向mysql数据库中插入数据时报错的问题

下面是我的代码

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:
com.mysql.jdbc.exceptions.jdbc4.MySQLIntegrityConstraintViolationException: Duplicate entry '1' for key 'PRIMARY'
请大神帮我看看，哪里有问题