Program execution is non sequential. Why?

Question

I was fooling around with how I could set up my encapsulation.

But my program is executing in an unexpected order. Here is my rather simple code:

The "Main":

package research.debug;

public class Main {

public static void main(String[] args) {

    Boolean b = Boolean.TRUE ;      

    Debug.black.printVariable( b, "b" ) ;
    Debug.red.printVariable( b, "b" ) ;

    System.out.println( "SUPPOSED to be inbetween..." ) ;

    Debug.black.println( "Hello" ) ;
    Debug.red.println( "Howdie" ) ;

}

}

"Debug":

package research.debug;

public class Debug {

public static final Output black = new Output( Output.BLACK ) ;
public static final Output red = new Output( Output.RED ) ;

}

And lastly, "Output":

package research.debug;

public class Output {

public static final int BLACK = 0 ;
public static final int RED = 1 ; 

private int outputMode = 0 ;

public Output( int outputMode ) {

    this.outputMode = outputMode ; 

}

public void println( String msg ) {

    if( outputMode == Output.BLACK ) {
        System.out.println( "Printed with black font: " + msg ) ;
    } else {
        System.err.println( "Printed with red font: " + msg ) ;
    }

}

public void printVariable( Object variable, String variableName ) {

    println( variableName + " = \"" + variable + "\"" ) ;

}

}

And the expected output would be:

Printed with black font: b = "true"

Printed with red font: b = "true"

SUPPOSED to be inbetween...

Printed with black font: Hello

Printed with red font: Howdie

But is instead out of the expected order, like this:

Printed with black font: b = "true"

SUPPOSED to be inbetween...

Printed with black font: Hello

Printed with red font: b = "true"

Printed with red font: Howdie

What's happening?

EDIT: Sometimes the "Supposed to be in between" message moves around. Without me changing the code.

Answer 1

System.out is buffered and System.err is not, they are two different streams, and some of your messages go to one, some to the other.

Hence, these mixed messages may not appear in the expected order as the prints to System.out are delayed until the buffer is flushed (manually or automatically), whereas those to System.err should be written right away.

You can flush a stream manually by calling the flush() method.

Answer 2

You are printing to System.err and System.out. Try to print only to System.out or use System.out.flush() to flush the buffers.