help with "Exception in thread "main" java.lang.NullP ointerException "

"Exception in thread "main" java.lang.NullP ointerException
at sortmergejoin.M ergeJoin.Field( MergeJoin.java: 204)

Look at the line #204 and the object what you are passing is NULL. that's why this error. Just initialize the object with a proper, non-null value and run it again.

tanks for your reply.
the correct code part is:
public String Field(Frame frame,int lineNo){
String []recLine ;
String line;
line = Line(frame,line No);
recLine = line.split(" ");
return recLine[POSITION];
}

and i still get this error.

tanks for your reply.
the correct code part is:
public String Field(Frame frame,int lineNo){
String []recLine ;
String line;
line = Line(frame,line No);
recLine = line.split(" ");
return recLine[POSITION];
}

and i still get this error.

That's fine. Use the "Code Tags" for the neat look and feel of Java code.

What is the POSITION here? What exatly the Line(a,b) method returns here? I thought it was a Constructor but then I got to know you are NOT following the standards and you have the all the methods with the first letter capital (Line(), Field() etc.,).

Just check the return value of Line(frame,line No) -- which you assign to "line". It is NULL and thats why you get a NullPointerExce ption in the next line when you invoke split() on it.