java多线程中，共享数据变量 为什么一定要定义private权限

publicclassThreadPlusAndMinus{privateintcount;privateintisRun;publicstaticvoidmain(String[]args){ThreadPlusAndMinustpm=newThreadPlusAndMinus();Threadt1=newThread(tpm.newMyThread(true));Threadt2=newThread(tpm.newMyThread(true));Threadt3=newThread(tpm.newMyThread(false));Threadt4=newThread(tpm.newMyThread(false));t1.start();t2.start();t3.start();t4.start();}privateclassMyThreadimplementsRunnable{privatebooleanisPlus;publicMyThread(booleanisPlus){this.isPlus=isPlus;}@Overridepublicvoidrun(){while(isRun++<100){if(isPlus){count++;}else{count--;}try{Thread.sleep(10);}catch(InterruptedExceptionex){Logger.getLogger(ThreadPlusAndMinus.class.getName()).log(Level.SEVERE,null,ex);}System.out.println(Thread.currentThread().getName()+":"+count);}}}}run方法不能加synchronized，加了就等于是单线程了。所以线程1必须等线程0完了才运行，另外每个线程预算完毕，释放cpu，让其他线程进来