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Java/DAO.md

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+# Database Access Object
+
+## DB
+
+Connection to the DB.
+
+```java
+package dao;
+
+import java.sql.Connection;
+import java.sql.DriverManager;
+import java.sql.SQLException;
+
+public class DB {
+
+    private Connection conn;  // db connection obj
+
+    private final String URL = "jdbc:<dbms>:<db_url>/<database>";
+    private final String USER = "";
+    private final String PWD = "";
+
+    public DB() {
+        this.conn = null;
+    }
+
+    public void connect() {
+        try {
+            this.conn = DriverManager.getConnection(URL, USER, PWD);
+        } catch (SQLException e) {
+            e.printStackTrace();
+        }
+    }
+
+    public void disconnect() {
+        if(conn != null) {
+            try {
+                this.conn.close();
+            } catch (SQLException e) {
+                e.printStackTrace();
+            }
+        }
+    }
+
+    public Connection getConn() {
+        return conn;
+    }
+}
+```
+
+## `I<Type>DAO`
+
+Interface for CRUD methods on a database.
+
+```java
+package dao;
+
+import java.sql.SQLException;
+import java.util.List;
+
+public interface I<Type>DAO {
+
+    String RAW_QUERY = "SELECT * ..."
+    public Type Query() throws SQLException;
+}
+
+```
+
+## `<Type>DAO`
+
+Class implementing `I<Type>DAO` and handling the comunication with the db.
+
+```java
+package dao;
+
+import java.sql.ResultSet;
+import java.sql.SQLException;
+import java.sql.Statement;
+
+public class <Type>DAO implements I<Type>DAO {
+
+    private <Type> results;
+
+    private Statement statement;  // SQL instruction contenitor
+    private ResultSet rs;  // Query results contenitor
+    private DB db;
+
+    public <Type>DAO() {
+        db = new DB();
+    }
+
+    @Override
+    public Type Query() throws SQLException {
+        // 1. connection
+        db.connect();
+
+        // 2. instruction
+        statement = db.getConn().createStatement();  // statement creation
+
+        // 3. query
+        rs = statement.executeQuery(RAW_QUERY);  // set the query
+
+        // 4. results
+        while(rs.next()) {
+            // create and valorize Obj
+            // add to results
+        }
+
+        // 5. disconnection
+        db.disconnect()
+
+        // 6. return results
+        return results;
+    }
+}
+```