public void push(){
  top++;
  if (top >= stack.length) {
    Mapping newstack[]=new Mapping[stack.length * 2];
    System.arraycopy(stack,0,newstack,0,stack.length);
    stack=newstack;
  }
  stack[top]=null;
}

public void push ( ) { top ++ ; if ( top >= stack . length ) { Mapping newstack [ ] = new Mapping [ stack . length * 2 ] ; System . arraycopy ( stack , 0 , newstack , 0 , stack . length ) ; stack = newstack ; } stack [ top ] = null ; }
create a new frame at the top of the stack .
