//get Statement
Statement st = conn.createStatement();
//get all records in table a
ResultSet rs = st.executeQuery("select * from a"
//insert every record into table b
while(rs.next()){
column1_type col1 = rs.getcolumn1_type("column1_name"
......
columnX_type colX = rs.getcolumnX_type("columnX_name"
st.executeUpdate("insert into b (column1_name,....,columnX_name) values ("+col1+","+....+colX+""
}