Thinking in Java笔记10

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#java #class #constructor #byte #string #vector

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本文探讨了Java中运行时类型识别(RTTI)的工作原理，重点介绍了Class对象的作用及其实现方式。通过示例代码展示了如何利用反射API创建对象并统计不同类型对象的数量。

（第11章笔记）

/* * 为理解RTTI 在Java 里如何工作，首先必须了解类型信息在运行期是如何表示的。这时要用到一个名为"Class 对象"的 * 特殊形式的对象，其中包含了与类有关的信息（有时也把它叫作“元类”）。事实上，我们要用Class 对象创建属于某个 * 类的全部"常规"或"普通"对象。对于作为程序一部分的每个类，它们都有一个 Class对象。换言之，每次写一个新类时， * 同时也会创建一个Class对象（更恰当地说，是保存在一个完全同名的.class文件中）。在运行期，一旦我们想生成那个 * 类的一个对象，用于执行程序的Java 虚拟机（JVM）首先就会检查那个类型的Class对象是否已经载入。若尚未载入， * JVM就会查找同名的.class文件，并将其载入。所以Java 程序启动时并不是完全载入的，这一点与许多传统语言都不同。 * 一旦那个类型的Class 对象进入内存，就用它创建那一类型的所有对象。 */ package com.ch11; import java.util.*; class Pet{} class Dog extends Pet{} class Pug extends Dog{} class Cat extends Pet{} class Rodent extends Pet{} class Gerbil extends Rodent{} class Hamster extends Rodent{} class Counter {int i;} class PetCount1 { static String[] typenames = { "Pet", "Dog", "Pug", "Cat", "Rodent", "Gerbil", "Hamster", }; public void test() { Vector pets = new Vector(); try{ Class[] petTypes = { Class.forName("com.ch11.Dog"), Class.forName("com.ch11.Pug"), Class.forName("com.ch11.Cat"), Class.forName("com.ch11.Rodent"), Class.forName("com.ch11.Gerbil"), Class.forName("com.ch11.Hamster"), }; for(int i = 0; i < 15; i++) pets.addElement(petTypes[(int)(Math.random()*petTypes.length)].newInstance()); }catch(InstantiationException e){ e.printStackTrace(); }catch(IllegalAccessException e){ e.printStackTrace(); }catch(ClassNotFoundException e){ e.printStackTrace(); } Hashtable h = new Hashtable(); for(int i = 0; i < typenames.length; i++) h.put(typenames[i], new Counter()); for(int i = 0; i < pets.size(); i++){ Object o = pets.elementAt(i); //Instanceof返回一个布尔值，反映对象是不是一个特定类型的实例 if(o instanceof Pet) ((Counter)h.get("Pet")).i++; if(o instanceof Dog) ((Counter)h.get("Dog")).i++; if(o instanceof Pug) ((Counter)h.get("Pug")).i++; if(o instanceof Cat) ((Counter)h.get("Cat")).i++; if(o instanceof Rodent) ((Counter)h.get("Rodent")).i++; if(o instanceof Gerbil) ((Counter)h.get("Gerbil")).i++; if(o instanceof Hamster) ((Counter)h.get("Hamster")).i++; } for(int i = 0; i < pets.size(); i++) System.out.println(pets.elementAt(i).getClass().toString()); for(int i = 0; i < typenames.length; i++) System.out.println(typenames[i] + " quantity: " + ((Counter)h.get(typenames[i])).i); } } class PetCount2{ public void test(){ Vector pets = new Vector(); Class[] petTypes = { Pet.class, Dog.class, Pug.class, Cat.class, Rodent.class, Gerbil.class, Hamster.class, }; try{ for(int i = 0; i < 15; i++){ int rnd = 1 + (int)(Math.random()*(petTypes.length - 1)); pets.addElement(petTypes[rnd].newInstance()); } }catch(InstantiationException e) {} catch(IllegalAccessException e) {} Hashtable h = new Hashtable(); for(int i = 0; i < petTypes.length; i++) h.put(petTypes[i].toString(), new Counter()); for(int i = 0; i < pets.size(); i++){ Object o = pets.elementAt(i); if(o instanceof Pet) ((Counter)h.get("class com.ch11.Pet")).i++; if(o instanceof Dog) ((Counter)h.get("class com.ch11.Dog")).i++; if(o instanceof Pug) ((Counter)h.get("class com.ch11.Pug")).i++; if(o instanceof Cat) ((Counter)h.get("class com.ch11.Cat")).i++; if(o instanceof Rodent) ((Counter)h.get("class com.ch11.Rodent")).i++; if(o instanceof Gerbil) ((Counter)h.get("class com.ch11.Gerbil")).i++; if(o instanceof Hamster) ((Counter)h.get("class com.ch11.Hamster")).i++; } for(int i = 0; i < pets.size(); i++) System.out.println(pets.elementAt(i).getClass().toString()); Enumeration keys = h.keys(); while(keys.hasMoreElements()){ String nm = (String)keys.nextElement(); Counter cnt = (Counter)h.get(nm); System.out.println(nm.substring(nm.lastIndexOf('.') + 1) + " quantity: " + cnt.i); } } } interface HasBatteries {} interface Waterproof {} interface ShootsThings {} class Toy { // Comment out the following default // constructor to see // NoSuchMethodError from (*1*) Toy() {} Toy(int i) {} } class FancyToy extends Toy implements HasBatteries, Waterproof, ShootsThings { FancyToy() { super(1); } } class ToyTest { public void test() { Class c = null; try { c = Class.forName("com.ch11.FancyToy"); } catch(ClassNotFoundException e) {} printInfo(c); Class[] faces = c.getInterfaces(); for(int i = 0; i < faces.length; i++) printInfo(faces[i]); Class cy = c.getSuperclass(); Object o = null; try { // Requires default constructor: o = cy.newInstance(); // (*1*) } catch(InstantiationException e) {} catch(IllegalAccessException e) {} printInfo(o.getClass()); } static void printInfo(Class cc) { System.out.println( "Class name: " + cc.getName() + " is interface? [" + cc.isInterface() + "]"); } } public class PetCount{ public static void main(String[] args){ PetCount1 p1 = new PetCount1(); p1.test(); System.out.println("----------------------------------------------"); PetCount2 p2 = new PetCount2(); p2.test(); System.out.println("----------------------------------------------"); ToyTest tt = new ToyTest(); tt.test(); } }

运行结果：

class com.ch11.Cat
class com.ch11.Dog
class com.ch11.Hamster
class com.ch11.Dog
class com.ch11.Dog
class com.ch11.Rodent
class com.ch11.Pug
class com.ch11.Pug
class com.ch11.Hamster
class com.ch11.Dog
class com.ch11.Pug
class com.ch11.Cat
class com.ch11.Pug
class com.ch11.Cat
class com.ch11.Dog
Pet quantity: 15
Dog quantity: 9
Pug quantity: 4
Cat quantity: 3
Rodent quantity: 3
Gerbil quantity: 0
Hamster quantity: 2
----------------------------------------------
class com.ch11.Gerbil
class com.ch11.Rodent
class com.ch11.Pug
class com.ch11.Rodent
class com.ch11.Cat
class com.ch11.Dog
class com.ch11.Hamster
class com.ch11.Pug
class com.ch11.Gerbil
class com.ch11.Hamster
class com.ch11.Dog
class com.ch11.Hamster
class com.ch11.Cat
class com.ch11.Cat
class com.ch11.Gerbil
Gerbil quantity: 3
Cat quantity: 3
Pet quantity: 15
Pug quantity: 2
Hamster quantity: 3
Rodent quantity: 8
Dog quantity: 4
----------------------------------------------
Class name: com.ch11.FancyToy is interface? [false]
Class name: com.ch11.HasBatteries is interface? [true]
Class name: com.ch11.Waterproof is interface? [true]
Class name: com.ch11.ShootsThings is interface? [true]
Class name: com.ch11.Toy is interface? [false]

package com.ch11; import java.lang.reflect.*; public class ShowMethods { static final String usage = "usage: /n" + "ShowMethods qualified.class.name/n" + "To show all methods in class or: /n" + "ShowMethods qualified.class.name word/n" + "To search for methods involving 'word'"; public void test(String[] s){ if(s.length < 1) { System.out.println(usage); System.exit(0); } try { Class c = Class.forName(s[0]); Method[] m = c.getMethods(); Constructor[] ctor = c.getConstructors(); if(s.length == 1) { for (int i = 0; i < m.length; i++) System.out.println(m[i].toString()); for (int i = 0; i < ctor.length; i++) System.out.println(ctor[i].toString()); } else { for (int i = 0; i < m.length; i++) if(m[i].toString().indexOf(s[1])!= -1) System.out.println(m[i].toString()); for (int i = 0; i < ctor.length; i++) if(ctor[i].toString().indexOf(s[1])!= -1) System.out.println(ctor[i].toString()); } } catch (ClassNotFoundException e) { System.out.println("No such class: " + e); } } public static void main(String[] args) { String[] s = {"java.lang.String"}; ShowMethods sm = new ShowMethods(); sm.test(s); } }

运行结果：

public boolean java.lang.String.equals(java.lang.Object)
public java.lang.String java.lang.String.toString()
public int java.lang.String.hashCode()
public int java.lang.String.compareTo(java.lang.String)
public int java.lang.String.compareTo(java.lang.Object)
public int java.lang.String.indexOf(int)
public int java.lang.String.indexOf(int,int)
public int java.lang.String.indexOf(java.lang.String)
public int java.lang.String.indexOf(java.lang.String,int)
public static java.lang.String java.lang.String.valueOf(int)
public static java.lang.String java.lang.String.valueOf(char)
public static java.lang.String java.lang.String.valueOf(boolean)
public static java.lang.String java.lang.String.valueOf(float)
public static java.lang.String java.lang.String.valueOf(char[],int,int)
public static java.lang.String java.lang.String.valueOf(double)
public static java.lang.String java.lang.String.valueOf(char[])
public static java.lang.String java.lang.String.valueOf(java.lang.Object)
public static java.lang.String java.lang.String.valueOf(long)
public char java.lang.String.charAt(int)
public int java.lang.String.codePointAt(int)
public int java.lang.String.codePointBefore(int)
public int java.lang.String.codePointCount(int,int)
public int java.lang.String.compareToIgnoreCase(java.lang.String)
public java.lang.String java.lang.String.concat(java.lang.String)
public boolean java.lang.String.contains(java.lang.CharSequence)
public boolean java.lang.String.contentEquals(java.lang.CharSequence)
public boolean java.lang.String.contentEquals(java.lang.StringBuffer)
public static java.lang.String java.lang.String.copyValueOf(char[])
public static java.lang.String java.lang.String.copyValueOf(char[],int,int)
public boolean java.lang.String.endsWith(java.lang.String)
public boolean java.lang.String.equalsIgnoreCase(java.lang.String)
public static java.lang.String java.lang.String.format(java.lang.String,java.lang.Object[])
public static java.lang.String java.lang.String.format(java.util.Locale,java.lang.String,java.lang.Object[])
public byte[] java.lang.String.getBytes(java.lang.String) throws java.io.UnsupportedEncodingException
public void java.lang.String.getBytes(int,int,byte[],int)
public byte[] java.lang.String.getBytes()
public byte[] java.lang.String.getBytes(java.nio.charset.Charset)
public void java.lang.String.getChars(int,int,char[],int)
public native java.lang.String java.lang.String.intern()
public boolean java.lang.String.isEmpty()
public int java.lang.String.lastIndexOf(java.lang.String)
public int java.lang.String.lastIndexOf(int,int)
public int java.lang.String.lastIndexOf(int)
public int java.lang.String.lastIndexOf(java.lang.String,int)
public int java.lang.String.length()
public boolean java.lang.String.matches(java.lang.String)
public int java.lang.String.offsetByCodePoints(int,int)
public boolean java.lang.String.regionMatches(boolean,int,java.lang.String,int,int)
public boolean java.lang.String.regionMatches(int,java.lang.String,int,int)
public java.lang.String java.lang.String.replace(java.lang.CharSequence,java.lang.CharSequence)
public java.lang.String java.lang.String.replace(char,char)
public java.lang.String java.lang.String.replaceAll(java.lang.String,java.lang.String)
public java.lang.String java.lang.String.replaceFirst(java.lang.String,java.lang.String)
public java.lang.String[] java.lang.String.split(java.lang.String)
public java.lang.String[] java.lang.String.split(java.lang.String,int)
public boolean java.lang.String.startsWith(java.lang.String)
public boolean java.lang.String.startsWith(java.lang.String,int)
public java.lang.CharSequence java.lang.String.subSequence(int,int)
public java.lang.String java.lang.String.substring(int)
public java.lang.String java.lang.String.substring(int,int)
public char[] java.lang.String.toCharArray()
public java.lang.String java.lang.String.toLowerCase()
public java.lang.String java.lang.String.toLowerCase(java.util.Locale)
public java.lang.String java.lang.String.toUpperCase()
public java.lang.String java.lang.String.toUpperCase(java.util.Locale)
public java.lang.String java.lang.String.trim()
public final native void java.lang.Object.wait(long) throws java.lang.InterruptedException
public final void java.lang.Object.wait() throws java.lang.InterruptedException
public final void java.lang.Object.wait(long,int) throws java.lang.InterruptedException
public final native java.lang.Class java.lang.Object.getClass()
public final native void java.lang.Object.notify()
public final native void java.lang.Object.notifyAll()
public java.lang.String()
public java.lang.String(java.lang.String)
public java.lang.String(char[])
public java.lang.String(char[],int,int)
public java.lang.String(int[],int,int)
public java.lang.String(byte[],int,int,int)
public java.lang.String(byte[],int)
public java.lang.String(java.lang.StringBuilder)
public java.lang.String(byte[],int,int,java.lang.String) throws java.io.UnsupportedEncodingException
public java.lang.String(byte[],int,int,java.nio.charset.Charset)
public java.lang.String(byte[],java.lang.String) throws java.io.UnsupportedEncodingException
public java.lang.String(byte[],java.nio.charset.Charset)
public java.lang.String(byte[],int,int)
public java.lang.String(byte[])
public java.lang.String(java.lang.StringBuffer)