Java GUI method error

Question

I want to begin to learn about GUI in java. However, when i try to copy a simple JFrame code from a tutorial website to Textpad and when I try to compile it, there is an error:

"C:\Programming\Java\Practice GUI\GUIPractice.java:7: error: invalid method declaration; return type required public MyFrame() { ^ 1 error"

This also happens when I also copy simple GUI code from other websites, What seems to be the problem? I know that a method must either be void or a return type, but why does the method not specify if void or if return-type, a datatype? This seems to be the syntax of GUI code for other sites.

Here is the code:

// file: EmptyFrame.java
// Adapted from Core Java, vol.1, by Horstmann & Cornell

import javax.swing.*;

class MyFrame extends JFrame {
  public MyFrame() {
    setTitle("My Empty Frame");
    setSize(300,200); // default size is 0,0
    setLocation(10,200); // default is 0,0 (top left corner)
  }

  public static void main(String[] args) {
    JFrame f = new MyFrame();
    f.show();
  }
}

I tried this with the Netbeans IDE and the same error shows up. what seems to be the problem?

this "method" is actually a constructor and has therefore no return type. But: a constructor needs to be named exactly the same like your class. There seems to be some confusion about your naming: the error is reported in a file called "GUIPractice.java", your source code gives "file: EmptyFrame.java", the class is actually called "MyFrame" - which is now the correct file name and class name? — Thomas Kläger
– Thomas Kläger, Commented Feb 21, 2017 at 12:50
I'm sorry about the confusion but I copied the code directly from the website, I actually renamed the class name when I compiled to Textpad. — JPAsensi
– JPAsensi, Commented Feb 21, 2017 at 13:18
Thank you @ThomasKläger, Renaming the constructor actually solved the problem, But why should the constructor have the same name as the class file? — JPAsensi
– JPAsensi, Commented Feb 21, 2017 at 13:24

Steve Smith · Accepted Answer · 2017-02-21 12:44:08Z

1

Your class is called MyFrame, but it is in a file called EmptyFrame.java. They need to be the same name. Because they are different, it thinks MyFrame() is a method.

answered Feb 21, 2017 at 12:44

Steve Smith

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4 Comments

Thomas Kläger Over a year ago

They just need to be named the same if the class is public (which the class in the example is not)

Steve Smith Over a year ago

But then he'd get an error from having a static method (i.e. static void main() ) which is only allowed in top-level classes.

Thomas Kläger Over a year ago

Sure, a static method is only allowed in a top-level class. But top-level classes can be public, protected or only package visible. And the class name only needs to match the file name if the class is public.

JPAsensi Over a year ago

I'm sorry about the confusion but I copied the code directly from the website, I actually renamed the class name when I compiled to Textpad.

Lemonov · Accepted Answer · 2017-02-21 12:46:59Z

0

Change GUIPractice.java to MyFrame.java or change the class name and constructor to GUIPractice.

answered Feb 21, 2017 at 12:46

Lemonov

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1 Comment

JPAsensi Over a year ago

I'm sorry about the confusion but I copied the code directly from the website, I actually renamed the class name when I compiled to Textpad.

Sweeper · Accepted Answer · 2017-02-21 13:01:19Z

0

In every .java file, there must be a class with the same name as the file name, otherwise a ClassNotFoundException will be thrown at runtime. The error you saw is pretty weird. I don't know why it complains about the return type of a constructor.

Just rename your file so that it has the same name as the class i.e. MyFrame.java. You can also make your class public.

answered Feb 21, 2017 at 13:01

Sweeper

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4 Comments

Thomas Kläger Over a year ago

Could you provide a link where this requirement is stated? As far as I know only public classes do need to reside in files with the same name. (Though it is best practice to place classes in similarly named files it is only required for public classes)

Sweeper Over a year ago

I just tested it and a ClassNotFoundException is thrown. @ThomasKläger

JPAsensi Over a year ago

I'm sorry about the confusion but I copied the code directly from the website, I actually renamed the class name when I compiled to Textpad.

Thomas Kläger Over a year ago

Are you sure? I just tried javac GUIPractice.java, followed by java MyFrame and the frame showed up

JPAsensi · Accepted Answer · 2017-02-21 13:30:02Z

0

The problem actually arose since After I copied the code to Textpad, I renamed the class name to GUIPractice to match the file name. However, I did not know that the method which contained the GUI Code is actually not a method but a constructor. However, what baffles me is why should the constructor have the same name as the class name.

answered Feb 21, 2017 at 13:30

JPAsensi

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2 Comments

Thomas Kläger Over a year ago

Well, that is how constructors are named. The Java Language Specifiction states: The SimpleTypeName in the ConstructorDeclarator must be the simple name of the class that contains the constructor declaration, or a compile-time error occurs.

JPAsensi Over a year ago

Thank you, actually, I didn't know this since we are still in the topic of Methods in our class. Thanks

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