在 Java 上运行 Main 之外的程序

Question

I'm a new CS student just starting out with Java, and currently I'm using IntelliJ as my IDE. At the moment, we aren't going over any crazy complicated programs, just basic in class projects. My problem is that when I go to create my project, I have no choice but to run it through the Main class, even though the instructions want the class name to be something specific. Also, whenever I try making a new class or running a java file I've downloaded, it still continues to run the Main class, even if I have Main closed. I have files that I created inside of VS Code (I'm aware that's not the best option, and that's why I switched to IntelliJ), and I'm unable to run them unless I create a brand new project, copy and paste the code into the Main file, and change the class name to Main.

I'm not sure if this is something inside of Java, IntelliJ, or just me not understanding how everything works yet, but I was hoping someone can help me out with this.

Here's an example. In the src folder, I have the package folder and inside of this is the Main java file along with a separate file that I'm trying to run. Main is also empty.

package com.ethan;

import java.util.Scanner;


public class Exercise02_19 {

    public static void main(String[] args) {
        Scanner input = new Scanner(System.in);

        //Enter 3 points
        System.out.println("Enter the 3 points for a triangle: ");

        //Point A
        double x1 = input.nextDouble();
        double y1 = input.nextDouble();

        //Point B
        double x2 = input.nextDouble();
        double y2 = input.nextDouble();

        //Point C
        double x3 = input.nextDouble();
        double y3 = input.nextDouble();


        //Find the sides and area

        double side1 = Math.sqrt((x1 - x2) * (x1 - x2) + (y1 - y2) * (y1 - y2));
        double side2 = Math.sqrt((x1 - x3) * (x1 - x3) + (y1 - y3) * (y1 - y3));
        double side3 = Math.sqrt((x3 - x2) * (x3 - x2) + (y3 - y2) * (y3 - y2));

        double s = (side1 + side2 + side3)/2;
        double area = Math.sqrt(s * (s - side1) * (s - side2) * (s - side3));

        System.out.println("The area of the triangle is: " + area);

    }
}

Thank you.

Answer 1

Having your files open on an editor or on an IDEA does not mean that they are closed or open for running. They are just open to modification.

In order to run a Java program, you need a main method (unless you are using a framework which in that case it is hidden from you). Everything runs through main, so if you want to use a custom class you have to call it through main.

The main method looks like this

public static void main(String[] args){
     // your code
}

You can run the program either by using the IDEA, by setting the JDK or by compiling it with javac and then running the .class file with java command. There are plenty of articles on how to run your program through CLI. I would recommend using the CLI at first in order to understand what's going on JVM behind the scenes.

Answer 2

It does not depend on the classes name but on the main method which is the entry point for code execution.

This method must be defined inside a class and looks exactly like this:

public static void main(String[] args){
// code goes here
}

Make sure this method exists only once in your application.

To create this method fast in IntelliJ type psvm and then tab.

The name of the class you use to invoke the JVM determines which main method is called when you define more than one main method.

Answer 3

@efan - Alexis Pavlidis is correct.

To elaborate:

1. In Java, every "program" consists of one or more " classes ". In your example, the class happens to be called Exercise02_19 .

2. Every program must "start somewhere". This is called an entry point .

3. In Java, the entry point is a method with the signature public static void main(String[] args) .

4. In Java, unlike many other languages, each class may have its own main() . You must choose which entry point at runtime. With your IDE, or with command line arguments.

   In your case, you choose the main() method in class Exercise02_19 .

5. As Alexis Pavlidis said:

   There is no Main class, the only requirement for a java application is a main method. So your class can be named whatever you want.

Answer 4

A java program will always start in the main() method. You can imagine it as your entry point.

If you want to create separate classes, you must call them from your main method. Here is an example:

Main.java

public class Main {

    public static void main(String[] args) {
        MyTestClass myInstanceOfIt = new MyTestClass();
        myInstanceOfIt.doSomething();
    }
}

MyTestClass.java

public class MyTestClass {

    public void doSomething() {
        //do something here
    }
}