Java 中的 Java.lang.Class 类|集合 1
原文:https://www . geesforgeks . org/Java-lang-class-class-Java-set-1/
Java 在 java.lang 包中提供了一个名为 Class 的类。类的实例表示运行的 Java 应用程序中的类和接口。基本的 Java 类型(布尔型、字节型、字符型、短整型、整型、长整型、浮点型和双精度型)和关键字 void 也表示为类对象。它没有公共构造函数。类对象由 Java 虚拟机( JVM )自动构造。这是最后一节课，所以我们不能延长。
类类方法在反射 API 中被广泛使用。
创建类对象
创建类对象有三种方式:
-
*Class.forName(“className”) :* Since class Class doesn’t contain any constructor, there is static factory method present in class Class, which is Class.forName() , used for creating object of class Class. Below is the syntax :
```java Class c = Class.forName(String className)
```
上面的语句为作为字符串参数(类名)传递的类创建了类对象。请注意，参数类名必须是要为其创建类对象的所需类的完全限定名。java 中返回相同类对象的任何类中的方法也称为工厂方法。要为其创建类对象的类名在运行时确定。 2. Myclass.class : When we write .class after a class name, it references the Class object that represents the given class. It is mostly used with primitive data types and only when we know the name of class. The class name for which Class object is to be created is determined at compile-time. Below is the syntax :
```java Class c = int.class
```
请注意，此方法与类名一起使用，而不是与类实例一起使用。例如
```java A a = new A(); // Any class A Class c = A.class; // No error Class c = a.class; // Error
``` 3. obj.getClass() : 这个方法出现在 Object 类中。它返回这个(obj)对象的运行时类。下面是语法:
```java A a = new A(); // Any class A Class c = a.getClass();
```**
*方法:*
-
*String toString() : This method converts the Class object to a string. It returns the string representation which is the string “class” or “interface”, followed by a space, and then by the fully qualified name of the class. If the Class object represents a primitive type, then this method returns the name of the primitive type and if it represents void* then it returns “void”.
```java Syntax : public String toString() Parameters : NA Returns : a string representation of this class object. Overrides : toString in class Object
```
```java // Java program to demonstrate toString() method public class Test { public static void main(String[] args) throws ClassNotFoundException { // returns the Class object for the class // with the specified name Class c1 = Class.forName("java.lang.String"); Class c2 = int.class; Class c3 = void.class;
System.out.print("Class represented by c1: ");
// toString method on c1 System.out.println(c1.toString());
System.out.print("Class represented by c2: ");
// toString method on c2 System.out.println(c2.toString());
System.out.print("Class represented by c3: ");
// toString method on c3 System.out.println(c3.toString()); } } ```
输出:
```java Class represented by c1: class java.lang.String Class represented by c2: int Class represented by c3: void
``` 2. Class<?> forName(String className) : As discussed earlier, this method returns the Class object associated with the class or interface with the given string name. The other variant of this method is discussed next.
```java Syntax : public static Class<?> forName(String className) throws ClassNotFoundException Parameters : className - the fully qualified name of the desired class. Returns : return the Class object for the class with the specified name. Throws : LinkageError : if the linkage fails ExceptionInInitializerError - if the initialization provoked by this method fails ClassNotFoundException - if the class cannot be located
```
```java // Java program to demonstrate forName() method public class Test { public static void main(String[] args) throws ClassNotFoundException { // forName method // it returns the Class object for the class // with the specified name Class c = Class.forName("java.lang.String");
System.out.print("Class represented by c : " + c.toString()); } } ```
输出:
```java Class represented by c : class java.lang.String
``` 3. Class<?> forName(String className,boolean initialize, ClassLoader loader) : This method also returns the Class object associated with the class or interface with the given string name using the given class loader.
指定的类加载器用于加载类或接口。如果参数加载器为空，则通过中的引导类加载器加载该类。只有当 initialize 参数为 true 并且该类之前没有初始化过时，该类才会初始化。
```java Syntax : public static Class<?> forName(String className,boolean initialize, ClassLoader loader) throws ClassNotFoundException Parameters : className - the fully qualified name of the desired class initialize - whether the class must be initialized loader - class loader from which the class must be loaded Returns : return the Class object for the class with the specified name. Throws : LinkageError : if the linkage fails ExceptionInInitializerError - if the initialization provoked by this method fails ClassNotFoundException - if the class cannot be located
```
```java // Java program to demonstrate forName() method public class Test { public static void main(String[] args) throws ClassNotFoundException { // returns the Class object for this class Class myClass = Class.forName("Test");
ClassLoader loader = myClass.getClassLoader();
// forName method // it returns the Class object for the class // with the specified name using the given class loader Class c = Class.forName("java.lang.String",true,loader);
System.out.print("Class represented by c : " + c.toString()); } } ```
输出:
```java Class represented by c : class java.lang.String
``` 4. T newInstance() : This method creates a new instance of the class represented by this Class object. The class is created as if by a new expression with an empty argument list. The class is initialized if it has not already been initialized.
```java Syntax : public T newInstance() throws InstantiationException,IllegalAccessException TypeParameters : T - The class type whose instance is to be returned Parameters : NA Returns : a newly allocated instance of the class represented by this object. Throws : IllegalAccessException - if the class or its nullary constructor is not accessible. InstantiationException - if this Class represents an abstract class, an interface, an array class, a primitive type, or void or if the class has no nullary constructor; or if the instantiation fails for some other reason. ExceptionInInitializerError - if the initialization provoked by this method fails. SecurityException - If a security manager, s, is present
```
```java // Java program to demonstrate newInstance() method public class Test { public static void main(String[] args) throws ClassNotFoundException, InstantiationException, IllegalAccessException { // returns the Class object for this class Class myClass = Class.forName("Test");
// creating new instance of this class // newInstance method Object obj = myClass.newInstance();
// returns the runtime class of obj System.out.println("Class of obj : " + obj.getClass()); } } ```
输出:
```java Class of obj : class Test
``` 5. boolean isInstance(Object obj) : This method determines if the specified Object is assignment-compatible with the object represented by this Class. It is equivalent to instanceof operator in java.
```java Syntax : public boolean isInstance(Object obj) Parameters : obj - the object to check Returns : true if obj is an instance of this class else return false
```
```java // Java program to demonstrate isInstance() method public class Test { public static void main(String[] args) throws ClassNotFoundException { // returns the Class object for the class // with the specified name Class c = Class.forName("java.lang.String");
String s = "GeeksForGeeks"; int i = 10;
// checking for Class instance // isInstance method boolean b1 = c.isInstance(s); boolean b2 = c.isInstance(i);
System.out.println("is s instance of String : " + b1); System.out.println("is i instance of String : " + b1);
} } ```
输出:
```java is s instance of String : true is i instance of String : false
``` 6. boolean isAssignableFrom(Class<?> cls) : This method determines if the class or interface represented by this Class object is either the same as, or is a superclass or superinterface, of the class or interface represented by the specified Class parameter.
```java Syntax : public boolean isAssignableFrom(Class<?> cls) Parameters : cls - the Class object to be checked Returns : true if objects of the type cls can be assigned to objects of this class Throws: NullPointerException - if the specified Class parameter is null.
```
```java // Java program to demonstrate isAssignableFrom() method public class Test extends Thread { public static void main(String[] args) throws ClassNotFoundException, InstantiationException, IllegalAccessException { // returns the Class object for this class Class myClass = Class.forName("Test");
// returns the Class object for the class // with the specified name Class c1 = Class.forName("java.lang.Thread"); Class c2 = Class.forName("java.lang.String");
// isAssignableFrom method on c1 // it checks whether Thread class is assignable from Test boolean b1 = c1.isAssignableFrom(myClass);
// isAssignableFrom method on c2 // it checks whether String class is assignable from Test boolean b2 = c2.isAssignableFrom(myClass);
System.out.println("is Thread class Assignable from Test : " + b1); System.out.println("is String class Assignable from Test : " + b2);
} } ```
输出:
```java is Thread class Assignable from Test : true is String class Assignable from Test : false
``` 7. boolean isInterface() : This method determines if the specified Class object represents an interface type.
```java Syntax : public boolean isInterface() Parameters : NA Returns : return true if and only if this class represents an interface type else return false
```
```java // Java program to demonstrate isInterface() method public class Test { public static void main(String[] args) throws ClassNotFoundException { // returns the Class object for the class // with the specified name Class c1 = Class.forName("java.lang.String"); Class c2 = Class.forName("java.lang.Runnable");
// checking for interface type
// isInterface method on c1 boolean b1 = c1.isInterface();
// is Interface method on c2 boolean b2 = c2.isInterface();
System.out.println("is java.lang.String an interface : " + b1); System.out.println("is java.lang.Runnable an interface : " + b2); } } ```
输出:
```java is java.lang.String an interface : false is java.lang.Runnable an interface : true
``` 8. boolean isPrimitive() : This method determines if the specified Class object represents a primitive type.
```java Syntax : public boolean isPrimitive() Parameters : NA Returns : return true if and only if this class represents a primitive type else return false
```
```java // Java program to demonstrate isPrimitive method public class Test { public static void main(String[] args) { // returns the Class object associated with an integer; Class c1 = int.class;
// returns the Class object associated with Test class Class c2 = Test.class;
// checking for primitive type
// isPrimitive method on c1 boolean b1 = c1.isPrimitive();
// isPrimitive method on c2 boolean b2 = c2.isPrimitive();
System.out.println("is "+c1.toString()+" primitive : " + b1); System.out.println("is "+c2.toString()+" primitive : " + b2); } } ```
输出:
```java is int primitive : true is class Test primitive : false
``` 9. boolean isArray() : This method determines if the specified Class object represents an array class.
```java Syntax : public boolean isArray() Parameters : NA Returns : return true if and only if this class represents an array type else return false
```
```java // Java program to demonstrate isArray method public class Test { public static void main(String[] args) { int a[] = new int[2];
// returns the Class object for array class Class c1 = a.getClass();
// returns the Class object for Test class Class c2 = Test.class;
// checking for array type
// isArray method on c1 boolean b1 = c1.isArray();
// is Array method on c2 boolean b2 = c2.isArray();
System.out.println("is "+c1.toString()+" an array : " + b1); System.out.println("is "+c2.toString()+" an array : " + b2);
} } ```
输出:
```java is class [I an array : true is class Test an array : false
``` 10. boolean is onymous class():当且仅当这个类是匿名类时，这个方法返回 true。匿名类类似于本地类，只是它们没有名称。
```java Syntax : public boolean isAnonymousClass() Parameters : NA Returns : true if and only if this class is an anonymous class. false,otherwise.
``` 11. 布尔 isLocalClass() :当且仅当此类是本地类时，此方法返回 true。本地类是在 Java 代码块中本地声明的类，而不是作为类的成员。
```java Syntax : public boolean isLocalClass() Parameters : NA Returns : true if and only if this class is a local class. false,otherwise.
``` 12. boolean isMemberClass() : This method returns true if and only if the this class is a Member class.A member class is a class that is declared as a non-static member of a containing class.
```java Syntax : public boolean isMemberClass() Parameters : NA Returns : true if and only if this class is a Member class. false,otherwise.
```
下面是解释 isAnonymousClass()方法、isLocalClass 方法和 isMemberClass()方法的使用的 Java 程序。
```java // Java program to demonstrate isAnonymousClass() ,isLocalClass // and isMemberClass() method
public class Test { // any Member class of Test class A{}
public static void main(String[] args) { // declaring an anonymous class Test t1 = new Test() { // class definition of anonymous class };
// returns the Class object associated with t1 object Class c1 = t1.getClass();
// returns the Class object associated with Test class Class c2 = Test.class;
// returns the Class object associated with A class Class c3 = A.class;
// checking for Anonymous class // isAnonymousClass method boolean b1 = c1.isAnonymousClass(); System.out.println("is "+c1.toString()+" an anonymous class : "+b1);
// checking for Local class // isLocalClass method boolean b2 = c2.isLocalClass(); System.out.println("is "+c2.toString()+" a local class : " + b2);
// checking for Member class // isMemberClass method boolean b3 = c3.isMemberClass(); System.out.println("is "+c3.toString()+" a member class : " + b3);
} } ```
输出:
```java is class Test$1 an anonymous class : true is class Test a local class : false is class Test$A a member class : true
``` 13. boolean isEnum() : This method returns true if and only if this class was declared as an enum in the source code.
```java Syntax : public boolean isEnum() Parameters : NA Returns : true iff this class was declared as an enum in the source code. false,otherwise.
```
```java // Java program to demonstrate isEnum() method
enum Color { RED, GREEN, BLUE; }
public class Test { public static void main(String[] args) { // returns the Class object associated with Color(an enum class) Class c1 = Color.class;
// returns the Class object associated with Test class Class c2 = Test.class;
// checking for Enum class // isEnum method boolean b1 = c1.isEnum(); boolean b2 = c2.isEnum();
System.out.println("is "+c1.toString()+" an Enum class : " + b1); System.out.println("is "+c2.toString()+" an Enum class : " + b2); } } ```
输出:
```java is class Color an Enum class : true is class Test an Enum class : false
``` 14. boolean isAnnotation() : This method determines if this Class object represents an annotation type. Note that if this method returns true, isInterface() method will also return true, as all annotation types are also interfaces.
```java Syntax : public boolean isAnnotation() Parameters : NA Returns : return true if and only if this class represents an annotation type else return false
```
```java // Java program to demonstrate isAnnotation() method
// declaring an Annotation Type @interface A { // Annotation element definitions }
public class Test { public static void main(String[] args) throws ClassNotFoundException { // returns the Class object associated with A annotation Class c1 = A.class;
// returns the Class object associated with Test class Class c2 = Test.class;
// checking for annotation type // isAnnotation method boolean b1 = c1.isAnnotation(); boolean b2 = c2.isAnnotation();
System.out.println("is "+c1.toString()+" an annotation : " + b1); System.out.println("is "+c2.toString()+" an annotation : " + b2);
} } ```
输出:
```java is interface A an annotation : true is class Test an annotation : false
``` 15. String getName() : This method returns the name of the entity (class, interface, array class, primitive type, or void) represented by this Class object, as a String.
```java Syntax : public String getName() Parameters : NA Returns : returns the name of the name of the entity represented by this object.
```
```java // Java program to demonstrate getName() method public class Test { public static void main(String[] args) { // returns the Class object associated with Test class Class c = Test.class;
System.out.print("Class Name associated with c : ");
// returns the name of the class // getName method System.out.println(c.getName()); } } ```
输出:
```java Class Name associated with c : Test
``` 16. String getSimpleName() : This method returns the name of the underlying class as given in the source code. It returns an empty string if the underlying class is anonymous class.
数组的简单名称是附加了“]”的组件类型的简单名称。特别是组件类型为匿名的数组的简单名称是“[]”。
```java Syntax : public String getSimpleName() Parameters : NA Returns : the simple name of the underlying class
```
```java // Java program to demonstrate getSimpleName() method public class Test { public static void main(String[] args) throws ClassNotFoundException { // returns the Class object for the class // with the specified name Class c1 = Class.forName("java.lang.String");
System.out.print("Class Name associated with c : ");
// returns the name of the class // getName method System.out.println(c1.getName());
System.out.print("Simple class Name associated with c : ");
// returns the simple name of the class // getSimpleName method System.out.println(c1.getSimpleName()); } } ```
输出:
```java Class Name associated with c : java.lang.String Simple class Name associated with c : String
``` 17. ClassLoader getClassLoader() : This method returns the class loader for this class. If the class loader is bootstrap classloader then this method returned null, as bootstrap classloader is implemented in native languages like C, C++. If this object represents a primitive type or void,then also null is returned.
```java Syntax : public ClassLoader getClassLoader() Parameters : NA Returns : the class loader that loaded the class or interface represented by this object represented by this object. Throws : SecurityException - If a security manager and its checkPermission method denies access to the class loader for the class.
```
```java // Java program to demonstrate getClassLoader() method public class Test { public static void main(String[] args) throws ClassNotFoundException { // returns the Class object for this class Class myClass = Class.forName("Test");
// returns the Class object for the class // with the specified name Class c1 = Class.forName("java.lang.String");
// returns the Class object for primitive int Class c2 = int.class;
System.out.print("Test class loader : ");
// getting the class loader for Test class // getClassLoader method on myClass System.out.println(myClass.getClassLoader());
System.out.print("String class loader : ");
// getting the class loader for String class // we will get null as String class is loaded // by BootStrap class loader // getClassLoader method on c1 System.out.println(c1.getClassLoader());
System.out.print("primitive int loader : ");
// getting the class loader for primitive int // getClassLoader method on c2 System.out.println(c2.getClassLoader()); } } ```
输出:
```java Test class loader : sun.misc.Launcher$AppClassLoader@73d16e93 String class loader : null primitive int loader : null
``` 18. TypeVariable>[ ] getTypeParameters() : This method returns an array of TypeVariable objects that represent the type variables declared by the generic declaration represented by this GenericDeclaration object, in declaration order
```java Syntax : public TypeVariable>[] getTypeParameters() Specified by: getTypeParameters in interface GenericDeclaration Parameters : NA Returns : an array of TypeVariable objects that represent the type variables declared by this generic declaration represented by this object. Throws : GenericSignatureFormatError - if the generic signature of this generic declaration does not conform to the format specified in JVM.
```
```java // Java program to demonstrate getTypeParameters() method
import java.lang.reflect.TypeVariable;
public class Test { public static void main(String[] args) throws ClassNotFoundException { // returns the Class object for the class // with the specified name Class c = Class.forName("java.util.Set");
// getting array of TypeVariable for myClass object // getTypeParameters method TypeVariable[] tv = c.getTypeParameters();
System.out.println("TypeVariables in "+c.getName()+" class : ");
// iterating through all TypeVariables for (TypeVariable typeVariable : tv) { System.out.println(typeVariable); }
} } ```
输出:
```java TypeVariables in java.util.Set class : E
``` 19. Class<? super T> getSuperclass() : This method returns the Class representing the superclass of the entity (class, interface, primitive type or void) represented by this Class. If this Class represents either the Object class, an interface, a primitive type, or void, then null is returned. If this object represents an array class then the Class object representing the Object class is returned.
```java Syntax : public Class<? super T> getSuperclass() Parameters : NA Returns : the superclass of the class represented by this object
```
```java // Java program to demonstrate getSuperclass() method
// base class class A { // methods and fields }
// derived class class B extends A {
}
// Driver class public class Test { public static void main(String[] args) throws ClassNotFoundException
{ // returns the Class object associated with Test class Class myClass = Test.class;
// returns the Class object for the class // with the specified name Class c1 = Class.forName("A"); Class c2 = Class.forName("B"); Class c3 = Class.forName("java.lang.Object");
// getSuperclass method returns the superclass of the class represented // by this class object
System.out.print("Test superclass : ");
// getSuperclass method on myClass System.out.println(myClass.getSuperclass());
System.out.print("A superclass : ");
// getSuperclass method on c1 System.out.println(c1.getSuperclass());
System.out.print("B superclass : ");
// getSuperclass method on c2 System.out.println(c2.getSuperclass());
System.out.print("Object superclass : ");
// getSuperclass method on c3 System.out.println(c3.getSuperclass()); } } ```
输出:
```java Test superclass : class java.lang.Object A superclass : class java.lang.Object B superclass : class A Object superclass : null
``` 20. Type getGenericSuperclass() : This method returns the Type representing the direct superclass of the entity (class, interface, primitive type or void) represented by this Class.
如果此类表示对象类、接口、基元类型或 void，则返回 null。如果此对象表示数组类，则返回表示对象类的类对象。
```java Syntax : public Type getGenericSuperclass() Parameters : NA Returns : the superclass of the class represented by this object Throws: GenericSignatureFormatError - if the generic class signature does not conform to the format specified in JVM TypeNotPresentException - if the generic superclass refers to a non-existent type declaration MalformedParameterizedTypeException - if the generic superclass refers to a parameterized type that cannot be instantiated for any reason
```
```java // Java program to demonstrate // getGenericSuperclass() method public class Test { public static void main(String[] args) throws ClassNotFoundException
{ // returns the Class object associated with Test class Class myClass = Test.class;
// returns the Class object for the class // with the specified name Class c1 = Class.forName("java.util.ArrayList"); Class c3 = Class.forName("java.lang.Object");
// getGenericSuperclass method returns the generic // superclass of the class represented // by this class object
System.out.print("Test superclass : ");
// getGenericSuperclass method on myClass System.out.println(myClass.getGenericSuperclass());
System.out.print("ArrayList superclass : ");
// getGenericSuperclass method on c1 System.out.println(c1.getGenericSuperclass());
System.out.print("Object superclass : ");
// getGenericSuperclass method on c3 System.out.println(c3.getGenericSuperclass()); } } ```
输出:
```java Test superclass : class java.lang.Object Set superclass : java.util.AbstractList Object superclass : null
``` 21. Class<?>[] getInterfaces() : This method returns the interfaces implemented by the class or interface represented by this object.
如果此对象表示不实现接口的类或接口，则方法返回长度为 0 的数组。 如果这个对象表示一个基元类型或者 void，那么这个方法返回一个长度为 0 的数组。
```java Syntax : public Class<?>[] getInterfaces() Parameters : NA Returns : an array of interfaces implemented by this class.
```
```java // Java program to demonstrate getInterfaces() method
// base interface interface A { // methods and constant declarations }
// derived class class B implements A { // methods implementations that were declared in A }
// Driver class public class Test { public static void main(String[] args) throws ClassNotFoundException
{ // returns the Class object for the class // with the specified name Class c1 = Class.forName("B"); Class c2 = Class.forName("java.lang.String");
// getInterface method on c1 // it returns the interfaces implemented by B class Class c1Interfaces[] = c1.getInterfaces();
// getInterface method on c2 // returns the interfaces implemented by String class Class c2Interfaces[] = c2.getInterfaces();
System.out.println("interfaces implemented by B class : ");
// iterating through B class interfaces for (Class class1 : c1Interfaces) { System.out.println(class1); }
System.out.println("interfaces implemented by String class : ");
// iterating through String class interfaces for (Class class1 : c2Interfaces) { System.out.println(class1); } } } ```
输出:
```java interfaces implemented by B class : interface A interfaces implemented by String class : interface java.io.Serializable interface java.lang.Comparable interface java.lang.CharSequence
``` 22. Type[] getGenericInterfaces() : This method returns the Types representing the interfaces directly implemented by the class or interface represented by this object.
如果此对象表示不实现接口的类或接口，则方法返回长度为 0 的数组。 如果这个对象表示一个基元类型或者 void，那么这个方法返回一个长度为 0 的数组。
```java Syntax : public Type[] getGenericInterfaces() Parameters : NA Returns : an array of interfaces implemented by this class Throws: GenericSignatureFormatError - if the generic class signature does not conform to the format specified in JVM TypeNotPresentException - if the generic superinterfaces refers to a non-existent type declaration MalformedParameterizedTypeException - if the generic superinterfaces refers to a parameterized type that cannot be instantiated for any reason
```
```java // Java program to demonstrate getGenericInterfaces() method
import java.lang.reflect.Type;
public class Test { public static void main(String[] args) throws ClassNotFoundException
{ // returns the Class object for the class // with the specified name Class c1 = Class.forName("java.util.Set");
// getGenericInterfaces() on c1 // it returns the interfaces implemented by Set interface Type c1GenericInterfaces[] = c1.getGenericInterfaces();
System.out.println("interfaces implemented by Set interface : ");
// iterating through Set class interfaces for (Type type : c1GenericInterfaces) { System.out.println(type); } } } ```
输出:
```java interfaces implemented by Set interface : java.util.Collection
``` 23. Package getPackage() : This method returns the package for this class. The classloader subsystem in JVM Architecture used this method to find the package of a class or interface.
```java Syntax : public Package getPackage() Parameters : NA Returns : the package of the class, or null if no package information is available from the archive or codebase.
```
```java // Java program to demonstrate getPackage() method public class Test { public static void main(String[] args) throws ClassNotFoundException { // returns the Class object for the class // with the specified name Class c1 = Class.forName("java.lang.String"); Class c2 = Class.forName("java.util.ArrayList");
// getting package of class
// getPackage method on c1 System.out.println(c1.getPackage());
// getPackage method on c2 System.out.println(c2.getPackage());
} } ```
输出:
```java package java.lang, Java Platform API Specification, version 1.8 package java.util, Java Platform API Specification, version 1.8
``` 24. Field[] getFields() : This method returns an array of Field objects reflecting all the accessible public fields of the class(and of all its superclasses) or interface(and of all its superclasses) represented by this Class object.
```java Syntax : public Field[] getFields() throws SecurityException Parameters : NA Returns : the array of Field objects representing the public fields and array of length 0 if the class or interface has no accessible public fields or if this Class object represents a primitive type or void. Throws : SecurityException - If a security manager, s, is present.
```
```java // Java program to demonstrate getFields() method
import java.lang.reflect.Field;
public class Test { public static void main(String[] args) throws SecurityException,ClassNotFoundException { // returns the Class object for the class // with the specified name Class c1 = Class.forName("java.lang.Integer");
// getFields method on c1 // it array of fields of Integer class Field F[] = c1.getFields();
System.out.println("Below are the fields of Integer class : ");
// iterating through all fields of String class for (Field field : F) { System.out.println(field); } } } ```
输出:
```java Below are the fields of Integer class : public static final int java.lang.Integer.MIN_VALUE public static final int java.lang.Integer.MAX_VALUE public static final java.lang.Class java.lang.Integer.TYPE public static final int java.lang.Integer.SIZE public static final int java.lang.Integer.BYTES
``` 25. Class<?>[ ] getClasses() : This method returns an array containing Class objects representing all the public classes and interfaces that are members of the class represented by this Class object. The array contains public class and interface members inherited from superclasses and public class and interface members declared by the class.
如果此类对象没有公共成员类或接口，则此方法返回长度为 0 的数组。 如果这个 Class 对象表示一个基元类型、一个数组类或 void，那么这个方法也返回一个长度为 0 的数组。
```java Syntax : Class<?>[ ] getClasses() Parameters : NA Returns : the array of Class objects representing the public members of this class Throws : SecurityException - If a security manager, s, is present.
```
```java // Java program to demonstrate getClasses() method
public class Test { // base interface public interface A { // methods and constant declarations }
// derived class public class B implements A { // methods implementations that were declared in A }
public static void main(String[] args) throws ClassNotFoundException
{ // returns the Class object associated with Test class Class c1 = Test.class;
// getClasses method on c1 // it returns the array of Class objects containing the all // public classes and interfaces represented by Test class Class[] c1Classes = c1.getClasses();
System.out.println("public members of Test class : ");
// iterating through all public members of Test class for (Class class1 : c1Classes) { System.out.println(class1); }
} } ```
输出:
```java public members of Test class : interface Test$A class Test$B
``` 26. Method[] getMethods() : This method returns an array of Method objects reflecting all the accessible public methods of the class or interface and those inherited from superclasses and super interfaces represented by this Class object.
```java Syntax : public Method[] getMethods() throws SecurityException Parameters : NA Returns : the array of Method objects representing the public methods and array of length 0 if the class or interface has no accessible public method or if this Class object represents a primitive type or void. Throws : SecurityException - If a security manager, s, is present.
```
```java // Java program to demonstrate getMethods() method
import java.lang.reflect.Method;
public class Test { public static void main(String[] args) throws SecurityException,ClassNotFoundException { // returns the Class object for the class // with the specified name Class c1 = Class.forName("java.lang.Object");
// getMethods method on c1 // it returns array of methods of Object class Method M[] = c1.getMethods();
System.out.println("Below are the methods of Object class : ");
// iterating through all methods of Object class for (Method method : M) { System.out.println(method); } } } ```
输出:
```java Below are the methods of Object class : 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 void java.lang.Object.wait(long) throws java.lang.InterruptedException public boolean java.lang.Object.equals(java.lang.Object) public java.lang.String java.lang.Object.toString() public native int java.lang.Object.hashCode() 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()
``` 27. Constructor<?>[] getConstructors() : This method returns an array of Constructor objects reflecting all the public constructors of the class represented by this Class object.
```java Syntax : public Constructor<?>[] getConstructors() throws SecurityException Parameters : NA Returns : the array of Constructor objects representing the public constructors of this class and array of length 0 if the class or interface has no accessible public constructor or if this Class object represents a primitive type or void. Throws : SecurityException - If a security manager, s, is present.
```
```java // Java program to demonstrate getConstructors() method
import java.lang.reflect.Constructor;
public class Test { public static void main(String[] args) throws SecurityException,ClassNotFoundException { // returns the Class object for the class // with the specified name Class c1 = Class.forName("java.lang.Boolean");
// getConstructor method on c1 // it returns an array of constructors of Boolean class Constructor C[] = c1.getConstructors();
System.out.println("Below are the constructors of Boolean class :");
// iterating through all constructors for (Constructor constructor : C) { System.out.println(constructor); } } } ```
输出:
```java Below are the constructors of Boolean class : public java.lang.Boolean(boolean) public java.lang.Boolean(java.lang.String)
``` 28. Field getField(String fieldName) : This method returns a Field object that reflects the specified public member field of the class or interface represented by this Class object.
```java Syntax : public Field getField(String fieldName) throws NoSuchFieldException,SecurityException Parameters : fieldName - the field name Returns : the Field object of this class specified by name Throws : NoSuchFieldException - if a field with the specified name is not found. NullPointerException - if fieldName is null SecurityException - If a security manager, s, is present.
```
```java // Java program to demonstrate getField() method
import java.lang.reflect.Field;
public class Test { public static void main(String[] args) throws SecurityException,ClassNotFoundException, NoSuchFieldException { // returns the Class object for the class // with the specified name Class c1 = Class.forName("java.lang.Integer");
// getField method on c1 // it checks a public field in Integer class with specified parameter Field f = c1.getField("MIN_VALUE");
System.out.println("public field in Integer class with MIN_VALUE name :"); System.out.println(f); } } ```
输出:
```java public field in Integer class with MIN_VALUE name : public static final int java.lang.Integer.MIN_VALUE
``` 29. Method getMethod(String methodName,Class… parameterTypes) : This method returns a Method object that reflects the specified public member method of the class or interface represented by this Class object.
```java Syntax : public Method getMethod(String methodName,Class... parameterTypes) throws NoSuchFieldException,SecurityException Parameters : methodName - the method name parameterTypes - the list of parameters Returns : the method object of this class specified by name Throws : NoSuchMethodException - if a method with the specified name is not found. NullPointerException - if methodName is null SecurityException - If a security manager, s, is present.
```
```java // Java program to demonstrate getMethod() method
import java.lang.reflect.Method;
public class Test { public static void main(String[] args) throws SecurityException,ClassNotFoundException, NoSuchMethodException { // returns the Class object for the class // with the specified name Class c1 = Class.forName("java.lang.Integer"); Class c2 = Class.forName("java.lang.String");
// getMethod method on c1 // it checks for a public Method in Integer class Method m = c1.getMethod("parseInt",c2);
System.out.println("method in Integer class specified by parseInt : "); System.out.println(m); } } ```
输出:
```java public method in Integer class specified by parseInt : public static int java.lang.Integer.parseInt(java.lang.String) throws java.lang.NumberFormatException
``` 30. Constructor<?> getConstructor(Class<?>… parameterTypes) : This method returns a Constructor object that reflects the specified public constructor of the class represented by this Class object.The parameterTypes parameter is an array of Class objects that identify the constructor’s formal parameter types, in declared order.
```java Syntax : public Constructor<?> getConstructor(Class<?>... parameterTypes) throws NoSuchMethodException,SecurityException Parameters : parameterTypes - the list of parameters Returns : the Constructor object of the public constructor that matches the specified parameterTypes Throws : NoSuchMethodException - if a Constructor with the specified parameterTypes is not found. SecurityException - If a security manager, s, is present.
```
```java // Java program to demonstrate // getConstructor() Constructor
import java.lang.reflect.Constructor;
public class Test { public static void main(String[] args) throws SecurityException,ClassNotFoundException, NoSuchMethodException { // returns the Class object for the class // with the specified name Class c1 = Class.forName("java.lang.Integer"); Class c2 = Class.forName("java.lang.String");
// getConstructor method on c1 // it checks a public Constructor in Integer class // with specified parameterTypes Constructor c = c1.getConstructor(c2);
System.out.println("Constructor in Integer class & String parameterType:"); System.out.println(c); } } ```
输出:
```java public Constructor in Integer class with String parameterType : public java.lang.Integer(java.lang.String) throws java.lang.NumberFormatException
```
注:方法 getFields()、getMethods()、getConstructors()、getField()、getMethod()、getConstructor()在 Reflection 中被广泛使用(参考这篇举例) 31. T cast(Object obj) : This method is used to casts an object to the class or interface represented by this Class object.
```java Syntax : public T cast(Object obj) TypeParameters : T - The class type whose object is to be cast Parameters : obj - the object to be cast Returns : the object after casting, or null if obj is null Throws : ClassCastException - if the object is not null and is not assignable to the type T.
```
```java // Java program to demonstrate cast() method class A { // methods and fields }
class B extends A { // methods and fields }
// Driver Class public class Test { public static void main(String[] args) { A a = new A();
System.out.println(a.getClass());
B b = new B();
// casting b to a // cast method a = A.class.cast(b);
System.out.println(a.getClass());
} } ```
输出:
```java class A class B
``` 32. Class<? extends U> asSubclass(Class clazz) : This method is used to cast this Class object to represent a subclass of the class represented by the specified class object.It always returns a reference to this class object.
```java Syntax : public Class<? extends U> asSubclass(Class class) TypeParameters : U - The superclass type whose object is to be cast Parameters : clazz - the superclass object to be cast Returns : this Class object, cast to represent a subclass of the specified class object. Throws : ClassCastException - if this Class object does not represent a subclass of the specified class (here "subclass" includes the class itself).
```
```java // Java program to demonstrate asSubclass() method class A { // methods and fields }
class B extends A { // methods and fields }
// Driver Class public class Test { public static void main(String[] args) { A a = new A();
// returns the Class object for super class(A) Class superClass = a.getClass();
B b = new B();
// returns the Class object for subclass(B) Class subClass = b.getClass();
// asSubclass method // it represent a subclass of the specified class object Class cast = subClass.asSubclass(superClass);
System.out.println(cast);
} } ```
输出:
```java class B
```
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