android怎样将activity放入全局栈

amdroid将Activity放入全局栈如下：
import java.util.Stack;
import android.app.Activity;
public class ScreenManager {
private static Stack activityStack;
private static ScreenManager instance;
private ScreenManager(){
}
public static ScreenManager getScreenManager(){
if(instance==null){
Activity是Android程序的表现层。程序的每一个显示屏幕就是一个Activity。正在运行的Activity处在栈的最顶端，它是运行状态的。

amdroid将Activity放入全局栈如下： import javautilStack; import androidappActivity; public class ScreenManager { private static Stack activityStack; private static ScreenManager instance; private ScreenManager(){ } public standroid怎样将activity放入全局栈