Why can't I keep the prototype class at the top of my java file?-CodePudding

For example, this works just fine.

package OOPS;

public class Pen {
    public static void main(String[] args) {
        PenMaker linc = new PenMaker();
        linc.color = "blue";
        linc.type = "ballpoint";

        linc.write("Hi mom!");
        linc.printTypeAndColor();
    }
}

class PenMaker {
    String color;
    String type;

    public void write(String str) {
        System.out.println(str);
    }

    public void printTypeAndColor() {
        System.out.println("I'm a "   this.type   " pen of "   this.color   " color.");
    }
}

But I want to keep my prototype class at the top of the file. Something like this.

package OOPS;

class PenMaker {
    String color;
    String type;

    public void write(String str) {
        System.out.println(str);
    }

    public void printTypeAndColor() {
        System.out.println("I'm a "   this.type   " pen of "   this.color   " color.");
    }
}

public class Pen {
    public static void main(String[] args) {
        PenMaker linc = new PenMaker();
        linc.color = "blue";
        linc.type = "ballpoint";

        linc.write("Hi mom!");
        linc.printTypeAndColor();
    }
}

But this doesn't compile and says "error: can't find main(String[]) method in class: OOPS.PenMaker".
In both cases the file name is Pen.java .

CodePudding user response：

both codes are correct and compile and run successfully

as long as your file has one public class (in this case Pen) it should compile it doesn't matter the order.

I would suggest to resave your second code and close your IDE or console and reopen again

CodePudding user response：

The argument you pass to java (the executable) is a fully qualified classname. In your second snippet, the main method is in the class named Pen which is in the package named OOPS, which therefore makes that class's fully qualified name OOPS.Pen. There's also the OOPS.PenMaker class, but it has no main method, which is exactly what your error says.

To run that main method, you'd type java OOPS.Pen. Because that class has a main method. The error message you are getting when you run java OOPS.PenMaker kinda spells this out. In general, assume error messages are telling you what's wrong.