java-向mysql数据库中插入数据时报错

<code>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);        }    }</code>

<code>com.mysql.jdbc.exceptions.jdbc4.MySQLIntegrityConstraintViolationException: Duplicate entry '1' for key 'PRIMARY'</code>