ThreadGroup类的activeCount()方法用于返回当前线程的线程组中活动线程的数量。 返回的值只是一个估计值，因为当此方法遍历内部数据结构时，线程数可能会动态更改。
语法
public static int activeCount()
返回值
它返回当前线程的线程组中活动线程的数量。
示例
package com.yiibai.threadgroup;
class NewThread extends Thread {
NewThread(String threadname, ThreadGroup tg) {
super(tg, threadname);
}
public void run() {
for (int i = 0; i < 10; i++) {
try {
Thread.sleep(10);
} catch (InterruptedException ex) {
System.out.println(Thread.currentThread().getName() + " interrupted");
}
}
System.out.println(Thread.currentThread().getName() + " completed executing");
}
}
public class ThreadGroupActiveCountExp {
public static void main(String arg[]) {
// creating the thread group
ThreadGroup tg1 = new ThreadGroup("parent thread group");
// creating the threads
NewThread t1 = new NewThread("Thread-1", tg1);
NewThread t2 = new NewThread("Thread-2", tg1);
NewThread t3 = new NewThread("Thread-3", tg1);
// this will call the run() method
t1.start();
t2.start();
t3.start();
// checking the number of active thread
System.out.println("Number of active thread: " + tg1.activeCount());
}
}
执行上面示例代码，得到以下结果：
Number of active thread: 3
Thread-1 completed executing
Thread-2 completed executing
Thread-3 completed executing
