This is a repost from our previous blog platform.

Jun 2, 2021 • Aleks Rutins

Java Reflection Tutorial, Part 1

First of all, this is the first time I’ve ever written a blog post. My apologies if the organization makes no sense.

Reflection in programming is the act of using code to write or read itself. For example, in Java, getting the name of a class or method at runtime is reflection.

In Java, most of the reflection classes and methods are in the java.lang.reflect package, and tools for dealing with annotations are in java.lang.annotation.

To get started, create a directory to hold files for this tutorial. Create a new basic Java file, and call it Basic.java:

import java.lang.reflect.*;
import java.lang.annotation.*;

public class Basic {
    public static void main(String[] args) {
        System.out.println("Hello Java Reflection");
    }
}

If you don’t know what that does, a comprehensive tutorial is here.

Run it with java Basic.java. You should see the output:

$ java Basic.java
Hello Java Reflection

Great! Your Java installation works. If it doesn’t, go ask the Oracle. Or ask OpenJDK, if you’re on Linux.

Getting the Name of a Class

Now, let’s do some reflection! Add some lines to main:

import java.lang.reflect.*;
import java.lang.annotation.*;

public class Basic {
    public static void main(String[] args) {
        System.out.println("Hello Java Reflection");
        Class<?> cls = Basic.class;
        System.out.println("The class's name is " + cls.getName());
    }
}

Run it again, and you should see this:

$ java Basic.java
Hello Java Reflection
The class's name is Basic

Whoa! How’d it know that? Let’s see.

First, take a look at this line:

Class<?> cls = Basic.class;

First of all, the ? in Class<?> tells the Java compiler to auto-detect the type argument. In this case, it could also have been written as Class<Basic>, but that would be more typing. I could also use var, which tells it to completely auto-detect the type:

var cls = Basic.class;

However, var is not supported in JDK 7, which is what FTC uses. If you have JDK 9 or higher, you can use var, though.

Next, Basic.class tells it to get the Class<T> (T is a type argument) instance for Basic. Class<T> is core for reflection. So essential, in fact, that it is included in java.lang.

On to the next line:

System.out.println("The class's name is " + cls.getName());

Everybody should be familiar with System.out.println. If not, I urge you once again to check out this tutorial. However, cls.getName() should be new. If not, I urge you to skip to the next heading, wait for part 2 (annotations), or seek out a more advanced tutorial.

getName is a method on Class<T> that gets the qualified name. For example, if Basic were in the com.not.a.domain package, cls.getName() would return "com.not.a.domain.Basic". Since Basic is not in a package, it just returns "Basic". If you need an unqualified name, without the package, use Class.getSimpleName.

Getting the Name of a Method

To access methods, we use the Method type, which is in java.lang.reflect. Methods can be accessed using Class.getMethod or Class.getMethods, which return a specific method or a list of all methods in the class, respectively.

Add a method to Basic:

public void doSomething(String whatever) {
    System.out.println("Doing something very interesting...");
    System.out.println("Whatever is " + whatever);
}

Now, just to make sure it exists, create an instance in main and try it:

Basic basic = new Basic();
basic.doSomething("whatever");

Run it, and the whole output should be:

$ java Basic.java
Hello Java Reflection
The class's name is Basic
Doing something very interesting...
Whatever is whatever

It works! Now, let’s get a Method instance for that method. Remember, cls is Basic.class. The try block is necessary because getMethod can throw a NoSuchMethodException if the method was not found.

try {
    Method doSomething = cls.getMethod("doSomething", String.class);
} catch(NoSuchMethodException e) {
    System.out.println("A whatsit happened: " + e.toString());
}

Now, let’s look at the interesting line:

Method doSomething = cls.getMethod("doSomething", String.class);

If you look at the description for Class.getMethod, you can see that the first argument is the name of the method, and any remaining arguments are parameter types, as Class<T> instances. In this case, since the method is called doSomething and it takes one String argument, the first argument is "doSomething" and the second argument is String.class.

Now that we have the Method instance, it’s easy enough to get the name (inside the try block):

System.out.println("The method's name is " + doSomething.getName());

Now, the output should be:

$ java Basic.java
Hello Java Reflection
The class's name is Basic
Doing something very interesting...
Whatever is whatever
The method's name is doSomething

It worked!

If something went wrong, here’s the full code of Basic.java (organized into sections):

import java.lang.reflect.*;
import java.lang.annotation.*;

public class Basic {
    public static void main(String[] args) {
        // SETUP //
        System.out.println("Hello Java Reflection");

        // GETTING THE NAME OF A CLASS //
        Class<?> cls = Basic.class;
        System.out.println("The class's name is " + cls.getName());
        
        // GETTING THE NAME OF A METHOD //
        Basic basic = new Basic();
        basic.doSomething("whatever");
        try {
            Method doSomething = cls.getMethod("doSomething", String.class);
            System.out.println("The method's name is " + doSomething.getName());
        } catch(NoSuchMethodException e) {
            System.out.println("A whatsit happened: " + e.toString());
        }
    }
    public void doSomething(String whatever) {
        System.out.println("Doing something very interesting...");
        System.out.println("Whatever is " + whatever);
    }
}

See you in Part 2, which will go over annotations!

Other Notes & Exercises

1. Try putting Basic in a package, and see what getName returns. Then, try to get the name without the package name.
2. If you aren’t sure how to set up your IDE, I use Visual Studio Code with the Java Extension Pack.
3. The java command-line tool, as used above, can also compile files on-the-fly. If you experience problems, you can use the longer version:
   $ javac Basic.java
$ java Basic ...
4. Use the documentation for Method to figure out how to get the name of the class that declared the method.