JMenuItem mItem[] ={copy,paste}; 
  for(int b=0;b<mItem.length;b++)
  mItem[b].addActionListener(new ActionListener(){
      public void actionPerformed(ActionEvent e)
        String mItemCommand = e.getActionCommand();
        JTextArea copypaste = new JTextArea(scr.getText());



        String num =copypaste.getText();
        try{double n=Double.parseDouble(num);
           isDecimal = false;
                isFirstDigit = false;
                if( n-((long)n)>0.0)  // I just didn't understand codes starting from here
                if(n>=Long.MAX_VALUE || n<=Long.MIN_VALUE){

                }catch(NumberFormatException ex){}

As far as I can tell, the person who wrote this code was trying to copy a number that was represented as a String and get rid of any decimals that it had.

I don't know where you got that code, but it's awful.
n as name for a variable ... doesn't really tells what it is or supposed to be.

}catch(NumberFormatException ex){}

about the worst thing you can do.

indentation should be better, just to make it more readable.


(same for the "Copy" check) ... clearly written by someone who doesn't know the basics of how Strings (and other objects) should be compared in Java.

my advice: don't just copy some code you find somewhere in order to try and learn to program. chances are that it is crappy code, like the code you posted here.

Horrible way of checking...

if(n-((long)n)>0.0) // check if the number contains decimal places
  scr.setText(""+n); // it does, write it out as is
  scr.setText(""+((long)n)); // it doesn't, display w/o decimal
if(n>=Long.MAX_VALUE || n<=Long.MIN_VALUE) { // check if out of `long` range
  scr.setText("0");  // it is out of range, display 0

Seriously, the checking for decimal should be done when it is still in String form (before converting to a double). The code is bad both in styling (as stultuske stated) and algorithm... (i.e. The check for value of n to be within long range comes after the part that attempts to convert double to long.)