如何将Android Studio中的主题从Noactionbar更改为ActionBar

Question

我在Android Studio启动了一个项目，显然，主题是theme.nameofproject.noactionbar。

这引起了以下错误：

java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.weoutdoor/com.example.weoutdoor.MainActivity}: java.lang.NullPointerException: Attempt to invoke virtual method 'void androidx.appcompat.app.ActionBar.setTitle(java.lang.CharSequence)' on a null object reference

project themes.xml：

<resources xmlns:tools="http://schemas.android.com/tools">
<!-- Base application theme. -->
<style name="Theme.WeOutdoor" parent="Theme.MaterialComponents.DayNight.DarkActionBar">
    <!-- Primary brand color. -->
    <item name="colorPrimary">@color/purple_500</item>
    <item name="colorPrimaryVariant">@color/purple_700</item>
    <item name="colorOnPrimary">@color/white</item>
    <!-- Secondary brand color. -->
    <item name="colorSecondary">@color/teal_200</item>
    <item name="colorSecondaryVariant">@color/teal_700</item>
    <item name="colorOnSecondary">@color/black</item>
    <!-- Status bar color. -->
    <item name="android:statusBarColor" tools:targetApi="l">?attr/colorPrimaryVariant</item>
    <!-- Customize your theme here. -->
</style>

<style name="Theme.WeOutdoor.NoActionBar">
    <item name="windowActionBar">true</item>
    <item name="windowNoTitle">true</item>
</style>

<style name="Theme.WeOutdoor.AppBarOverlay" parent="ThemeOverlay.AppCompat.Dark.ActionBar" />

<style name="Theme.WeOutdoor.PopupOverlay" parent="ThemeOverlay.AppCompat.Light" />

和androidmanifest.xml：

<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:tools="http://schemas.android.com/tools"
package="com.example.weoutdoor">

<application
    android:allowBackup="true"
    android:dataExtractionRules="@xml/data_extraction_rules"
    android:fullBackupContent="@xml/backup_rules"
    android:icon="@mipmap/ic_launcher"
    android:label="@string/app_name"
    android:roundIcon="@mipmap/ic_launcher_round"
    android:supportsRtl="true"
    android:theme="@style/Theme.WeOutdoor.AppBarOverlay"
    tools:targetApi="31">
    <activity
        android:name=".RegisterActivity"
        android:exported="false" />
    <activity
        android:name=".MainActivity"
        android:exported="true"
        android:theme="@style/Theme.WeOutdoor.NoActionBar">
        <intent-filter>
            <action android:name="android.intent.action.MAIN" />

            <category android:name="android.intent.category.LAUNCHER" />
        </intent-filter>

    </activity>
    <activity android:name=".UserProfileActivity"
        android:exported="true"
        android:theme="@style/Theme.WeOutdoor.AppBarOverlay">

    </activity>
    <activity android:name=".LoginActivity"
        android:exported="true"
        android:theme="@style/Theme.WeOutdoor.AppBarOverlay">

    </activity>

</application>

我尝试将样式的父和主题更改为themoverlay.appcompat.dark.actionalbar

，但是每当我尝试使用代码getsupportactionActionBar（）。 settitle（“ weoutdoor”）;

我在上面有上述错误。

在尝试在此帖子的答案中应用一些建议后，我最终遇到了一个新错误：

 Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.Button.setOnClickListener(android.view.View$OnClickListener)' on a null object reference

目前的主题如下：

   <resources xmlns:tools="http://schemas.android.com/tools">
   <!-- Base application theme. -->
   <style name="MyTheme" 
   parent="Theme.MaterialComponents.DayNight.DarkActionBar">
    <item name="windowActionBar">true</item>
   

以及我的androidMenifest.xml：

<?xml version="1.0" encoding="utf-8"?>
<manifest 
xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:tools="http://schemas.android.com/tools"
package="com.example.weoutdoor">

<application
    android:allowBackup="true"
    android:dataExtractionRules="@xml/data_extraction_rules"
    android:fullBackupContent="@xml/backup_rules"
    android:icon="@mipmap/ic_launcher"
    android:label="@string/app_name"
    android:roundIcon="@mipmap/ic_launcher_round"
    android:supportsRtl="true"
    android:theme="@style/MyTheme"
    tools:targetApi="31">
    <activity
        android:name=".RegisterActivity"
        android:theme="@style/MyTheme"
        android:exported="false" />
    <activity
        android:name=".MainActivity"
        android:exported="true"
        android:theme="@style/MyTheme">
        <intent-filter>
            <action android:name="android.intent.action.MAIN" />

            <category android:name="android.intent.category.LAUNCHER" />
        </intent-filter>

    </activity>
    <activity android:name=".UserProfileActivity"
        android:exported="true"
        android:theme="@style/MyTheme">

    </activity>
    <activity android:name=".LoginActivity"
        android:exported="true"
        android:theme="@style/MyTheme">

    </activity>

</application>

请记住，在每个活动中，我都使用getsupportactionbar（）;

我在错误中突出显示的登录性：

public class LoginActivity extends AppCompatActivity {

private EditText editTextLoginEmail, editTextLoginPwd;
private ProgressBar progressBar;
private FirebaseAuth authProfile;
private static final String TAG = "LoginActivity";

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    //setContentView(R.layout.login_activity);

    getSupportActionBar();

    editTextLoginEmail = findViewById(R.id.editText_login_email);
    editTextLoginPwd = findViewById(R.id.editText_login_password);

    progressBar = findViewById(R.id.progressBar);

    authProfile = FirebaseAuth.getInstance();

    //Login Tab
    Button buttonLogin = findViewById(R.id.button_login);
    buttonLogin.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View v) {
            String textEmail = editTextLoginEmail.getText().toString();
            String textPwd = editTextLoginPwd.getText().toString();

            if(TextUtils.isEmpty(textEmail)){
                Toast.makeText(LoginActivity.this, "Please enter your email", Toast.LENGTH_SHORT).show();
                editTextLoginEmail.setError("Email is required");
                editTextLoginEmail.requestFocus();
            } else if (!Patterns.EMAIL_ADDRESS.matcher(textEmail).matches()){
                Toast.makeText(LoginActivity.this, "Please re-enter your email", Toast.LENGTH_SHORT).show();
                editTextLoginEmail.setError("Valid Email is required");
                editTextLoginEmail.requestFocus();
            } else if (TextUtils.isEmpty(textPwd)){
                Toast.makeText(LoginActivity.this, "Please enter your password", Toast.LENGTH_SHORT).show();
                editTextLoginPwd.setError("Password is required");
                editTextLoginPwd.requestFocus();

            } else {
                progressBar.setVisibility(View.VISIBLE);
                loginUser (textEmail, textPwd);

            }
        }
    });

在登录中很明显。java，有些行被评论并引起了NPE。

所有校正和动作栏都可以正常工作。

Answer 1

您正在尝试使用themoverlay，它与完整的主题不同。但是，老实说，只需坚持noactionbar主题，然后在顶部创建自己的工具栏即可。它只是标题中包含textView的布局，以及一些image button s for Icons。