在 ActivityA 里面设置一个静态的变量instance,初始化为this
在 ActivityB 里面, ActivityA.instance.finish();

2.

也可以通过ActivityManager 

ActivityManager manager = (ActivityManager)getSystemService(ACTIVITY_SERVICE);   

manager.restartPackage(packageName); 

package 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);

  }

}

 

package com.activity.yuzhenbei;

import android.app.Activity;

import android.os.Bundle;

import android.view.KeyEvent;

import android.view.View;

public class  ActivityB extends Activity {

@Override

public void onCreate(Bundle savedInstanceState) {

super.onCreate(savedInstanceState);

setContentView(R.layout.activityb);

}

// 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

ActivityB.this.finish();

ActivityA.instance.finish();

}

return super.onKeyDown(keyCode, event);

}

}