1. Overview
In Java, every type is either a reference or a primitive. Classes, enums, and arrays (which all inherit from java.lang.Object), as well as interfaces, are all reference types. Examples of reference types include java.lang.String, all of the wrapper classes for primitive types such as java.lang.Double, the interface java.io.Serializable, and the enum javax.swing.SortOrder. There is a fixed set of primitive types: boolean, byte, short, int, long, char, float, and double.
java.lang.Class is the entry point for all the reflection operations. For every type of object, JVM instantiates an immutable instance of java.lang.Class that provides methods to examine the runtime properties of the object and create new objects, invoke its method and get/set object fields.
The source code for this post is available on GitHub.
The below class diagram shows a list of Reflection APIs offered by java.lang.Class.
The source code for this post is available on GitHub.
The below class diagram shows a list of Reflection APIs offered by java.lang.Class.
 |
| Java Reflection for Classes |
2. Reflection Class API Examples
- Get Class Object
- Get Super Class
- Get Public Member Classes
- Get Declared Classes
- Get Declaring Class
- Getting Package Name
- Getting Class Modifiers
- Get Type Parameters
- Get Implemented Interfaces
- Get All Public Methods
- Get All Public Constructors
- Get All Public Fields
- Get All Annotations
In this article, we will look into all the above important methods of Class. Let's first create BaseInterface, BaseClass and ConcreteClass then we will apply all the reflection API's on these reference types.
BaseInterface.java
package com.javaguides.reflection.classes;
/**
* Base Interface
* @author javaguides.net
*
*/
public interface BaseInterface {
void method1();
void method2(String str);
void method3(int a);
}
BaseClass.java
package com.javaguides.reflection.classes;
/**
*
* @author javaguides.net
*
*/
public class BaseClass {
public void method4(){
System.out.println("method4");
}
public void method5(){
System.out.println("method5");
}
public void method6(){
System.out.println("method6");
}
}
ConcreteClass.java
package com.javaguides.reflection.classes;
/**
* Concrete class
* @author javaguides.net
*
*/
@Deprecated
public class ConcreteClass extends BaseClass implements BaseInterface {
public int id;
private String name;
public ConcreteClass(int id){
this.id = id;
}
public void method1() {
System.out.println("ConcreteClass :: method 1");
}
public void method2(String str) {
System.out.println("ConcreteClass :: method 2");
}
public void method3(int a) {
System.out.println("ConcreteClass :: method 3");
}
// Inner classes
private class InnerPrivateClass {
}
public class InnerPublicClass {
}
protected class InnerProtectedClass {
}
class InnerDefaultClass {
}
// Member enum
public enum ConcreClassMemberEnum {
}
// Member Interface
public interface ConcreteMemberInterface {
}
}
2.1 Get Class Object
There are 3 ways to get the instance of Class class. They are as follows:
- forName() method of Class class
- getClass() method of Object class
- the .class syntax
Examples to demonstrate all 3 ways to get the instance of Class class.
Class<BaseClass> baseClass = BaseClass.class;
System.out.println(baseClass.getCanonicalName());
// using forName() method
// Returns the Class object associated with the class or interface with
// the given string name
Class<?> concreteClass = Class.forName("com.javaguides.reflection.classes.ConcreteClass");
System.out.println(concreteClass.getSimpleName());
// for primitive types
Class<?> booleanClass = boolean.class;
System.out.println(booleanClass.getCanonicalName()); // prints boolean
// wrapper classes
Class<?> cDouble = Double.TYPE;
System.out.println(cDouble.getCanonicalName()); // prints double
// For arrays
Class<?> cDoubleArray = Class.forName("[D");
System.out.println(cDoubleArray.getCanonicalName()); // prints double[]
// Two Dimensional array.
Class<?> twoDStringArray = String[][].class;
System.out.println(twoDStringArray.getCanonicalName()); // prints
//
java.lang.String[][]
// For Collections
Set<String> s = new HashSet<String>();
Class c4 = s.getClass();
System.out.println(c4.getCanonicalName());
// For String
Class c = "foo".getClass();
System.out.println(c.getCanonicalName());
Output:
com.javaguides.reflection.classes.BaseClass
ConcreteClass
boolean
double
double[]
java.lang.String[][]
java.util.HashSet
java.lang.String
2.2 Get Super Class
getSuperclass() method on a Class object returns the superclass of the 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.
// Get super class BaseClass of ConcreteClass.
Class<?> concreteClass = Class.forName("com.javaguides.reflection.classes.ConcreteClass");
System.out.println(concreteClass.getSuperclass());
// Get super class of Object class
System.out.println(Object.class.getSuperclass()); // prints null
// Get super class of String class
System.out.println(String.class.getSuperclass());
// Get super class of String[]
System.out.println(String[].class.getSuperclass());
// Get Super class of Integer class
System.out.println(Integer.class.getSuperclass());
Output:
class com.javaguides.reflection.classes.BaseClass
null
class java.lang.Object
class java.lang.Object
class java.lang.Number
2.3 Get Public Member Classes
getClasses() method of a Class representation of object returns an array containing Class objects representing all the public classes, interfaces and enums that are members of the class represented by this Class object. This includes public class and interface members inherited from superclasses and public class and interface members declared by the class. This method returns an array of length 0 if this Class object has no public member classes or interfaces or if this Class object represents a primitive type, an array class, or void.
Class<?> concreteClass = Class.forName("com.javaguides.reflection.classes.ConcreteClass");
Class<?>[] classes = concreteClass.getClasses();
for (Class<?> class1 : classes) {
System.out.println(class1.getCanonicalName());
}
Output:
com.javaguides.reflection.classes.ConcreteClass.ConcreClassMemberEnum
com.javaguides.reflection.classes.ConcreteClass.ConcreteMemberInterface
com.javaguides.reflection.classes.ConcreteClass.InnerPublicClass
2.4 Get Declared Classes
getDeclaredClasses() method returns an array of Class objects reflecting all the classes and interfaces declared as members of the class represented by this Class object. The returned array doesn’t include classes declared in inherited classes and interfaces.
Class<?> concreteClass = Class.forName("com.javaguides.reflection.classes.ConcreteClass");
Class<?>[] classes = concreteClass.getDeclaredClasses();
for (Class<?> class1 : classes) {
System.out.println(class1.getCanonicalName());
}
Output:
com.javaguides.reflection.classes.ConcreteClass.ConcreClassMemberEnum
com.javaguides.reflection.classes.ConcreteClass.ConcreteMemberInterface
com.javaguides.reflection.classes.ConcreteClass.InnerDefaultClass
com.javaguides.reflection.classes.ConcreteClass.InnerPrivateClass
com.javaguides.reflection.classes.ConcreteClass.InnerProtectedClass
com.javaguides.reflection.classes.ConcreteClass.InnerPublicClass
2.6 Get Declaring Class
getDeclaringClass() method returns the Class object representing the class in which it was declared.
Class<?> concreteClass = Class.forName("com.javaguides.reflection.classes.ConcreteClass$InnerProtectedClass");
System.out.println(concreteClass.getDeclaringClass().getCanonicalName());
// prints com.javaguides.reflection.classes.ConcreteClass
Output:
com.javaguides.reflection.classes.ConcreteClass
2.7 Getting Package Name
getPackage() method returns the package for this class. The class loader of this class is used to find the package. We can invoke getName() method of Package to get the name of the package.
Class<?> concreteClass = Class.forName("com.javaguides.reflection.classes.ConcreteClass");
System.out.println(concreteClass.getPackage());
Output:
package com.javaguides.reflection.classes
2.8 Getting Class Modifiers
getModifiers() method returns the int representation of the class modifiers, we can use java.lang.reflect.Modifier.toString() method to get it in the string format as used in source code.
Class<?> concreteClass = Class.forName("com.javaguides.reflection.classes.ConcreteClass");
System.out.println(Modifier.toString(concreteClass.getModifiers()));
// prints public
Output:
public
2.9 Get Type Parameters
getTypeParameters() returns the array of TypeVariable if there are any Type parameters associated with the class. The type parameters are returned in the same order as declared.
//Get Type parameters (generics)
TypeVariable<?>[] typeParameters = Class.forName("java.util.HashMap").getTypeParameters();
for(TypeVariable<?> t : typeParameters)
System.out.print(t.getName()+",");
2.10 Get Implemented Interfaces
getGenericInterfaces() method returns the array of interfaces implemented by the class with generic type information. We can also use getInterfaces() to get the class representation of all the implemented interfaces.
Type[] interfaces = Class.forName("java.util.HashMap").getGenericInterfaces();
// prints "[java.util.Map<K, V>, interface java.lang.Cloneable,
// interface java.io.Serializable]"
System.out.println(Arrays.toString(interfaces));
// prints "[interface java.util.Map, interface java.lang.Cloneable,
// interface java.io.Serializable]"
System.out.println(Arrays.toString(Class.forName("java.util.HashMap").getInterfaces()));
Output:
[java.util.Map<K, V>, interface java.lang.Cloneable, interface java.io.Serializable]
[interface java.util.Map, interface java.lang.Cloneable, interface java.io.Serializable]
2.11 Get All Public Methods
getMethods() method returns the array of public methods of the Class including public methods of it’s superclasses and super interfaces.
Class<?> concreteClass = Class.forName("com.javaguides.reflection.classes.ConcreteClass");
Method[] methods = concreteClass.getMethods();
for (Method method : methods) {
System.out.println(method.getName());
}
Output:
method1
method2
method3
method4
method5
method6
wait
wait
wait
equals
toString
hashCode
getClass
notify
notifyAll
2.12 Get All Public Constructors
getConstructors() method returns the list of public constructors of the class reference of the object.
Class<?> concreteClass = Class.forName("com.javaguides.reflection.classes.ConcreteClass");
Constructor<?>[] constructors = concreteClass.getConstructors();
for (Constructor constructor : constructors) {
System.out.println(constructor.getName());
}
Output:
com.javaguides.reflection.classes.ConcreteClass
2.13 Get All Public Fields
getFields() method returns the array of public fields of the class including public fields of its superclasses and super interfaces.
Class<?> concreteClass = Class.forName("com.javaguides.reflection.classes.ConcreteClass");
Field[] fields = concreteClass.getFields();
for (Field field : fields) {
System.out.println(field.getName());
}
Output:
id
2.14 Get All Annotations
getAnnotations() method returns all the annotations for the element, we can use it with class, fields, and methods also.
Class<?> concreteClass = Class.forName("com.javaguides.reflection.classes.ConcreteClass");
Annotation[] annotations = concreteClass.getAnnotations();
for (Annotation annotation : annotations) {
System.out.println(annotation);
}
Output:
@java.lang.Deprecated()
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