Class Reflection
In this chapter you will learn:
- What is java.lang.Class
- How to load class from class name
- How to get the class name
- How to check the type of a class
- How to create new instance with reflection
java.lang.Class
Instances of the Class represent classes and interfaces.
An enum is a kind of class and an annotation is a kind of interface.
Every array belongs to a class.
Load Class from class name
We can use the following two methods from
Class to load a class by the class name.
static Class<?> forName(String className)returns the Class object for given string name.static Class<?> forName(String name, boolean initialize, ClassLoader loader)returns the Class object using the given class loader.
Use Class.forName to create Class from a String
public class Main {
/* j av a 2 s. co m*/
public static void main(String[] args) {
try {
Class c = Class.forName("java.lang.String");
} catch (ClassNotFoundException e) {
e.printStackTrace();
}
}
}
Get class name
Once we have the Class object we can get the class name by using the following
methods. getCanonicalName() returns the class name along with its package name.
String getCanonicalName()returns the canonical name of a class.String getName()returns the name of the entity (class, interface, array class, primitive type, or void).String getSimpleName()returns the simple name of the class.
The following code loads the class for String and get its name, simple name and Canonical Name.
public class Main {
/*from ja v a2 s .c o m*/
public static void main(String[] args) {
Class c = new String().getClass();
String s = c.getName();
System.out.println(s);
s = c.getSimpleName();
System.out.println(s);
s = c.getCanonicalName();
System.out.println(s);
}
}
The output:
Check class type
java.lang.Class represents not only the class. It can also represent the annotation, array, enum, interface. The following table lists methods we can use to
boolean isAnnotation()is it an annotation type.boolean isAnnotationPresent(Class<? extends Annotation> annotationClass)Is an annotation present.boolean isAnonymousClass()Is it an anonymous class.boolean isArray()Is it an array class.boolean isAssignableFrom(Class<?> cls)Is it assigned from.boolean isEnum()Is it an enum.boolean isInstance(Object obj)Is it instance of another object.boolean isInterface()Is it an interface type.boolean isLocalClass()Is it a local class.boolean isMemberClass()Is it a member class.boolean isPrimitive()Is it a primitive type.boolean isSynthetic()Is it a synthetic class.
The following code calls isInterface() method
to see if java.lang.String is an interface.
public class Main {
/* j a v a 2s.c om*/
public static void main(String[] args) {
Class c = new String().getClass();
System.out.println(c.isInterface());
}
}
The output:
Create new instance
T newInstance() creates a new instance of the class represented by this Class object.
public class Main {
// j a v a 2 s . com
public static void main(String[] args) {
Class c = new String().getClass();
try {
Object obj = c.newInstance();
} catch (InstantiationException e) {
e.printStackTrace();
} catch (IllegalAccessException e) {
e.printStackTrace();
}
}
}
Next chapter...
What you will learn in the next chapter: