# Db.set_h_hash

`

###

    import com.sleepycat.db.*;

    public interface DbHash
    {
    	public abstract int hash(Db db, byte[] data, int len);
    }
    public class Db
    {
    	public void set_h_hash(DbHash h_hash)
    		throws DbException;
    	...
    }

# Description

Set a user defined hash method; if no hash method is specified, a default hash method is used. Since no hash method performs equally well on all possible data, the user may find that the built-in hash method performs poorly with a particular data set. User specified hash functions must take a pointer to a byte string and a length as arguments and return a value of type int. The hash function must handle any key values used by the application (possibly including zero-length keys).

If a hash method is specified, Db.open will attempt to determine if the hash method specified is the same as the one with which the database was created, and will fail if it detects that it is not.

The Db.set_h_hash interface may only be used to configure Berkeley DB before the Db.open interface is called.

The Db.set_h_hash method throws an exception that encapsulates a non-zero error value on failure.

# Errors

EINVAL
  An invalid flag value or parameter was specified.

  Called after Db.open was called.

### Class

Db

# See Also

Db.close, Db.cursor, Db.del, Db.fd, Db.get, Db.get_byteswapped, Db.get_type, Db.join, Db.key_range, Db.open, Db.put, Db.remove, Db.set_bt_minkey, Db.set_cachesize, Db.set_errcall, Db.set_errpfx, Db.set_flags, Db.set_h_ffactor, Db.set_h_nelem, Db.set_lorder, Db.set_pagesize, Db.set_q_extentsize, Db.set_re_delim, Db.set_re_len, Db.set_re_pad, Db.set_re_source, Db.stat, Db.sync, Db.upgrade and Db.verify.`

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