Query Builder Java

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package main;

import com.mysql.jdbc.Connection;
import com.mysql.jdbc.Statement;
import java.awt.List;
import java.sql.DriverManager;
import java.sql.PreparedStatement;
import java.sql.ResultSet;
import java.sql.SQLException;
import java.util.ArrayList;
import java.util.Map;
import java.util.logging.Level;
import java.util.logging.Logger;

/**
 *
 * @author Sianipar
 */
public class QueryBuilder {

    private Connection koneksi = null;
    private ResultSet rs = null;
    private Statement stmt = null;
    private PreparedStatement pst = null;

    private String _field = "*";
    private String _table = "";
    private String __join = "";
    private String _where = "";
    private String _group = "";
    private String _order = "";
    private String _limit = "";

    public QueryBuilder() {
        try {
            String url = "jdbc:mysql://192.168.43.201:3306/absensi";
            DriverManager.registerDriver(new com.mysql.jdbc.Driver());
            koneksi = (Connection) DriverManager.getConnection(url, "absen", "absen123");
        } catch (SQLException ex) {
            Logger.getLogger(QueryBuilder.class.getName()).log(Level.SEVERE, null, ex);
        }
    }

    public QueryBuilder select(String field) {
        this._field = field;
        return this;
    }

    public QueryBuilder table(String table) {
        this._table = table;
        return this;
    }

    public QueryBuilder join(String type, String table, String on) {
        this.__join += " " + type + " " + table + " ON (" + on + ")";
        return this;
    }

    public QueryBuilder where(String field, String condition, String value) {
        this._where += (this._where.contains("WHERE")) ? " AND " : " ";
        this._where += (this._where.contains("WHERE")) ? " " : " WHERE ";
        this._where += field + " " + condition + " '" + value + "'";
        return this;
    }

    public QueryBuilder where(String field, String value) {
        this._where += (this._where.contains("WHERE")) ? " AND " : " ";
        this._where += (this._where.contains("WHERE")) ? " " : " WHERE ";
        this._where += field + " = '" + value + "'";
        return this;
    }

    public QueryBuilder group(String group) {
        this._group = " GROUP BY " + group;
        return this;
    }

    public QueryBuilder order(String order) {
        this._order = " ORDER BY " + order;
        return this;
    }

    public QueryBuilder limit(String limit) {
        this._limit = " LIMIT " + limit;
        return this;
    }

    public ResultSet execute() {
        try {
            String query = "SELECT " + this._field + " FROM " + this._table + this.__join + this._where + this._group + this._order + this._limit;
            System.out.println(query);
            stmt = (Statement) koneksi.createStatement();
            rs = stmt.executeQuery(query);

            this._field = "*";
            this._table = "";
            this.__join = "";
            this._where = "";
            this._group = "";
            this._order = "";
            this._limit = "";

            return rs;
        } catch (SQLException ex) {
            Logger.getLogger(QueryBuilder.class.getName()).log(Level.SEVERE, null, ex);
        }
        return null;
    }

    public void insert(Map<String, String> param) {
        try {
            String args = "";
            String field = "";
            int i = 0;

            for (Map.Entry<String, String> entry : param.entrySet()) {
                field += (i < param.size() - 1) ? entry.getKey() + "," : entry.getKey();
                args += (i < param.size() - 1) ? "?," : "?";
                i++;
            }

            String query = "INSERT INTO " + this._table + " (" + field + ") VALUES (" + args + ")";
            System.out.println(query);
            pst = koneksi.prepareStatement(query);

            i = 1;
            for (Map.Entry<String, String> entry : param.entrySet()) {
                pst.setString(i, entry.getValue());
                i++;
            }

            pst.execute();
            this._table = "";
        } catch (SQLException ex) {
            Logger.getLogger(QueryBuilder.class.getName()).log(Level.SEVERE, null, ex);
        }
    }

    public void update(Map<String, String> param) {
        try {
            String set = "";
            int i = 0;

            for (Map.Entry<String, String> entry : param.entrySet()) {
                set += entry.getKey() + "=?";
                set += (i < param.size() - 1) ? "," : "";
                i++;
            }

            String query = "UPDATE " + this._table + " SET " + set + this._where;
            System.out.println(query);
            pst = koneksi.prepareStatement(query);

            i = 1;
            for (Map.Entry<String, String> entry : param.entrySet()) {
                pst.setString(i, entry.getValue());
                i++;
            }

            pst.execute();
            this._table = "";
            this._where = "";
        } catch (SQLException ex) {
            Logger.getLogger(QueryBuilder.class.getName()).log(Level.SEVERE, null, ex);
        }
    }

    public void delete() {
        try {
            String query = "DELETE FROM " + this._table + this._where;
            System.out.println(query);
            pst = koneksi.prepareStatement(query);
            pst.execute();

            this._table = "";
            this._where = "";
        } catch (SQLException ex) {
            Logger.getLogger(QueryBuilder.class.getName()).log(Level.SEVERE, null, ex);
        }
    }

}