必须添加 rs.next(); 否则报错

org.postgresql.util.PSQLException: ResultSet not positioned properly, perhaps you need to call next.

代码实例

import java.sql.DriverManager;import java.sql.Connection;import java.sql.SQLException;import java.sql.ResultSet;import java.sql.Statement;public class PG{	public static void main(String []args){		System.out.println("PostgreSQL JDBC...");		try {			Class.forName("org.postgresql.Driver");		}catch (ClassNotFoundException e){			System.out.println("could not find postgreSQL JDBC Driver");			e.printStackTrace();			return;		}		Connection connection = null;		try {			connection = DriverManager.getConnection(					"jdbc:postgresql://localhost:5432/postgres", 					"postgres",					"");		}catch (SQLException e){			System.out.println("Connection Failed");			e.printStackTrace();			return;		}		if (connection != null){			System.out.println("connect db successful!");		}else {			System.out.println("Failed to make conn!");		}		try {			Statement  stmt = connection.createStatement();			ResultSet rs = stmt.executeQuery("SELECT 1 AS A");			if(rs!=null){				rs.next();  // 必须添加				System.out.println(rs.getInt("a"));			}else {				System.out.println("null");			}			rs.close();			stmt.close();			connection.close();		}catch (Exception exp){			exp.printStackTrace();		}	}}