Reflection API in Java

Question 1

What is meant by reflection?

Answer 1

Reflection is the ability for a class or object to examine itself.

Question 2

What is the reflection API?

Answer 2

The reflection API is an interface used to write Java code to look at the class of an object and determine its structure.

Question 3

The ability to break open objects is typically guarded by the…

Answer 3

Security manager

Question 4

What is the standard rule for reflection?

Answer 4

Another object shouldn’t be able to use the reflection API to do something it couldn’t do without ordinary compiled Java code.

Question 5

What is the entry point for the reflection API?

Answer 5

The immutable java.lang.Class object generated for each class by the JVM at runtime.

Question 6

If you don’t know the type name, how would you get the Class object of a class?

Answer 6

You can use object.getClass(), which is inherited by all classes and returns this instance’s class’s class object.

Question 7

If you do know the type name, how would you get the Class object of a class?

Answer 7

You can simply use the .class keyword.
e.g. MainClass.class

Question 8

If, for some reason, you have a string with a reference to the class namespace, how could you get the Class object of a class?

Answer 8

You can use Class.forName(String str), where str is your string class reference.

Question 9

What is the difference between getFields and getField?

Answer 9

getFields eturns an array of all Field instances, detailing all public attributes associated with the class, including inherited ones.

getField returns only one Field, which is the one specified by the String argument.

Question 10

What is the difference between getFields and getDeclaredFields?

HINT: What does Declared mean in this case?

Answer 10

getFields returns Field object(s) for attributes associated with the class, even inherited ones.

getDeclaredFields does the same, but not including inherited ones.

Declared, in this case, means Fields declared within the class.

Question 11

What is the difference between getConstructors and getDeclaredConstructors?

HINT: Different from getFields

Answer 11

getConstructors returns public constructors associated with that class, while getDeclaredConstructors returns all constructors associated with that class.

Question 12

What does getMethods do?

Answer 12

getMethods returns an array of Method instances, containing details of all public methods associated with this class, including inherited ones.

Question 13

What does the parameterised definition of getMethods do?

Answer 13

When parameters are given, getMethods returns only one method, relating to the specified public method, which may be inherited, whose name matches the String name and whose arguments match the types in …argumentTypes.

Question 14

Will getDeclaredFields return inherited Fields?

Answer 14

No.

Question 15

Will getDeclaredConstructors return inherited Constructors?

Answer 15

No. You cannot inherit constructors, so this mode instead returns all constructors regardless of access level.

Question 16

Will getMethods, given parameters, return inherited Fields?

Answer 16

Yes, as long as the given argument matches a method given.

Question 17

Will getMethods be able to return a protected method?

Answer 17

No. getMethods, regardless of how it is parameterised, will always return only methods with public access modifiers.

Question 18

What is the difference between Class.newInstance() and Constructor.newInstance()?

Answer 18

Class.newInstance can only invoke a constructor with no arguments, and requires that the constructor be visible.

Constructor.newInstance, however, may invoke any constructor, irrespective of the number of parameters and/or access modifiers (granted you have the Constructor object instance).

Question 19

How may you access a private variable from outside of that class using the reflection API?

Answer 19

Field, Method and Constructor all extend AccessibleObject, which contains a method called setAccessible(), which allows you to deactivate normal security when accessing that member.

So, you can find the variable using getDeclaredField or getFields, then use setAccessible to turn off security, then you may access it.

Question 20

Will the security manager always allow you to change member accessibility?

Answer 20

No. If you are running a security policy that denies this method from being invoked, you may not use this functionality. This can only be done when running no security policy, or running one that leaves this option open, for some reason.