java-向mysql数据库中插入数据时报错

public class categorydao { public static void save(category c) throws SQLException{ Connection conn=(Connection) DB.getConnection(); String sql=null; if(c.getId()==-1){ sql="insert into category values(null,?,?,?,?,?)"; }else{ sql="insert into category values("+c.getId()+",?,?,?,?,?)"; } PreparedStatement ps=conn.prepareStatement(sql); try { ps.setString(1,c.getName() ); ps.setString(2,c.getDescr()); ps.setInt(3, c.getPid()); ps.setInt(4, c.isIsleaf()?0:1); ps.setInt(5, c.getGrade()); ps.executeUpdate(); } catch (Exception e) { // TODO Auto-generated catch block e.printStackTrace(); }finally{ DB.free(null, ps, conn); } }

错误如下：（反正插入不了）

com.mysql.jdbc.exceptions.jdbc4.MySQLIntegrityConstraintViolationException: Duplicate entry '1' for key 'PRIMARY'