What is mean by precasting in java can any one give me simple example please...

Not sure exactly what you are referring to with "precasting". There is casting

List pointList = new ArrayList();
pointList.add( new Point(0,0) );
Point p = (Point)pointList.get(0); // you must cast Object to Point here

or you can use generics to "pre-cast" and declare the type of collection

List<Point> pointList = new ArrayList<Point>();
pointList.add( new Point(0,0) );
Point p0 = pointList.get(0);  // no need for the cast here

Is that what you are wondering about?