The Class API

#Introduction

So far we've learned what Reflection is, what the Class class is, how to obtain a Class object, and how class loading works. Now we'll actually start using Reflection. Everything begins with one object:

Class<?> cls = Employee.class;

Once we have it, we can ask the JVM questions like: What is the class name? Which package does it belong to? Is it an interface or an enum? Which superclass does it extend? Which interfaces does it implement? Is it abstract, final, public? All this is called class metadata.

#Sample Class

Throughout this chapter we'll use:

package com.demo;

public class Employee extends Person implements Serializable {
    private String name;

    public void work() {
    }
}

Class<?> cls = Employee.class;

#Names

getName()

Returns the fully qualified class name (FQCN):

System.out.println(cls.getName());

com.demo.Employee

A fully qualified class name is package name + class name — com.demo.Employee instead of Employee. Spring, Hibernate, JDBC and Class.forName("com.demo.Employee") all use fully qualified names.

getSimpleName()

Returns only the class name:

System.out.println(cls.getSimpleName());

Employee

getCanonicalName()

System.out.println(Employee.class.getCanonicalName());

com.demo.Employee

For normal top-level classes both are identical. Arrays and nested classes differ:

Canonical names are easier for humans to read.

#getPackageName()

System.out.println(cls.getPackageName());

com.demo

Older Java versions used cls.getPackage() (which returns a Package object); Java 9 introduced getPackageName(), which returns a String.

#getSuperclass()

class Person {
}

class Employee extends Person {
}

System.out.println(cls.getSuperclass());

class com.demo.Person

String.class.getSuperclass();   // class java.lang.Object
Object.class.getSuperclass();   // null

Object.class.getSuperclass() returns null because Object has no parent. (Interfaces and primitive types also return null.)

#getInterfaces()

Returns all directly implemented interfaces.

class Employee implements Serializable, Cloneable {
}

Class<?>[] interfaces = cls.getInterfaces();

for (Class<?> c : interfaces) {
    System.out.println(c.getName());
}

java.io.Serializable
java.lang.Cloneable

#getModifiers()

Returns modifiers as an integer bit mask.

public final class Employee {
}

int mod = cls.getModifiers();
System.out.println(mod);

17

17 = PUBLIC (1) + FINAL (16). Since that isn't meaningful directly, Java provides the Modifier utility class:

import java.lang.reflect.Modifier;

System.out.println(Modifier.isPublic(mod));
System.out.println(Modifier.isFinal(mod));
System.out.println(Modifier.isAbstract(mod));

true
true
false

Common Modifier Methods

Modifier.isPublic()
Modifier.isPrivate()
Modifier.isProtected()
Modifier.isFinal()
Modifier.isStatic()
Modifier.isAbstract()
Modifier.isInterface()

These are commonly asked in interviews. (Modifier.toString(mod) gives "public final".)

#Type Checks

Spring internally checks annotations like @Component, @Service and @Repository using Reflection.

#isAssignableFrom()

One of the most frequently asked Reflection methods.

class Animal {
}

class Dog extends Animal {
}

System.out.println(Animal.class.isAssignableFrom(Dog.class));

true

Meaning: a Dog object can be assigned to an Animal reference — Animal a = new Dog(); is valid.

Dog.class.isAssignableFrom(Animal.class);   // false

…because Dog d = new Animal(); is invalid.

#getClassLoader()

Returns the class loader that loaded the class.

System.out.println(Employee.class.getClassLoader());

jdk.internal.loader.ClassLoaders$AppClassLoader@4e0e2f2a

(The hash after @ varies; on Java 8 it prints sun.misc.Launcher$AppClassLoader@….)

System.out.println(String.class.getClassLoader());

null

Why? Because String is loaded by the Bootstrap ClassLoader, which is implemented natively and isn't represented by a regular Java object.

#getDeclaredClasses()

Returns nested classes.

class Employee {
    class Address {
    }
}

Class<?>[] classes = Employee.class.getDeclaredClasses();
System.out.println(Arrays.toString(classes));

[class com.demo.Employee$Address]

#Complete Example

Class<?> cls = Employee.class;

System.out.println(cls.getName());
System.out.println(cls.getSimpleName());
System.out.println(cls.getPackageName());
System.out.println(cls.getSuperclass());
System.out.println(cls.isInterface());
System.out.println(cls.isEnum());
System.out.println(cls.isAnnotation());
System.out.println(cls.isArray());
System.out.println(cls.isPrimitive());
System.out.println(cls.getClassLoader());

com.demo.Employee
Employee
com.demo
class com.demo.Person
false
false
false
false
false
jdk.internal.loader.ClassLoaders$AppClassLoader@4e0e2f2a

#Methods Covered in This Chapter

getName()
getSimpleName()
getCanonicalName()
getPackageName()
getSuperclass()
getInterfaces()
getModifiers()
isInterface()
isEnum()
isAnnotation()
isArray()
isPrimitive()
isAssignableFrom()
getClassLoader()
getDeclaredClasses()

#Key Points to Remember

• Class provides metadata about a loaded class.
• getName() returns the fully qualified class name; getSimpleName() only the class name.
• getSuperclass() returns the parent class (null for Object).
• getInterfaces() lists implemented interfaces.
• Modifier decodes modifier flags.
• isAssignableFrom() checks type compatibility.
• getClassLoader() identifies which class loader loaded the class.