//代碼1
package?communication;

import?java.util.Random;

public?class?Flag?{
	public?static?void?main(String[]?args)?{
		FlagSender?f?=?new?FlagSender();
		FlagRec?r?=?new?FlagRec(f);
		Thread?t1?=?new?Thread(f);
		Thread?t2?=?new?Thread(r);
		t1.start();
		t2.start();
	}
}
class?FlagSender?implements?Runnable{
	private?int?theValue;
	private?boolean?flag;
	
	public?int?getTheValue()?{
		return?theValue;
	}

	public?void?setTheValue(int?theValue)?{
		this.theValue?=?theValue;
	}

	public?boolean?isFlag()?{
		return?flag;
	}

	public?void?setFlag(boolean?flag)?{
		this.flag?=?flag;
	}

	@Override
	public?void?run()?{
		for?(int?i?=?0;?i?<?5;?i++)?{
			while(flag){
				Thread.yield();
			}
			theValue?=?new?Random().nextInt(1000);
			System.out.println("send?the?value?is?"+theValue);
			flag?=?true;
		}
	}
}
class?FlagRec?implements?Runnable{
	private?FlagSender?f;?
	public?FlagRec(FlagSender?f)?{
		super();
		this.f?=?f;
	}

	@Override
	public?void?run()?{
		for?(int?i?=?0;?i?<?5;?i++)?{
			while(!f.isFlag()){
				Thread.yield();
			}
			System.out.println("receive?the?value?is?"+f.getTheValue());
			f.setFlag(false);
		}
	}
}

//代碼2
package?communication;

import?java.util.Random;

public?class?WrongOfFlag?{
	public?static?void?main(String[]?args)?{
		FlagSender1?f?=?new?FlagSender1();
		FlagRec1?r?=?new?FlagRec1();
		Thread?t1?=?new?Thread(f);
		Thread?t2?=?new?Thread(r);
		t1.start();
		t2.start();
	}
}
class?FlagSender1?implements?Runnable{
	private?int?theValue;
	private?boolean?flag;
	
	public?int?getTheValue()?{
		return?theValue;
	}

	public?void?setTheValue(int?theValue)?{
		this.theValue?=?theValue;
	}

	public?boolean?isFlag()?{
		return?flag;
	}

	public?void?setFlag(boolean?flag)?{
		this.flag?=?flag;
	}

	@Override
	public?void?run()?{
		for?(int?i?=?0;?i?<?5;?i++)?{
			while(flag){
				Thread.yield();
			}
			theValue?=?new?Random().nextInt(1000);
			System.out.println("send?the?value?is?"+theValue);
			flag?=?true;
		}
	}
}
class?FlagRec1?implements?Runnable{
	FlagSender?f?=?new?FlagSender();
	@Override
	public?void?run()?{
		for?(int?i?=?0;?i?<?5;?i++)?{
			while(!f.isFlag()){
				Thread.yield();
			}
			System.out.println("receive?the?value?is?"+f.getTheValue());
			f.setFlag(false);
		}
	}
}

為什么要讓FlagSender作為FlagRec的成員，像代碼二中直接new了FlagSender為什么結果只輸出一個send the value is 138，然后就死循環空程序了，結果如下圖