java 找不到自定义类的函数符号...谢谢

import java.io.*;

public class PipeStreamTest
{
public static void main(String[] agrs)
{

Thread t1 = new Sender();
Thread t2 = new Receiver();
PipedOutputStream out = t1.getOutputStream();
PipedInputStream in = t2.getInputStream();
out.connect(in);
t1.start();
t2.start();

}
}

class Sender extends Thread
{
private PipedOutputStream out = new PipedOutputStream();
public PipedOutputStream getOutputStream()
{
return out;
}
public void run()
{
String s = new String("Hello,receiver");
try
{
out.write(s.getBytes());
out.close();
}
catch(IOException e)
{
System.out.println(e.getMessage());
}
}
}

class Receiver extends Thread
{
private PipedInputStream in = new PipedInputSt

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