. list.add(elem); public static void print(box<number> b){ 10. System.out.println(b.get()); public static void main(string[] args) { 13. Box <Number> box1 = new Box<>(); 14. box1.add(new Integer(3)); 15. box1.add(new Double(3.2)); 16. box1.print(box1); Box <Double> box2 = new Box<>(); 19. box1.print(box2);

11 use an upper bounded wildcard to relax the restrictions on a variable want to write a method that works on List<Integer>, List<Double>, and List<Number> Any subclasses include itself 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 (b) ArrayList in JDK 1.5

12 public class Test3 { public static void print(arraylist<? extends Animal> animals){ System.out.println(animals); public static void main(string[] args) { ArrayList<Dog> dogs = new ArrayList<Dog>(); dogs.add(new Dog()); dogs.add(new Dog()); ArrayList<Cat> cats = new ArrayList<Cat>(); cats.add(new Cat()); cats.add(new Cat()); animals.add(new Dog());//complie error, cannot add print(dogs); print(cats); สมาช กใน list ท ส งไปม type เป นต วเองหร อเป นล กหลาน Ok Animal, Dog, Cat แต add ไม ได 12

13 import java.util.*; public class Test { public static void addanimals(arraylist<? super Dog> animals){ animals.add(new Dog());//ok public static void main(string[] args) { // make ArrayLists instead of arrays for Dog, Cat, Bird ArrayList<Dog> dogs = new ArrayList<Dog>(); dogs.add(new Dog()); dogs.add(new Dog()); ArrayList<Cat> cats = new ArrayList<Cat>(); cats.add(new Cat()); cats.add(new Cat()); ArrayList<Animal> ams = new ArrayList<Animal>(); ams.add(new Animal()); ams.add(new Animal()); addanimals(dogs); addanimals(ams); addanimals(cats);//compile error สมาช กใน list ท ส งไปม type เป นต วเองหร อ super class จะ OK Dog, Animal, Object และ add ได เฉพาะ type ของต วเอง add ได เฉพาะ Dog เท าน น 13

14 import java.util.*; public class Test5 { public static void print(arraylist<object> list){ System.out.println(list); public static void main(string[] args) { ArrayList<Dog> dogs = new ArrayList<Dog>(); dogs.add(new Dog()); dogs.add(new Dog()); ArrayList<Cat> cats = new ArrayList<Cat>(); cats.add(new Cat()); cats.add(new Cat()); print(dogs); //no print(cats); //no 14

15 import java.util.*; public class Test5 { public static void print(arraylist<?> list){ System.out.println(list); // list.add(new Dog());//compile error // list.add(new Object());//compile error public static void main(string[] args) { ArrayList<Dog> dogs = new ArrayList<Dog>(); dogs.add(new Dog()); dogs.add(new Dog()); ArrayList<Cat> cats = new ArrayList<Cat>(); cats.add(new Cat()); cats.add(new Cat()); print(dogs); print(cats); ArrayList<Integer> num = new ArrayList<>(); num.add(2); num.add(3); print(num); 15

16 GenericStack<E> -elements: E[] -size: int +GenericStack() +GenericStack(initialCapacity: int) +getsize(): int +peek(): E +pop(): E +push(o: E): E +isempty(): boolean An array to store elements. The number of the elements in this stack. Creates an empty stack with default initial capacity 16. Creates an empty stack with the specified initial capacity. 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. Return true if the stack is empty. GenericStack 16