Android开发中在一个Activity中封锁另一个Activity

例子如下：ActivityApackage com.activity.yuzhenbei;import android.os.Bundle;import android.app.Activity;import android.content.Intent;import android.view.KeyEvent;import android.view.Menu;import android.view.MenuItem;public class MainActivity extends Activity {    public static ActivityA instance = null;   @Override    public void onCreate(Bundle savedInstanceState) {        super.onCreate(savedInstanceState);        setContentView(R.layout.activitya);        instance = this;        Intent intent = new Intent();intent.setClass(ActivityA.this, ActivityB.class);ActivityA.this.startActivity(intent);    }        // Menu  // 当点击Menu按钮时，调用该方法  @Override  public boolean onCreateOptionsMenu(Menu menu) {  menu.add(0, 1, 1, R.string.help).setIcon(  android.R.drawable.ic_menu_close_clear_cancel);  return super.onCreateOptionsMenu(menu);  }  // 选中某个菜  @Override  public boolean onOptionsItemSelected(MenuItem item) {  if (item.getItemId() == 1) {  Intent intent = new Intent();intent.setClass(ActivityA.this, ActivityB.class);ActivityA.this.startActivity(intent);  }  return super.onOptionsItemSelected(item);  }        // 返回键  @Override  public boolean onKeyDown(int keyCode, KeyEvent event) {  if (keyCode == KeyEvent.KEYCODE_BACK) { // 如果是手机上的返回键  ActivityA.this.finish();  }  return super.onKeyDown(keyCode, event);  }}ActivityBpackage com.activity.yuzhenbei;import android.app.Activity;import android.os.Bundle;import android.view.KeyEvent;import android.view.View;public class  ActivityB extends Activity {@Overridepublic void onCreate(Bundle savedInstanceState) {super.onCreate(savedInstanceState);setContentView(R.layout.activityb);}// 返回键@Overridepublic boolean onKeyDown(int keyCode, KeyEvent event) {if (keyCode == KeyEvent.KEYCODE_BACK) { // 如果是手机上的返回键ActivityB.this.finish();ActivityA.instance.finish();}return super.onKeyDown(keyCode, event);} }