我正在尝试在这个非常简单的活动中启用后退按钮（由另一个活动实例化），但我总是得到空指针异常，因此当我启动活动时我的应用程序崩溃（没有后退按钮）动作栏活动效果很好）。 我试过在互联网上发布了许多不同的解决方案，但没有一个有效 我正在关注本指南：Android Developer providing up navigation
这里是清单文件：
<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
package="com.example.android.projectcalculator">
<application
android:allowBackup="true"
android:icon="@mipmap/ic_launcher"
android:label="@string/app_name"
android:supportsRtl="true"
android:theme="@style/Theme.AppCompat.Light.NoActionBar"
android:screenOrientation="portrait">
<!--attività principale-->
<activity
android:name=".MainActivity"
android:launchMode="singleInstance">
<intent-filter>
<action android:name="android.intent.action.MAIN" />
<category android:name="android.intent.category.LAUNCHER" />
</intent-filter>
</activity>
<!--attività infixToPostfix-->
<activity
android:name=".InfixToPostfixActivity"
android:launchMode="singleTask"
android:label="@string/infix_to_postfix"/>
<!--attività figlia di infixToPostfix-->
<activity
android:name=".ShowPostfixProcedureActivity"
android:launchMode="standard"
android:label="@string/postfix_procedure"
android:parentActivityName=".InfixToPostfixActivity" />
</application>
</manifest>
这里是活动的java文件（ShowPostfixProcedureActivity）：
package com.example.android.projectcalculator;
import android.content.Intent;
import android.os.Bundle;
import android.support.v4.app.NavUtils;
import android.support.v7.app.AppCompatActivity;
import android.support.v7.widget.Toolbar;
import android.text.Html;
import android.util.Log;
import android.view.Menu;
import android.view.MenuItem;
import android.widget.TextView;
import java.util.Stack;
import static java.lang.Character.isDigit;
public class ShowPostfixProcedureActivity extends AppCompatActivity {
private static String equation;
private static int openBracket;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.show_infix_to_postfix_jurney);
Toolbar myToolbar = (Toolbar) findViewById(R.id.infix_to_postfix_toolbar_procedure);
setSupportActionBar(myToolbar);
getSupportActionBar().setDisplayHomeAsUpEnabled(true);
TextView txview = (TextView)findViewById(R.id.postfix_jurney_text_wall);
Stack<String> s = new Stack<String>();
s = DataProcessor.ReturnProcess();
txview.setText(s.pop());
while (!s.empty())
{
if (s.peek().charAt(0) == '<')
{
txview.append(Html.fromHtml(s.pop()));
}
else
{
txview.append(s.pop());
}
}
//Log.d("SPPA/onCreate","text = "+DataProcessor.ReturnProcess());
}
public boolean onCreateOptionsMenu(Menu menu) {
getMenuInflater().inflate(R.menu.action_bar_postfix_procedure, menu);
return true;
}
@Override
public boolean onOptionsItemSelected(MenuItem item)
{
switch (item.getItemId()) {
// Respond to the action bar's Up/Home button
case android.R.id.home:
NavUtils.navigateUpFromSameTask(this);
return true;
}
return super.onOptionsItemSelected(item);
}
}
与布局相关的XML文件（show_infix_to_postfix_jurney）：
<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:app="http://schemas.android.com/apk/res-auto"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:orientation="vertical">
<!--ActionBar in alto-->
<android.support.v7.widget.Toolbar
android:id="@+id/infix_to_postfix_toolbar_procedure"
android:layout_width="match_parent"
android:layout_height="?attr/actionBarSize"
android:background="#42f480"
android:elevation="4dp"
android:theme="@style/ThemeOverlay.AppCompat.ActionBar"
app:popupTheme="@style/ThemeOverlay.AppCompat.Light"/>
<ScrollView
android:layout_width="match_parent"
android:layout_height="match_parent"
android:scrollbars="vertical">
<TextView
android:id="@+id/postfix_jurney_text_wall"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:padding="10sp"
android:textSize="20sp"
android:gravity="center"
android:background="#f2f2f2"/>
</ScrollView>
</LinearLayout>
最后是与导航栏相关的XML文件（action_bar_postfix_procedure）：
<?xml version="1.0" encoding="utf-8"?>
<menu xmlns:android="http://schemas.android.com/apk/res/android">
</menu>
这里出现错误信息：
10-29 17:14:43.841 18822-18822/com.example.android.projectcalculator E/AndroidRuntime: FATAL EXCEPTION: main
Process: com.example.android.projectcalculator, PID: 18822
Theme: themes:{}
java.lang.NullPointerException: Attempt to invoke interface method 'android.view.MenuItem android.view.MenuItem.setEnabled(boolean)' on a null object reference
at com.example.android.projectcalculator.ShowPostfixProcedureActivity.onCreateOptionsMenu(ShowPostfixProcedureActivity.java:51)
at android.app.Activity.onCreatePanelMenu(Activity.java:2852)
at android.support.v4.app.FragmentActivity.onCreatePanelMenu(FragmentActivity.java:340)
at android.support.v7.view.WindowCallbackWrapper.onCreatePanelMenu(WindowCallbackWrapper.java:85)
at android.support.v7.app.AppCompatDelegateImplBase$AppCompatWindowCallbackBase.onCreatePanelMenu(AppCompatDelegateImplBase.java:258)
at android.support.v7.view.WindowCallbackWrapper.onCreatePanelMenu(WindowCallbackWrapper.java:85)
at android.support.v7.app.ToolbarActionBar.populateOptionsMenu(ToolbarActionBar.java:454)
at android.support.v7.app.ToolbarActionBar$1.run(ToolbarActionBar.java:61)
at android.os.Handler.handleCallback(Handler.java:739)
at android.os.Handler.dispatchMessage(Handler.java:95)
at android.os.Looper.loop(Looper.java:148)
at android.app.ActivityThread.main(ActivityThread.java:5461)
at java.lang.reflect.Method.invoke(Native Method)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:726)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:616)
答案 0 :(得分：0)
我不知道NavUtils.navigateUpFromSameTask(this);行应该做什么。如果要在按下后退按钮时关闭活动，请使用方法调用onBackPressed()
@Override
public boolean onOptionsItemSelected(MenuItem item)
{
switch (item.getItemId()) {
// Respond to the action bar's Up/Home button
case android.R.id.home:
onBackPressed()
return true;
}
return super.onOptionsItemSelected(item);
}