OOP with Java 通知 Project 7 :6 月 14 日晚 9 点 6 月 5 日复习 6 月 12 日考试

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1 OOP with Java Yuanbin Wu

2 OOP with Java 通知 Project 7 :6 月 14 日晚 9 点 6 月 5 日复习 6 月 12 日考试

3 复习 I/O 流 InputStream/Reader read() OutputStream/Writer write() 抽象 : 数据的来源 / 数据的目的地 ByteArrayStream, FileStream StringStream ObjectStream

4 import java.io.fileinputstream; import java.io.fileoutputstream; import java.io.ioexception; public class CopyBytes { public static void main(string[] args) throws IOException { FileInputStream in = null; FileOutputStream out = null; try { in = new FileInputStream("xanadu.txt"); out = new FileOutputStream("outagain.txt"); int c; while ((c = in.read())!= -1) out.write(c); finally { if (in!= null) in.close(); if (out!= null) out.close();

5 装饰器 FilterInputStream/FilterOutputStream BufferedInputStream DataInputStream FileInputStream fin = new FileInputStream("xanadu.txt"); BufferedInputStream bf = new BufferedInputStream(fin); DataInputStream din = new DataInputStream(bf); din.read(); din.readint(); din.readdouble(); FileOutputStream fout = new FileOutputStream("xanadu.txt"); BufferedOutputStream bf = new BufferedOutputStream(fout); DataOutputStream dout = new DataOutputStream(bf); dout.write(1); dout.writeint(10); dout.writedouble(3.14);

6 复习 InputStream ByteArrayInputStream FileInputStream StringBufferInputStream FilterInputStream BufferdInputStream DataInputStream 装饰器

7 OOP with Java 运行时类型信息泛型

8 RTTI 运行时类型信息 RunTime Type Information (RTTI) Class 类每一个类都包含一个 Class 类的对象该对象包含该类的信息类名字类的有哪些方法类的有哪些成员

9 RTTI 每个类的静态成员所有对象共享一个 Class 对象每个对象调用 getclass() 获得 Class 类的对象 String s = hello ; Class c = s.getclass(); 每个类通过.class 获得 Class 类的对象 Class c = String.class

10 RTTI Class 类的方法 getname() getinterfaces() getsuperclass() newinstance()

11 public class ToyTest { interface A { interface B { interface C { class Toy { Toy() { Toy(int i) { class FancyToy extends Toy implements A, B, C { FancyToy() { super(1); static void printinfo(class c) { System.out.println("Class name: " + c.getname() ); System.out.println( Is interface? + c.isinterface()); System.out.println("Simple name: " + c.getsimplename()); System.out.println("Canonical name : " + c.getcanonicalname()); public static void main(string[] args) { Class c = null; try { c = Class.forName("typeinfo.toys.FancyToy"); catch(classnotfoundexception e) { System.out.println("Can t find FancyToy");System.exit(1); printinfo(c); for(class face : c.getinterfaces()) printinfo(face); Class up = c.getsuperclass(); Object obj = null; try { // Requires default constructor: obj = up.newinstance(); catch(instantiationexception e) { System.out.println("Cannot instantiate"); System.exit(1); catch(illegalaccessexception e) { System.out.println("Cannot access"); System.exit(1); printinfo(obj.getclass());

12 RTTI RTTI 的用途给定 Object 引用, 判断它的类型 (Downcasting) String s = new String( hello ); Object o = s; String type = o.getclass().getname(); String t = (String)o;

13 泛型容器 new ArrayList(); class Apple { private static long counter; private final long id = counter++; public long id() { return id; class Orange { public class ApplesAndOrangesWithoutGenerics { public static void main(string[] args) { ArrayList apples = new ArrayList(); for(int i = 0; i < 3; i++) apples.add(new Apple()); // Not prevented from adding an Orange to apples: apples.add(new Orange()); for(int i = 0; i < apples.size(); i++) ((Apple)apples.get(i)).id(); // Orange is detected only at run time

14 泛型类型安全的容器 new ArrayList<Apple>(); public class ApplesAndOrangesWithGenerics { public static void main(string[] args) { ArrayList<Apple> apples = new ArrayList<Apple>(); for(int i = 0; i < 3; i++) apples.add(new Apple()); // Compile error! // apples.add(new Orange()); for(int i = 0; i < apples.size(); i++) apples.get(i).id(); for(apple c: apples) System.out.println(c.id());

15 泛型不同类型间的相互替代? Upcasting 与多态 Class A { void f (A arg) { // f(new A()); Class A { Class B { void f (A arg) { // f(new B()); Class A { Class B { Class C extends A { void f (A arg) { // f(new C());

16 泛型不同类型间的相互替代? 泛型 : 更灵活的方式

17 基本概念泛型 (generics) 在定义类的成员和方法时, 类型为可变参数 class MyType {? member = null; void f (? arg) { //

18 基本概念语法 class MyType {? member = null; void f (? arg) { // class MyType<T> { T member = null; <T> void f (T arg) { //

19 Example: A simple Holder class class Apple { public class Holder { private Apple a; public Holder(Apple a) { this.a = a; public void set(apple a) { this.a = a; public Apple get() { return a; public static void main(string[] args) { Holder h = new Holder(new Apple()); Apple a = h.get(); // h.set("not an Apple"); // Error // h.set(1); // Error

20 Example: A simple Holder class class Apple { public class Holder<T> { private T a; public Holder(T a) { this.a = a; public void set(t a) { this.a = a; public T get() { return a; public static void main(string[] args) { Holder<String> hs = new Holder<String>(new String( hello )); String s = hs.get(); Holder<Integer> hi = new Holder<Integer>(1); int I = hi.get(); Holder<Apple> h = new Holder<Apple>(new Apple()); Apple a = h.get(); // h.set("not an Apple"); // Error // h.set(1); // Error

21 Example: A simple Holder class class Apple { public class Holder<T> { private T a; public Holder(T a) { this.a = a; public void set(t a) { this.a = a; public T get() { return a; class Apple { public class Holder { private Object a; public Holder(Object a) { this.a = a; public void set(object a) { this.a = a; public Object get() { return a; public static void main(string[] args) { Holder<Apple> h = new Holder<Apple>( new Apple()); Apple a = h.get(); public static void main(string[] args) { Holder h = new Holder(new Apple()); Apple a = (Apple)h.get();

22 基本概念参数化类型形参实参类型参数

23 Example: A tuple library Tuple 不可变数组 ( 数组元素不可变 ) 用途 : 函数返回多个值 Data transfer object

24 Example: A tuple library public class TwoTuple<A,B> { public final A first; public final B second; public TwoTuple(A a, B b) { first = a; second = b; public String tostring() { return "(" + first + ", " + second + ")";

25 Example: A tuple library public class ThreeTuple<A,B,C> extends TwoTuple<A,B> { public final C third; public ThreeTuple(A a, B b, C c) { super(a, b); third = c; public String tostring() { return "(" + first + ", " + second + ", " + third +")"; public class FourTuple<A,B,C,D> extends ThreeTuple<A,B,C> { public final D four; public ThreeTuple(A a, B b, C c, D d) { super(a, b, c); third = d; public String tostring() { return "(" + first + ", " + second + ", " + third + "," + four + ")";

26 Example: A tuple library class Apple { class Orange { public class TupleTest { static TwoTuple<String,Integer> f() { return new TwoTuple<String,Integer>("hi", 47); // Autoboxing static ThreeTuple<Apple,String,Integer> g() { return new ThreeTuple<Apple, String, Integer>(new Apple(), "hi", 47); static FourTuple<Orange,Apple,String,Integer> h() { return new FourTuple<Orange,Apple,String,Integer>(new Orange(), new Apple(), "hi", 47); public static void main(string[] args) { TwoTuple<String,Integer> ttsi = f(); System.out.println(ttsi); // ttsi.first = "there"; // Compile error: final System.out.println(g()); System.out.println(h());

27 泛型接口带有类型参数的接口 public interface Generator<T> { T next(); public class Coffee { private static long counter = 0; private final long id = counter++; public String tostring() { return getclass().getsimplename() + " " + id; public class Latte extends Coffee { public class Mocha extends Coffee { public class Cappuccino extends Coffee { public class Americano extends Coffee { public class Breve extends Coffee {

28 import java.util.*; public class CoffeeGenerator implements Generator<Coffee> { private Class[] types = { Latte.class, Mocha.class, Cappuccino.class, Americano.class, Breve.class, ; private static Random rand = new Random(47); private int size = 0; public CoffeeGenerator() { public CoffeeGenerator(int sz) { size = sz; public Coffee next() { try { return (Coffee) types[rand.nextint(types.length)].newinstance(); // Report programmer errors at run time: catch(exception e) { throw new RuntimeException(e); public static void main(string[] args) { CoffeeGenerator gen = new CoffeeGenerator(); for(int i = 0; i < 5; i++) System.out.println(gen.next());

29 // public class CoffeeGenerator implements Generator<Coffee> public class Fibonacci implements Generator<Integer> { private int count = 0; public Integer next() { return fib(count++); private int fib(int n) { if(n < 2) return 1; return fib(n-2) + fib(n-1); public static void main(string[] args) { Fibonacci gen = new Fibonacci(); for(int i = 0; i < 18; i++) System.out.print(gen.next() + " ");

30 泛型方法方法的参数, 返回值带有类型参数语法 public <T> void f(t x) { // <T> 说明该方法带有类型参数 T 需要放在返回值说明之前

31 泛型方法 public class GenericMethods { public <T> void f(t x) { System.out.println(x.getClass().getName()); public static void main(string[] args) { GenericMethods gm = new GenericMethods(); gm.f(""); gm.f(1); gm.f(1.0); gm.f(1.0f); gm.f( c ); gm.f(gm); 方法的类型参数可以与类的类型参数无关 public class GenericMethods<E> { E member; public <T> void f(t x) { System.out.println(x.getClass().getName());

32 类型推理编译器推理出类型变量的值 public class New { public static <K, V> Map<K, V>() { return new HashMap<K, V>(); public static void main(string []args){ Map<String, List<String>> s = New.Map(); 1. 由 Map<String, List<String>> s = New.Map(); 推断出 Map() 方法中的 K=String, V=List<String> 2. 可以通过语法明确泛型方法调用时的类型参数 Map<String, List<String>> s = New.<String, List<String>>Map(); // Diamond ArrayList<String> s = new ArrayList<>(); HashMap<String, Integer> h = new HashMap<>();

33 小结泛型类泛型接口 public class TwoTuple<A, B> { //... 泛型方法 public interface Generator<T> { T next(); public <T> void f(t x) { //

34 public class Generators { public static <T> Collection<T> fill(collection<t> coll, Generator<T> gen, int n) { for(int i = 0; i < n; i++) coll.add(gen.next()); return coll; public static void main(string[] args) { Collection<Coffee> coffee = fill(new ArrayList<Coffee>(), new CoffeeGenerator(), 4); for(coffee c : coffee) System.out.println(c); Collection<Integer> fnumbers = fill(new ArrayList<Integer>(), new Fibonacci(), 12); for(int i : fnumbers) System.out.print(i + ", ");

35 Type Erasure 当给定不同的类型参数时, 泛型的类型是否相同? class A { class B extends A{ public class Test { public static void main(string []args){ ArrayList<String> strlist = new ArrayList<>(); ArrayList<Integer> intl = strlist; ArrayList<B> blist = new ArrayList<>(); ArrayList<A> alist = blist; 编译错误! ArrayList<String>, ArrayList<Integer>, ArrayList<B>, ArrayList<A> 是不同类型!

36 Type Erasure 当给定不同的类型参数时, 泛型的类型是否相同? Class Object import java.util.*; public class ErasedTypeEquivalence { public static void main(string[] args) { Class c1 = new ArrayList<String>().getClass(); Class c2 = new ArrayList<Integer>().getClass(); System.out.println(c1 == c2); 输出 : true!

37 Type Erasure import java.util.*; class Frob { class Fnorkle { class Quark<Q> { class Particle<POSITION,MOMENTUM> { 输出 : [E] [K, V] [Q] [POSITION, MOMENTUM] public class LostInformation { public static void main(string[] args) { List<Frob> list = new ArrayList<Frob>(); Map<Frob,Fnorkle> map = new HashMap<Frob,Fnorkle>(); Quark<Fnorkle> quark = new Quark<Fnorkle>(); Particle<Long,Double> p = new Particle<Long,Double>(); System.out.println(Arrays.toString(list.getClass().getTypeParameters())); System.out.println(Arrays.toString(map.getClass().getTypeParameters())); System.out.println(Arrays.toString(quark.getClass().getTypeParameters())); System.out.println(Arrays.toString(p.getClass().getTypeParameters()));

38 Type Erasure 泛型是如何实现的? ArrayList<String> 与 ArrayList <Integer> 的区别在哪里?

39 Type Erasure 类型擦除 (Type Erasure), T 仅作为占位符, 不包含任何具体类型的信息所有 T 类型的对象引用最终实现为 Object 对象引用

40 Type Erasure public class HasF { public void f() { System.out.println("HasF.f()"); class Manipulator<T> { private T obj; public Manipulator(T x) { obj = x; // Error: cannot find symbol: method f(): public void manipulate() { obj.f(); public class Manipulation { public static void main(string[] args) { HasF hf = new HasF(); Manipulator<HasF> manipulator = new Manipulator<HasF>(hf); manipulator.manipulate();

41 Type Erasure public class Erased<T> { private final int SIZE = 100; public static void f(object arg) { if(arg instanceof T) { T var = new T(); // compile error // compile error T[] array = new T[SIZE]; // compile error

42 Type Erasure Type Erasure 实现原理 1: 所有引用最后都变为 Object 引用编译器自动添加 Downcasting 代码 class Apple { public class Holder<T> { private T a; public Holder(T a) { this.a = a; public void set(t a) { this.a = a; public T get() { return a; class Apple { public class Holder { private Object a; public Holder(Object a) { this.a = a; public void set(object a) { this.a = a; public Object get() { return a; public static void main(string[] args) { Holder<Apple> h = new Holder<Apple>( new Apple()); Apple a = h.get(); public static void main(string[] args) { Holder h = new Holder(new Apple()); Apple a = (Apple)h.get(); 生成相同的机器代码编译器在 get() 返回处自动添加了类型转换代码

43 Type Erasure Type Erasure 的实现原理 2: 编译器只做静态检查 class A { class B extends A{ public class Test { public static void main(string []args){ ArrayList<String> strlist = new ArrayList<>(); ArrayList<Integer> intl = strlist; ArrayList<B> blist = new ArrayList<>(); ArrayList<A> alist = blist; 1. 编译器通过静态检查报错 (ArrayList<String> 与 ArrayList<Integer> 类型不同 ) 2. 但实际存储中, 无论 <String> 还是 <Integer> 都存储的是 Object 类型的引用

44 Type Erasure Type Erasure 类型变量仅仅对编译器的静态检查有用当通过静态检查, 所有类型参数被擦除

45 Type Erasure 如何解决 Type Erasure 带来的问题? Class 对象 public class Erased<T> { private final int SIZE = 100; public static void f() { T var = new T(); // compile error public class TypeCapture<T> { Class<T> kind; public TypeCapture(Class<T> kind){ this.kind = kind; public T f() { T r = null; try{ r = kind.newinstance(); catch (Exception e){ System.out.println("fail"); return r; public static void main(string []args) { TypeCapture<String> c = new TypeCapture<>(String.class); System.out.println(c.f());

46 Type Erasure 如何解决 Type Erasure 带来的问题? public class HasF { public void f() { System.out.println("HasF.f()"); class Manipulator<T> { private T obj; public Manipulator(T x) { obj = x; // Error: cannot find symbol: method f(): public void manipulate() { obj.f(); class Manipulator<T extends HasF> { private T obj; public Manipulator(T x) { obj = x; public void manipulate() { obj.f(); public class Manipulation { public static void main(string[] args) { HasF hf = new HasF(); Manipulator<HasF> manipulator = new Manipulator<HasF>(hf); manipulator.manipulate();

47 被限定的类型参数限定类型参数的范围语法 : class A <T extends B> { // 表示类型参数 T 只能是 B 类型或者 B 的子类型作用 : 静态检查时, 可以合法引用 T 的方法 ( 但最终仍然是 Object ) 多个限定类型 class A <T extends B & C> { // 如果有类和接口, 类应该出现在第一个位置

48 被限定的类型参数 interface HasColor { java.awt.color getcolor(); class Colored<T extends HasColor> { T item; Colored(T item) { this.item = item; T getitem() { return item; // The bound allows you to call a method: java.awt.color color() { return item.getcolor(); class Dimension { public int x, y, z; // Multiple bounds: class ColoredDimension<T extends Dimension & HasColor> { T item; ColoredDimension(T item) { this.item = item; T getitem() { return item; java.awt.color color() { return item.getcolor(); int getx() { return item.x; int gety() { return item.y; int getz() { return item.z;

49 通配符 class Fruit { class Apple extends Fruit{ class Orange extends Fruit{ public class Test { public static void main(string []args){ List<Apple> alist = new ArrayList<Apple>(); // compile error // List<Fruit> flist = new ArrayList<Apple>(); 如何表达 ArrayList 中的元素是 Fruit 的某个子类?

50 通配符 class Fruit { class Apple extends Fruit{ class Orange extends Fruit{ public class Test { public static void main(string []args){ List<Apple> alist = new ArrayList<Apple>(); // compile error // List<Fruit> flist = new ArrayList<Apple>(); List<? extends Fruit> flist = new ArrayList<Apple>(); // Compile Error: can t add any type of object: // flist.add(new Apple()); // flist.add(new Fruit()); // flist.add(new Object()); flist.add(null); // Legal but uninteresting // We know that it returns at least Fruit: Fruit f = flist.get(0);

51 通配符类型参数为某个 B 的子类型具体是哪一个无法确定 List<? extends Fruit> 无法 add 任何对象 List 中存储的对象类型无法确定可以 get 对象 Fruit 类型 <? extends B>

52 通配符类型参数为某个 B 的父类型具体是哪一个无法确定 List<? super Apple> 无法 get 任何对象无法确定是哪个父类 <? super B> 可以 add Apple 的子类对象

53 import java.util.*; public class SuperTypeWildcards { static void writeto(list<? super Apple> apples) { apples.add(new Apple()); apples.add(new Jonathan()); // apples.add(new Fruit()); // Error

54 总结 Type Erasure 类型变量仅仅对编译器的静态检查有用当通过静态检查, 所有类型参数被擦除被限制的类型参数通配符 class A <T extends B & C> { // <? extends B> <? super B>

OOP with Java 通知 Project 7 :6 月 27 日晚 9 点 6 月 14 日复习 6 月 21 日考试答疑 6 月 20 日 13 : : 00 理科楼 B911

OOP with Java Yuanbin Wu cs@ecnu OOP with Java 通知 Project 7 :6 月 27 日晚 9 点 6 月 14 日复习 6 月 21 日考试答疑 6 月 20 日 13 : 00 15 : 00 理科楼 B911 abstract class demo { public int a; demo() { a = 10; abstract public