“你好，阿米戈！今天，你将学习如何执行一项新任务：替换 System.out 对象。”
System.out 是 System 类中名为 out 的 static PrintStream 变量。此变量包含 final 修饰符，因此你可以直接为其赋予新值。但是，System 类包含一个可以执行此操作的特殊方法：setOut(PrintStream stream)。这就是我们将要使用的方法。

“真有趣。我们用什么替换它？”

“我们需要一些可以收集输出数据的对象。ByteArrayOutputStream 最适合此项工作。这是一个特殊的类，它是一个动态（可调整大小的）数组，并实现 OutputStream 接口。”

“数组和 OutputStream 之间的适配器？”

“差不多。代码看起来如下所示。”

代码
public static void main(String[] args) throws Exception
{
 //Save the current PrintStream in a special variable
 PrintStream consoleStream = System.out;

 //Create a dynamic array
 ByteArrayOutputStream outputStream = new ByteArrayOutputStream();
 //Create an adapter for the PrintStream class
 PrintStream stream = new PrintStream(outputStream);
 //Set it as the current System.out
 System.setOut(stream);

 //Call a function that knows nothing about our changes
 printSomething();

 //Convert the data written to our ByteArray into a string
 String result = outputStream.toString();

 //Put everything back to the way it was
 System.setOut(consoleStream);
}

public static void printSomething()
{
 System.out.println("Hi");
 System.out.println("My name is Amigo");
 System.out.println("Bye-bye!");
}

“我们将如何处理此行？”

“嗯，怎么处理都行。例如，可以将其反转。则代码如下所示：”

代码
public static void main(String[] args) throws Exception
{
 //Save the current PrintStream in a special variable
 PrintStream consoleStream = System.out;

 //Create a dynamic array
 ByteArrayOutputStream outputStream = new ByteArrayOutputStream();
 //Create an adapter for the PrintStream class
 PrintStream stream = new PrintStream(outputStream);
 //Set it as the current System.out
 System.setOut(stream);

 //Call a function that knows nothing about our changes
 printSomething();

 //Convert the data written to our ByteArray into a string
 String result = outputStream.toString();

 //Put everything back to the way it was
 System.setOut(consoleStream);

 //Reverse the string
 StringBuilder stringBuilder = new StringBuilder(result);
 stringBuilder.reverse();
 String reverseString = stringBuilder.toString();

 //Output it to the console
 System.out.println(reverseString);
}

public static void printSomething()
{
 System.out.println("Hi");
 System.out.println("My name is Amigo");
 System.out.println("Bye-bye!");
}

“太有趣了！现在，我开始对这些小型类提供的强大功能有所了解了。”
“比拉博，谢谢你给我上了有趣的一课。”

Java webinar