Java From entry to abandonment 38—Class class

01 Class taxonomical concept

Class Class is Reflection The root of , For anything you want to load dynamically , Operation of class , You have to get the corresponding first Class object

stay Object Class defines the following methods , This method is inherited by all subclasses

public final Class getClass()// Back here Object Runtime class for . From the running results of the program, it is easy to understand , namely ： You can get the name of the class through object reflection .

understand Class：

• Class It’s also a class
• Class Objects can only be created by the system
• A loaded class is in JVM There will only be one Class example
• One Class Object corresponds to a load into JVM Medium .class file
• Every instance of a class remembers who it is by Class The case claims that
• adopt Class You can get all the loaded structures in a class completely
• Class Class is Reflection The root of , For anything you want to load dynamically , Operation of class , You have to get the corresponding first Class object

Which types have Class object

• class： External class , member （ Member inner class , Static inner class ）, Local inner classes , Anonymous inner class
• interface： Interface
• []： Array
• enum： enumeration
• annotation： annotation
• primitive type： Basic data type
• void

02 Class Common methods of class

03 Create an object of a runtime class

• obtain Class The way the class is instantiated ：

  • If a specific class is known , By class class Property acquisition , This method is the most secure and reliable , Program performance is the highest .

  Class clazz = Person.class
  

  • Knowing an instance of a class , Call the getClass Method

  Class clazz = person.getClass()
  

  • Know the full class name of a class , And the class is under the classpath , It can be done by Class Class static methods forName() obtain , May throw out ClassNotFoundException

  Class clazz = Class.forName("demo01.Student")
  

  • The built-in basic data type can use the class name directly .Type
  • It can also be used ClassLoader

Summary :

• No matter how many instances a class creates , There is only one Class object .
• obtain Class Object method
  • adopt Class.forName(“ Package name + Class name ”) You can only get one Class object ( Reflection )
  • adopt Object.getClass( ) You can also get one Class object ( The normal way to create objects is to get Class object )
  • Through the static member variables of the class class get （ Each class has static member variables class）
  • Only the default base type has a method ： Through static member variables Type（ By static final modification ） get （ Eight basic data types ：byte int short double float char boolean）
  • adopt Object.getClass.getSuperclass Get the type of the parent class

Program example

Create runtime Class The object of

public class ReflectionTest2 {
public static void main(String[] args) {
//1. adopt getClass
Person p1 = new Student();
Class c1 = p1.getClass();
System.out.println(c1);
//2. Recommend to use through forName
try {
Class c2 = Class.forName("org.westos.demo2.Person");
System.out.println(c2);
} catch (ClassNotFoundException e) {
e.printStackTrace();
}
//3. It is recommended to use static variables class
Class c3 = Person.class;
System.out.println(c3);
//4. The unique method of the eight basic data types Type
Class c4 = Integer.TYPE;
System.out.println(c4);
//5.getSuperclass Get the parent class class
Class c5 = p1.getClass().getSuperclass();
System.out.println(c5);
}
}
class Person{
String name;
public Person() {
}
public Person(String name) {
this.name = name;
}
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
}
class Student extends Person{
public Student(){
this.name=" Student ";
}
}
class Teacher extends Person{
public Teacher(){
this.name=" Teachers' ";
}
}

Running results

class org.westos.demo2.Student
class org.westos.demo2.Person
class org.westos.demo2.Person
int
class org.westos.demo2.Person

If the array type is the same , The same dimension , only one class object

public class ReflectionTest3 {
public static void main(String[] args) {
Class c1 = Object.class;
Class c2 = Comparable.class;
Class c3 = Integer.class;
Class c4 = Void.class;
Class c5 = ElementType.class;
Class c6 = Override.class;
Class c7 = Class.class;
Class c8 = int[].class;
Class c9 = int[][].class;
Class c10 = String[].class;
System.out.println(c1 );
System.out.println(c2 );
System.out.println(c3 );
System.out.println(c4 );
System.out.println(c5 );
System.out.println(c6 );
System.out.println(c7 );
System.out.println(c8 );
System.out.println(c9 );
System.out.println(c10);
}
}

Running results

class java.lang.Object
interface java.lang.Comparable
class java.lang.Integer
class java.lang.Void
class java.lang.annotation.ElementType
interface java.lang.Override
class java.lang.Class
class [I
class [[I
class [Ljava.lang.String;

Get the class name by reflection , attribute , Methods and construction methods

public class ReflectionTest {
public static void main(String[] args) throws
ClassNotFoundException {
// Write the copy class of array
Class c1 = Class.forName("java.util.concurrent.CopyOnWriteArrayList");
// Get class name
System.out.println(c1.getName());
System.out.println(c1.getSimpleName());
// Get attributes , Member variables
Field[] declaredFields = c1.getDeclaredFields();
for (Field declaredField : declaredFields) {
System.out.println(" attribute ："+declaredField);
}
// How to get
Method[] declaredMethods = c1.getDeclaredMethods();
for (Field declaredField : declaredFields) {
System.out.println(" Method ："+declaredField);
}
// Acquired construction method
Constructor[] constructors = c1.getConstructors();
for (Constructor constructor : constructors) {
System.out.println(" Construction method ："+constructor);
}
}
}

Running results

java.util.concurrent.CopyOnWriteArrayList
CopyOnWriteArrayList
attribute ：private static final long java.util.concurrent.CopyOnWriteArrayList.serialVersionUID
attribute ：final transient java.util.concurrent.locks.ReentrantLock java.util.concurrent.CopyOnWriteArrayList.lock
attribute ：private transient volatile java.lang.Object[] java.util.concurrent.CopyOnWriteArrayList.array
attribute ：private static final sun.misc.Unsafe java.util.concurrent.CopyOnWriteArrayList.UNSAFE
attribute ：private static final long java.util.concurrent.CopyOnWriteArrayList.lockOffset
Method ：private static final long java.util.concurrent.CopyOnWriteArrayList.serialVersionUID
Method ：final transient java.util.concurrent.locks.ReentrantLock java.util.concurrent.CopyOnWriteArrayList.lock
Method ：private transient volatile java.lang.Object[] java.util.concurrent.CopyOnWriteArrayList.array
Method ：private static final sun.misc.Unsafe java.util.concurrent.CopyOnWriteArrayList.UNSAFE
Method ：private static final long java.util.concurrent.CopyOnWriteArrayList.lockOffset
Construction method ：public java.util.concurrent.CopyOnWriteArrayList(java.util.Collection)
Construction method ：public java.util.concurrent.CopyOnWriteArrayList()
Construction method ：public java.util.concurrent.CopyOnWriteArrayList(java.lang.Object[])

Using attributes through reflection , Methods and construction methods

public class ReflectionTest2 {
public static void main(String[] args) throws
ClassNotFoundException, IllegalAccessException,
InstantiationException, NoSuchMethodException,
InvocationTargetException, NoSuchFieldException {
Class c1 = Class.forName("org.westos.demo4.Student");
// Using the nonparametric construction method
Student s1 = (Student) c1.newInstance();
System.out.println(s1);
// Using the parametric construction method
Constructor constructor = c1.getConstructor(String.class, int.class);
Student s2 = (Student) constructor.newInstance("Java", 11);
System.out.println(s2);
// Usage method
Method method = c1.getMethod("setAge", int.class);
method.invoke(s2,10);
System.out.println(s2.getAge());
// Use attributes
Field field1 = c1.getDeclaredField("name");
Field field2 = c1.getDeclaredField("age");
// Turn off permission access detection , The default is false[ Open permission access detection ] If it's not turned off, it could happen IllegalAccessException
field1.setAccessible(true);
field2.setAccessible(true);
// Set the value through the field ( object , value )
field1.set(s1,"JOHN");
field2.set(s1,23);
// Get value by field
System.out.println(field1.get(s1)+"|"+field2.get(s1));
}
}
class Student{
private String name;
private int age;
public Student(){
}
public Student(String name, int age) {
this.name = name;
this.age = age;
}
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
public int getAge() {
return age;
}
public void setAge(int age) {
this.age = age;
}
@Override
public String toString() {
return this.getName()+"|"+this.getAge();
}
}

Running results

null|0
Java|11
10
JOHN|23