Gather

void Intracomm.Gather(Object sendbuf, int sendoffset,
                      int sendcount, Datatype sendtype,
                      Object recvbuf, int recvoffset,
                      int recvcount, Datatype recvtype, int root)

sendbuf	send buffer array
sendoffset	initial offset in send buffer
sendcount	number of items to send
sendtype	datatype of each item in send buffer
recvbuf	receive buffer array
recvoffset	initial offset in receive buffer
recvcount	number of items in receive buffer
recvtype	datatype of each item in receive buffer
root	rank of receiving process

Each process sends the contents of its send buffer to the root process. Java binding of the MPI operation MPI_GATHER.

void Intracomm.Gatherv(Object sendbuf, int sendoffset,
                       int sendcount, Datatype sendtype,
                       Object recvbuf, int recvoffset,
                       int [] recvcounts, int [] displs,
                       Datatype recvtype, int root)

sendbuf	send buffer array
sendoffset	initial offset in send buffer
sendcount	number of items to send
sendtype	datatype of each item in send buffer
recvbuf	receive buffer array
recvoffset	initial offset in receive buffer
recvcounts	number of elements received from each process
displs	displacements at which to place incoming data
recvtype	datatype of each item in receive buffer
root	rank of receiving process

Extends functionality of Gather by allowing varying counts of data from each process. Java binding of the MPI operation MPI_GATHERV. The sizes of arrays recvcounts and displs should be the size of the group. Entry $i$ of displs specifies the displacement relative to element recvoffset of recvbuf at which to place incoming data. Note that if recvtype is a derived data type, elements of displs are in units of the derived type extent, (unlike recvoffset, which is a direct index into the buffer array).