How can I get as an output the name of a variable and it's type in Java?

Question

I have the following class:

public class Example {

    private String name;
    private int year;

    public Example() {

        name = "Clifford the big red dog";
        year = 2020;
    }

    public void showData() {
        // ?????
    }
}

Imagine in the main class I create an object of the previous class (Example) and execute it's constructor. Then I want to execute showData() and I would like to see the following output (given this example):

String name Clifford the big red dog
int year 2020

The main would look like this:

public static void main () {

    Example example = new Example();

    example.showData();
}

Is there any way I can do this?

Yes, with [reflection](https://docs.oracle.com/javase/tutorial/reflect/index.html). — Kayaman, Mar 25 '20 at 18:15
Reflection is a bit advanced, and it's not a general purpose tool. Is there a reason you want to print out the variable types, names and values? Is it for debugging purposes? — Kayaman, Mar 25 '20 at 18:20
Not for debugging, I'm connecting remotely to my SQL database manager and it would help on the creation of tables through the Java program — user157629, Mar 25 '20 at 18:30
Because I was wondering if I could create a function that could work on any class so I can create as many tables as I need on the SQL database without typing each variable name manually — user157629, Mar 25 '20 at 18:34
So you want to create a table programmatically based on a class? You can get the fields with `Class.getDeclaredFields();`, so in your case `Example.class.getDeclaredFields();`, it returns `Field[]` containing names, types and other info. — Kayaman, Mar 25 '20 at 18:38
I created this class as an example but yeah, `showData()` shouldn't be inside the class for what I know. Thanks for the comments, I'll start trying this with `getDeclaredFields();` as you guys say. — user157629, Mar 25 '20 at 18:44

Arvind Kumar Avinash · Accepted Answer · 2020-03-25T18:49:09.753

Do it as follows:

class Example {

    private String name;
    private int year;

    public Example() {

        name = "Clifford the big red dog";
        year = 2020;
    }

    public void showData() {
        System.out.println(name.getClass().getSimpleName() + " name " + name);
        System.out.println(((Object) year).getClass().getSimpleName() + " year " + year);
    }
}

public class Main {
    public static void main(String[] args) {
        Example example = new Example();
        example.showData();
    }
}

Output:

String name Clifford the big red dog
Integer year 2020

However, since showData is a method of class Example which already knows about its instance variables, you can simply do it as:

class Example {

    private String name;
    private int year;

    public Example() {

        name = "Clifford the big red dog";
        year = 2020;
    }

    public void showData() {
        System.out.println("String name " + name);
        System.out.println("int year " + year);
    }
}

public class Main {
    public static void main(String[] args) {
        Example example = new Example();
        example.showData();
    }
}

Output:

String name Clifford the big red dog
int year 2020

Thanks for the answer! But if you didn't know how many variables the class had, would still exist a way to do this? (using a loop or something)? — user157629, Mar 25 '20 at 18:36
Thanks, @user85421 for the valuable feedback. I've updated my answer to incorporate it. — Arvind Kumar Avinash, Mar 25 '20 at 18:44
@user157629 - You can get the list of instance variables using reflection. Check https://stackoverflow.com/questions/2989560/how-to-get-the-fields-in-an-object-via-reflection/ — Arvind Kumar Avinash, Mar 25 '20 at 18:53

How can I get as an output the name of a variable and it's type in Java?

1 Answers1