预定义接口的抽象方法如Connection，Statement等如何在没有主体的情况下执行某些任务？ [英] How does abstract method of predefined interface like Connection, Statement etc. perform some task without having body?

问题描述

java中有许多预定义的接口，如 ResultSet ，连接，声明等。接口只能有抽象方法（未实现的方法）。所以我们为什么要使用那些方法而不先定义它们。

There are many predefined interfaces in java like ResultSet, Connection, Statement etc.An Interface can have only abstract methods (unimplemented methods).So why do we use there methods without defining them first.

例如在下面代码jdbc

for example in following code of jdbc

public class JDBCSample {

public static void main( String args[]) {

String connectionURL = "jdbc:postgresql://localhost:5432/movies;
user=java;password=samples";`

try {

    Class.forName("org.postgresql.Driver");  


    Connection con = DriverManager.getConnection (connectionURL);


    Statement stmt = con.createStatement();


    ResultSet rs = stmd.executeQuery("select moviename, releasedate from movies");

    while (rs.next())
        {....do something.....}
}catch (SQLException e)
 {e.printStackTrace();}
catch (Exception e) 
{ e.printStackTrace();}}

这里我们调用abstract createStatement（）和 executeQuery（） <$ c $的方法c>连接和声明界面？如果是，则抽象方法（没有身体的方法）如何执行某项任务？

here are we calling abstract createStatement() and executeQuery() method of Connection and Statement interface? If yes then how an abstract method(method without body) can perform some task?

推荐答案

当您致电 DriverManager.getConnection ，它返回 org.postgresql.jdbc4.Jdbc4Connection 的实例，这是一个实现 java.sql.Connection interface。

When you call DriverManager.getConnection, it returns an instance of org.postgresql.jdbc4.Jdbc4Connection, which is a class that implements the java.sql.Connection interface.

Jdbc4Connection 有一个 createStatement 的方法体，它返回 org.postgresql.jdbc4.Jdbc4Statement 的一个实例，它是一个实现 java.sql.Statement 接口的类。

Jdbc4Connection has a method body for createStatement that returns an instance of org.postgresql.jdbc4.Jdbc4Statement, which is a class that implements the java.sql.Statement interface.

Jdbc4Statement 有一个 executeQuery 的方法体返回 org.postgresql.jdbc4.Jdbc4ResultSet 的实例，这是一个实现 java.sql.ResultSet 的类界面。

Jdbc4Statement has a method body for executeQuery which returns an instance of org.postgresql.jdbc4.Jdbc4ResultSet, which is a class that implements the java.sql.ResultSet interface.

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