Java基礎之反射和Class類以及類的初始化，重載，this關鍵字，

1.理解Class類對象。

 Class類被創建後的對象就是Class對象，注意，Class對象表示的是自己手動編寫類的類型信息，比如創建一個Shapes類，那麼，JVM就會創建一個Shapes對應Class類的Class對象，該Class對象保存了Shapes類相關的類型信息。實際上在Java中每個類都有一個Class對象，每當我們編寫並且編譯一個新創建的類就會產生一個對應Class對象並且這個Class對象會被保存在同名.class文件裏(編譯後的字節碼文件保存的就是Class對象)，那爲什麼需要這樣一個Class對象呢？是這樣的，當我們new一個新對象或者引用靜態成員變量時，Java虛擬機(JVM)中的類加載器子系統會將對應Class對象加載到JVM中，然後JVM再根據這個類型信息相關的Class對象創建我們需要實例對象或者提供靜態變量的引用值。需要特別注意的是，手動編寫的每個class類，無論創建多少個實例對象，在JVM中都只有一個Class對象，即在內存中每個類有且只有一個相對應的Class對象，挺拗口，通過下圖理解（內存中的簡易現象圖）：

到這我們也就可以得出以下幾點信息：

• Class類也是類的一種，與class關鍵字是不一樣的。
• 手動編寫的類被編譯後會產生一個Class對象，其表示的是創建的類的類型信息，而且這個Class對象保存在同名.class的文件中(字節碼文件)，比如創建一個Shapes類，編譯Shapes類後就會創建其包含Shapes類相關類型信息的Class對象，並保存在Shapes.class字節碼文件中。
• 每個通過關鍵字class標識的類，在內存中有且只有一個與之對應的Class對象來描述其類型信息，無論創建多少個實例對象，其依據的都是用一個Class對象。
• Class類只存私有構造函數，因此對應Class對象只能有JVM創建和加載
• Class類的對象作用是運行時提供或獲得某個對象的類型信息，這點對於反射技術很重要(關於反射稍後分析)。
   

例子：

package com.ssl;

public class Person {
	private  int age;
	public  String name;
	
	
	public Person() {
		System.out.println("Person的無參構造");
		// TODO Auto-generated constructor stub
	}
	
	
	
	public Person(int age, String name) {
		super();
		this.age = age;
		this.name = name;
	}



	public int getAge() {
		return age;
	}
	public void setAge(int age) {
		this.age = age;
	}
	
	public void say(){
		System.out.println("Person的say方法");
	}
	
	
}

package com.ssl;

public class Student extends Person{
	private double height;
	double weight;
	protected int classid;
	public char gender;
	public double getHeight() {
		return height;
	}
	public void setHeight(double height) {
		this.height = height;
	}
	public double getWeight() {
		return weight;
	}
	public void setWeight(double weight) {
		this.weight = weight;
	}
	public int getClassid() {
		return classid;
	}
	public void setClassid(int classid) {
		this.classid = classid;
	}
	public char getGender() {
		return gender;
	}
	public void setGender(char gender) {
		this.gender = gender;
	}
	public Student(double height, double weight, int classid, char gender) {
		super();
		this.height = height;
		this.weight = weight;
		this.classid = classid;
		this.gender = gender;
	}
	public Student(int age, String name, double height, double weight,
			int classid, char gender) {
		super(age, name);
		this.height = height;
		this.weight = weight;
		this.classid = classid;
		this.gender = gender;
	}
	private Student(double weight, int classid, char gender) {
		super();
		this.weight = weight;
		this.classid = classid;
		this.gender = gender;
	}
	public Student() {
		super();
	}
	
	public void sleep(){
		System.out.println("Student.sleep()");
	}
	//
	
	
}

package com.ssl;

import java.lang.reflect.Modifier;

public class Test {

	public static void main(String[] args) throws ClassNotFoundException {
		// TODO Auto-generated method stub
		//獲取Class類對象的方法
		//方法一：getclass
		Student stu = new Student();
		Class c1 = stu.getClass();
		Class c2 = stu.getClass();
		System.out.println(c1);
		System.out.println(c2);
		System.out.println(c1==c2);
		//方法二：
		Class c3 = Student.class;
		Class c4 = Student.class;
		System.out.println(c3);
		System.out.println(c4);
		System.out.println(c3==c4);
		//方法三：常用
		Class c5 = Class.forName("com.ssl.Student");
		Class c6 = Class.forName("com.ssl.Student");
		System.out.println(c5);
		System.out.println(c6);
		System.out.println(c5==c6);
		//常見方法
		//獲取類的全稱
		System.out.println(c5.getName());
		//獲取類的簡稱
		System.out.println(c5.getSimpleName());
		//獲取類的修飾符
		System.out.println(c5.getModifiers());
		System.out.println(Modifier.toString(c5.getModifiers()));
		//獲取父類
		System.out.println(c5.getSuperclass());
	}

}

反射機制_Class類_反射操縱類中屬性，方法，構造器

package com.ssl.copy;

public class Person {
	private  int age;
	public  String name;
	
	public Person() {
		System.out.println("Person的無參構造");
		// TODO Auto-generated constructor stub
	}
	
	public Person(int age, String name) {
		super();
		this.age = age;
		this.name = name;
	}

	public int getAge() {
		return age;
	}
	public void setAge(int age) {
		this.age = age;
	}
	
	public void say(){
		System.out.println("Person的say方法");
	}
}

package com.ssl.copy;

public class Student extends Person{
	private double height;
	double weight;
	protected int classid;
	public char gender;
	public double getHeight() {
		return height;
	}
	public void setHeight(double height) {
		this.height = height;
	}
	public double getWeight() {
		return weight;
	}
	public void setWeight(double weight) {
		this.weight = weight;
	}
	public int getClassid() {
		return classid;
	}
	public void setClassid(int classid) {
		this.classid = classid;
	}
	public char getGender() {
		return gender;
	}
	public void setGender(char gender) {
		this.gender = gender;
	}
	public Student(double height, double weight, int classid, char gender) {
		super();
		this.height = height;
		this.weight = weight;
		this.classid = classid;
		this.gender = gender;
	}
	public Student(int age, String name, double height, double weight,
			int classid, char gender) {
		super(age, name);
		this.height = height;
		this.weight = weight;
		this.classid = classid;
		this.gender = gender;
	}
	private Student(double weight, int classid, char gender) {
		super();
		this.weight = weight;
		this.classid = classid;
		this.gender = gender;
	}
	public Student() {
		super();
		System.out.println("student無參構造");
		
	}
	
	public void sleep(){
		System.out.println("Student.sleep()");
	}	
	
}

package com.ssl.copy;

import java.lang.reflect.Constructor;
import java.lang.reflect.InvocationTargetException;

public class Test {

	public static void main(String[] args) throws ClassNotFoundException, InstantiationException, IllegalAccessException, NoSuchMethodException, SecurityException, IllegalArgumentException, InvocationTargetException {
		Class c1 = Class.forName("com.ssl.copy.Student");
		//創建對象，默認使用無參構造，如果沒有無參構造會報錯。如果無參構造爲私有的，那麼也會報錯
		c1.newInstance();
		//獲取構造函數,private形式的構造方法這種方法是獲取不到的。
		System.out.println("=======================================");
		Constructor[] constructors = c1.getConstructors();
		for ( int i=0;i<constructors.length;i++){
			System.out.println(constructors[i]);
		}
		//獲取構造函數，所有的都可以獲取到
		System.out.println("==========================================");
		Constructor[] declaredConstructors = c1.getDeclaredConstructors();
		for ( int i=0;i<declaredConstructors.length;i++){
			System.out.println(declaredConstructors[i]);
		}
		//獲取指定的構造函數
		System.out.println("=============================================");
		//獲取指定的空構造函數
		Constructor con1= c1.getConstructor(null);
		System.out.println(con1);
		//根據參數獲取指定的構造函數
		System.out.println("=============================================");
		Constructor con2 = c1.getDeclaredConstructor(new Class[]{double.class,double.class,int.class,char.class});
		System.out.println(con2);
		//創建對象
		Object o1 = con1.newInstance(null);
		System.out.println(o1);
		Object o2 = con2.newInstance(1.0,2.0,20,'男');
		System.out.println(o2);
	}

}

反射操縱類中的屬性：

package com.ssl.copy;

import java.lang.reflect.Field;

public class TestDemo {

	public static void main(String[] args) throws ClassNotFoundException, NoSuchFieldException, SecurityException, InstantiationException, IllegalAccessException {
		// TODO Auto-generated method stub
		Class c1 = Class.forName("com.ssl.copy.Student");
		//獲取屬性
		//獲取子類和父類的public修飾的屬性、
		Field[] fields = c1.getFields();
		for (int i=0;i<fields.length;i++){
			System.out.println(fields[i]);
		}
		System.out.println("=====================");
		//獲去子類所有的屬性
		Field[] fields2 = c1.getDeclaredFields();
		for (int i=0;i<fields2.length;i++){
			System.out.println(fields2[i]);
		}
		System.out.println("=====================");
		//獲取特定屬性
		Field field = c1.getField("gender");
		System.out.println("field");
		
		Field field2 = c1.getDeclaredField("weight");
		System.out.println(field2);
		
		Field field3 = c1.getDeclaredField("height");
		System.out.println(field3);
		//給屬性賦值
		Student s = (Student)c1.newInstance();
		field.set(s, "男");
		System.out.println(s);
		field3.setAccessible(true);
		field3.setDouble(s, 12.1);
		System.out.println(s);
		
		
	}

}

反射操縱類中的方法：

package com.ssl.copy;

import java.lang.reflect.InvocationTargetException;
import java.lang.reflect.Method;

public class Test2 {
	public static void main(String[] args) throws ClassNotFoundException, NoSuchMethodException, SecurityException, InstantiationException, IllegalAccessException, IllegalArgumentException, InvocationTargetException {
		Class c1 = Class.forName("com.ssl.copy.Student");
		//獲取子類和父類的public的方法
		Method[] methods = c1.getMethods();
		for (int i=0;i<methods.length;i++){
			System.out.println(methods[i]);
		}
		System.out.println("方法的個數是："+methods.length);
		//獲取子類的所有方法
		Method[] methods1 = c1.getDeclaredMethods();
		for (int i=0;i<methods1.length;i++){
			System.out.println(methods[i]);
		}
		System.out.println("方法的個數是："+methods1.length);
		
		//獲取自動方法
		Method m1 = c1.getMethod("toString",new Class[]{});
		System.out.println(m1);
		System.out.println("==================");
		
		Method m2 = c1.getMethod("setClassid", new Class[]{int.class});
		System.out.println(m2);
		
		System.out.println("==============");
		//執行方法：
		Object o1 = c1.newInstance();
		System.out.println(m1.invoke(o1,new Class[]{}));
	}
}