OOP with Java 通知 Project 4: 4 月 19 日晚 9 点

OOP with Java Yuanbin Wu cs@ecnu

OOP with Java 通知 Project 4: 4 月 19 日晚 9 点

复习 类的复用 组合 (composition): has-a 关系 class MyType { public int i; public double d; public char c; public void set(double x) { d = x; public double get() { return d; public class MyCompType { private MyType m = new MyType(); private String s; public MyCompType(){ s = new String( Hello );

复习 继承 (inheritance) is-a 关系 class MyType { public int i; public double d; public char c; public void set(double x) { d = x; public double get() { return d; public class MySubType extends MyType{ String s = new String( Hello ); public double add(double d){return this.d + d; public double add(string s){return this.s + s; public void set(double x){ i = (int)x; public double get() { return i; public static void main(string [ ]args){ MySubType ms = new MySubType(); System.out.println(ms.get()); System.out.println(ms.add(1.0)); System.out.println(ms.add( World ));

复习 继承 子类有父类的所有方法和数据 子类可以定义新的方法和数据 子类可以重写 (override) 父类的方法 super 关键字 每一个子类对象都隐含包含一个父类对象 Object 对象 Single root class hierarchy tree 方法 : boolean equals(object o) String tostring() class MyType { public int i; public double d; public char c; public void set(double x) { d = x; public double get() { return d; public static void main(string [ ]args){ MyType m = new MyType(); MyType n = new MyType(); String s = hello ; m.equals(n); m.equals(s);

OOP with Java protected upcasting final 关键字

protected 访问控制 package access public private

protected 函数重写 class MyType { public int i; public double d; public char c; public void set(double x) { d = x; public void set(int y) {i = y; public double get() { return d; public class MySubType extends MyType{ public double foo(){ return get(); public void set(double x){ i = (int)x; public void set(char z) {c = z; public static void main(string [ ]args){ MySubType ms = new MySubType(); ms.set(1.0); System.out.println(ms.get()); System.out.println(ms.i); System.out.println(ms.d);

protected 函数重写? class MyType { public int i; public double d; public char c; private void set(double x) { d = x; private void set(int y) {i = y; private double get() { return d; public class MySubType extends MyType{ // can not access!! // public double foo(){ return get(); public void set(double x){ i = (int)x; public void set(char z) {c = z; public static void main(string [ ]args){ MySubType ms = new MySubType(); ms.set(1.0); System.out.println(ms.get()); System.out.println(ms.i); System.out.println(ms.d);

Protected 父类的方法 public private 是否有可能被子类访问而不被外界访问?

protected protected 可以被子类 / 同一包中的类访问, 不能被其他类访问 弱化的 private 同时赋予 package access

protected class MyType { public int i; public double d; public char c; protected void set(double x) { d = x; protected void set(int y) {i = y; protected double get() { return d; public class MySubType extends MyType{ public double foo(){ return get(); public void set(double x){ i = (int)x; public void set(char z) {c = z; public static void main(string [ ]args){ MySubType ms = new MySubType(); ms.set(1.0); System.out.println(ms.get()); System.out.println(ms.i); System.out.println(ms.d);

Protected 访问控制 package access public private protected

Upcasting 继承 is-a 关系 子类有父类所有的数据和方法 类型关系 : 子类是一种父类 the sub-class is a type of the base class

Upcasting 例子 class Instrument { public void play() { static void tune(instrument i) { //... i.play(); public class Wind extends Instrument { public static void main(string[] args) { Wind flute = new Wind(); Instrument.tune(flute); Upcasting

Upcasting 例子 public class MySubType extends MyType{ String s = new String( Hello ); public double add(double d){return this.d + d; public double add(string s){return this.s + s; public static void main(string [ ]args){ MySubType ms = new MySubType(); MyType m = ms; System.out.println(m.get()); System.out.println(ms.add("World")); m.set(1.0); System.out.println(m.get()); System.out.println(ms.get());

Upcasting Upcasting 需要父类对象 引用, 函数参数 可以用子类对象带入 安全的类型转换 子类拥有父类所有的数据和方法

Upcasting Upcasting Object Upcasting B MyType A Downcasting MySubType

Upcasting 子类重写了父类方法? class MyType { public int i; public double d; public char c; protected void set(double x) { d = x; protected void set(int y) {i = y; public double get() { return d; public class MySubType extends MyType{ public void set(double d){ System.out.println( Sub-class set ); i = int(d); public static void main(string [ ]args){ MySubType ms = new MySubType(); MyType m = ms; m.set(1.0); 多态

Upcasting 类型转化 基本类型 int double ( 安全, 自动转换 ) double int ( 损失精度, 强制转换 ) 基本类型与 wrapper 类 int Integer (autoboxing) Integer int (unboxing) 不支持强制转化 子类 父类 ( 安全, upcasting) 父类 子类 (downcasting)

Upcasting Downcasting MySubType ms = (MySubType)m; 仅在 m 确实指向子类对象时才能进行 运行时类型信息 (RTTI) 例子 : 重写 equals 方法

Upcasting 总结 子类是一种父类 (is-a) 父类的引用可以指向子类对象

final 关键字 final 关键字 不同的环境下有不同含义 基本意义为 : 不能被改变

final 关键字 final 数据 编译时常数 一旦被赋值就不能被修改

final 关键字 final 数据 例子 class MyType { public int i; public final double d = 1; public char c; public double get() { return d; public void set(double x) {d = x; public static void main(string []args){ MyType m = new MyType(); // m.d = 2.0;

final 关键字 final 数据 final 引用 class MyType { public int i; public final double d = 1; public char c; public final int [ ] a = new int[10]; public double get() { return d; public void set(double x) {d = x; public static void main(string []args){ MyType m = new MyType(); m.a[0] = 1.0; //m.a = new int[10];

final 关键字 final 数据 final + static static final int i = 1; 仅有一个不可变的存储空间

final 关键字 final 数据 Blank final class MyType { public int i; public final double d; public char c; public double get() { return d; public MyType(double x){ d = x; final 成员在定义时可以不给初值必须在构造函数中初始化 public static void main(string []args){ MyType m = new MyType(1.0); System.out.println(m.get()); // m.d = 2.0;

final 关键字 final 参数 函数不能修改参数的引用. class FinalArgs { public static void set(final int [ ] a) { a[0] = 1; // a = new int [10]; public static void main(string []args){ int [ ]a = new int[10]; FinalArgs.set(a);

final 关键字 final method 不能被重写 class MyType { public int i; public double d; public char c; final void set(double x) { d = x; protected void set(int y) {i = y; public double get() { return d; public class MySubType extends MyType{ // can t overide /* public void set(double d){ System.out.println( Sub-class set ); i = int(d); */ public static void main(string [ ]args){ MySubType ms = new MySubType(); MyType m = ms; m.set(1.0);

final 关键字 final class 不能被继承 final class MyType { public int i; public double d; public char c; final void set(double x) { d = x; protected void set(int y) {i = y; public double get() { return d; // can not be extended /* public class MySubType extends MyType{ public void set(double d){ System.out.println( Sub-class set ); i = int(d); public static void main(string [ ]args){ MySubType ms = new MySubType(); MyType m = ms; m.set(1.0); */

final 关键字 不可变 (immutable) 不可变对象 : 一旦创建就不能更改其状态 优点 : 易于使用, 易于 debug, 易于维护 缺点 : 空间 / 时间消耗 final 帮助构造不可变对象