<?php$servername = "localhost";$username = "username";$password = "password";$dbname = "database_name";$conn = new mysqli($servername, $username, $password, $dbname);if ($conn->connect_error) { die("Connection failed: " . $conn->connect_error);}$sql = "SELECT * FROM table_name";$result = $conn->query($sql);if ($result->num_rows > 0) { while($row = $result->fetch_assoc()) { echo $row["column_name"]. "<br>"; }} else { echo "0 results";}$conn->close();?> 将这段代码更换为java

这段代码是使用PHP语言编写的，如果想要将其转换为Java版本，需要进行如下步骤：

1. 连接数据库

PHP使用mysqli扩展提供MySQL数据库连接功能，而Java可以使用JDBC（Java Database Connectivity）来连接MySQL数据库。

在Java中，需要导入MySQL的JDBC驱动包，然后使用以下代码连接数据库：

import java.sql.*;

public class Main {
  public static void main(String[] args) {
    String url = "jdbc:mysql://localhost:3306/database_name";
    String username = "username";
    String password = "password";

    try {
      Connection conn = DriverManager.getConnection(url, username, password);
      System.out.println("Connected to the database");
      // 其他操作
      conn.close();
    } catch (SQLException e) {
      System.out.println(e.getMessage());
    }
  }
}

2. 执行SQL查询

在Java中，可以使用Statement或PreparedStatement对象执行SQL查询语句，与PHP中的mysqli_query()函数类似。

例如，将原来的SQL查询代码转换为Java代码如下：

Statement stmt = conn.createStatement();
ResultSet rs = stmt.executeQuery("SELECT * FROM table_name");
if (rs.next()) {
  do {
    System.out.println(rs.getString("column_name"));
  } while (rs.next());
} else {
  System.out.println("0 results");
}

完整的Java代码如下：

import java.sql.*;

public class Main {
  public static void main(String[] args) {
    String url = "jdbc:mysql://localhost:3306/database_name";
    String username = "username";
    String password = "password";

    try {
      Connection conn = DriverManager.getConnection(url, username, password);
      System.out.println("Connected to the database");

      Statement stmt = conn.createStatement();
      ResultSet rs = stmt.executeQuery("SELECT * FROM table_name");
      if (rs.next()) {
        do {
          System.out.println(rs.getString("column_name"));
        } while (rs.next());
      } else {
        System.out.println("0 results");
      }

      conn.close();
    } catch (SQLException e) {
      System.out.println(e.getMessage());
    }
  }
}