How to call inner class from main method

[John Sanderval]

https://www.reddit.com/r/learnprogramming/comments/8foof1/javahow_to_call_innerclass_from_main_method/

Please help me, regardless of whether or not this is bad practice...i need help trying to figure this issue out

[Carey Brown]

Not quite sure what your question is.

[John Sanderval]

Its....how do you call an inner class method from a main method that is outside the scope of the outer/inner class.

Like how would you call it from the main method

[Carey Brown]

public class Main{
   public static void main(String [] args){
      Top.Util.readFile("studentData.txt");
   }

Top is the outer class
Util is the inner class
readFile() is a static method of the Util inner class.

[John Sanderval]

Is the syntax for calling readFile() correct in this case?

Its giving me an error

[Carey Brown]

Hmmmm. I suggest you find some other tutorial on the subject, this example is riddled with syntax errors and won't even compile. It has so many errors I don't see any way to resolve them. Who the heck writes this stuff?

[John Sanderval]

Can you maybe tell me how ican fix these errors?

[Carey Brown]

John Sanderval wrote:Can you maybe tell me how ican fix these errors?

Here's a "fix". It violates general principles of object oriented design, but it will compile. I stripped out unused methods and added a bunch of 'static' modifiers. Yuck.

import java.io.File;
import java.io.IOException;
import java.util.Scanner;

public class Top
{
	static int numLabs = 6;
	static int numStuds = 17;
	static int num[][] = new int[numStuds][numLabs];

	static class Student
	{
		static void add( Scanner in )
		{
			for( int row = 0 ; row <= numStuds - 1 ; row++ )
			{
				for( int col = 0 ; col <= numLabs - 1 ; row++ )
				{
					num[row][col] = in.nextInt();
					if( !in.hasNextInt() )
					{
						return;
					}
				}
			}
		}
	}

	static class Util
	{
		static void readFile( String fileName ) throws IOException
		{
			File file = new File( "fileName.txt" );
			Scanner in = new Scanner( file );

			Student.add( in );
		}
	}
}

class Main
{
	public static void main( String[] args ) throws IOException
	{
		Top.Util.readFile( "studentData.txt" );
	}
}

[John Sanderval]

Is there any way this would work without static methods/variables?

Why are static keywords needed so badly?

[John Sanderval]

Is there any way this would work without static methods/variables?

Why are static keywords needed so badly?

Is it possible to create an instance somehow in the main method such that static wouldn't need to be used?

[Carey Brown]

Better, but it's starting to smell like Student and Util should not be inner classes.

import java.io.File;
import java.util.Scanner;

public class Top
{
	static int numLabs = 6;
	static int numStuds = 17;
	
	Student[] students = new Student[numStuds];
	
	void add( Scanner in ) throws Exception
	{
		for( int i=0 ; i < numStuds ; i++ )
		{
			students[i] = new Student( in );
		}
	}
	
	class Student
	{
		int[] labs = new int[numLabs];
		
		public Student( Scanner in ) throws Exception
		{
			for( int i = 0 ; i < numLabs ; i++ )
			{
				labs[i] = in.nextInt();
				if( !in.hasNextInt() )
				{
					throw new Exception( "unexpected end of file" );
				}
			}
		}
	}

	static class Util
	{
		static Top readFile( String fileName ) throws Exception
		{
			File file = new File( "fileName.txt" );
			Scanner in = new Scanner( file );

			Top top = new Top();
			top.add( in );
			return top;
		}
	}
}

class Main
{
	public static void main( String[] args ) throws Exception
	{
		Top top = Top.Util.readFile( "studentData.txt" );
	}
}

[Carey Brown]

What do you think?

[Carey Brown]

From your link..........

BUT, the real solution here is not to use inner classes. They add nothing exception complication and you don't need them and don't want them. They aren't all that common in the first place and almost without exception if you want to access them outside the parent class you are Doing It Wrong. So there.

[Campbell Ritchie]

John Sanderval wrote:. . .  a main method . . . outside the scope of the outer/inner class. . . .

You cannot write main methods in an inner class because inner classes are restricted as to what sort of static members they can have.