public BlockId(String fileName,long blkNum){
  this.fileName=fileName;
  this.blkNum=blkNum;
  myString="[file " + fileName + ", block "+ blkNum+ "]";
  myHashCode=myString.hashCode();
}

public BlockId ( String fileName , long blkNum ) { this . fileName = fileName ; this . blkNum = blkNum ; myString = "[file " + fileName + ", block " + blkNum + "]" ; myHashCode = myString . hashCode ( ) ; }
constructs a block id for the specified filename and block number .
