I run a file within a project but it keeps giving me the output of the previous file. How do I fix this?

Question

I'm super new to programming btw!

So I'm taking a Java Course and it tells me to make project and then I create 4 files and each is different. One prints hello world, the other shows me what errors look like, finding the max of some words, and then the grade of a class. but when I try to find the max of some words or grade of a class it keeps outputting the previous codes answers. Am I doing something wrong?

Answer 1

In your project, you can have as many classes or functions as you want.

However, when you run your project, there can be only one entry point, that is where the execution of the code begins.

Usually, it is called a main function (in languages like C/C++, Java, C#, etc). If all of your classes have a main function, then you need to tell the compiler, which class's main function to use.

Probably an easier option is to have your 4 classes, exercises if you want to call them that and have a 5th class, that is to store the program class with the main function. This main function will then instantiate (call the constructor of) one of your classes (you can edit which one before you run the project).

Below I demonstrate this with one Main and 2 other classes.

Main.java

public class Main
{
    public static void main(String[] args)
    {
        Hello hello = new Hello();
        //Error error = new Error();
        // Other classes commented out here
    }
}

Hello.java

public class Hello
{
    public Hello()
    {
        System.out.println("Hello");
    }
}

Error.java

public class Error
{
    public Error()
    {
        System.out.println("Integral division by zero is not allowed!");
        int error = 2 / 0;
    }
}

Comments

How do you navigate between files to edit code or run only one java file in a project?