小结

现在的Android有三个键： 任务键，Home键，返回键，也就是没有菜单键了，那么如何如何触发onCreateOptionsMenu(Menu menu)这个方法呢？测试了两种办法来触发这个onCreateOptionsMenu(Menu menu)方法，以达到显示菜单的效果。

问题及解决

无法触发onCreateOptionsMenu(Menu menu)

如果没有正确设置，是不会触发onCreateOptionsMenu(Menu menu)这个方法，也就是菜单无法触发显示不出来。

修改配置文件解决

参考CSDN: Android Studio 没有 gradle.properties 文件，需要在工程根目录下创建gradle.properties这个文件，内容如下：

android.useAndroidX=true
android.enableJetifier=true

参考关于在Fragment中设置Menu无效问题，在AndroidManifest.xml里添加android:theme="@style/ThemeOverlay.AppCompat.ActionBar"这行，以激活ActionBar的使用。
AndroidManifest.xml的内容如下：

<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"package="example.menu"android:versionCode="1"android:versionName="1.0"android:theme="@style/ThemeOverlay.AppCompat.ActionBar"><application android:icon="@drawable/icon" android:label="@string/app_name"><activity android:name=".MenuTest"android:label="@string/app_name"><intent-filter><action android:name="android.intent.action.MAIN" /><category android:name="android.intent.category.LAUNCHER" /></intent-filter></activity></application>
</manifest> 

dependencies是这样设置的：

dependencies {compile fileTree(dir: 'libs', include: '*.jar')//implementation 'androidx.appcompat:appcompat:1.0.0'//implementation 'android.support.v7.widget'//compile 'com.android.support:appcompat-v7:21.0.+'implementation 'com.android.support:appcompat-v7:29.0.+'
}

碰到以下问题：
uses-sdk:minSdkVersion 1 cannot be smaller than version 14 declared in library [androidx.appcompat:appcompat:1.0.0] C:\Users\LENOVO\.gradle\caches\transforms-3\9bee862a3eadd8338cd2918c9dad0196\transformed\appcompat-1.0.0\AndroidManifest.xml as the library might be using APIs not available in 1 Suggestion: use a compatible library with a minSdk of at most 1, or increase this project's minSdk version to at least 14, or use tools:overrideLibrary="androidx.appcompat" to force usage (may lead to runtime failures)
参考Android: App Manifest Overview 和 Stackoverlfow: Manifest merger failed : uses-sdk:minSdkVersion 1 cannot be smaller than version 14 declared in library

在build.gradle里添加

    defaultConfig {
// other config options...minSdkVersion 14}

添加了以下的包引用：

import androidx.appcompat.widget.Toolbar;
import androidx.appcompat.app.AppCompatActivity;

参考Stackoverflow: You need to use a Theme.AppCompat theme (or descendant) with this activity，注意：
MenuTest 是继承自Activity，而不是AppCompatActivity或者ActionBarActivity .

public class MenuTest extends Activity
//public class MenuTest extends AppCompatActivity

最后的效果，点以下红框中三个点的地方，可以触发菜单。

触发菜单如下红框所示：

使用一个按钮来触发

参考Stackoverflow: Handling the missing MENU button in new versions of Android (3.x and up)
和
Android: Progamatically Open and Close an Activity’s Option Menu

在界面中添加一个按钮Pop Up Menu，可以在按钮事件中添加 openOptionsMenu();这个方法来触发菜单的。在``方法中添加以下监听器：

		Button button = (Button) findViewById(R.id.button_send);button.setOnClickListener(new View.OnClickListener() {public void onClick(View v) {// Do something in response to button clickopenOptionsMenu();}});

点以下红框中的按钮，可以触发菜单。

点以上的按钮也是可以触发菜单的，效果跟先前Actionbar触发菜单是一样的。

最后，整个的build.gradle文件内容如下：

buildscript {repositories {google()mavenCentral()}dependencies {classpath 'com.android.tools.build:gradle:4.2.0'}
}
apply plugin: 'android'dependencies {compile fileTree(dir: 'libs', include: '*.jar')//implementation 'androidx.appcompat:appcompat:1.0.0'//implementation 'android.support.v7.widget'//compile 'com.android.support:appcompat-v7:21.0.+'implementation 'com.android.support:appcompat-v7:29.0.+'}allprojects {repositories {google()//jcenter()}}android {compileSdk 29buildToolsVersion '29.0.0'defaultConfig {
// other config options...minSdkVersion 14}sourceSets {main {manifest.srcFile 'AndroidManifest.xml'java.srcDirs = ['src']resources.srcDirs = ['src']aidl.srcDirs = ['src']renderscript.srcDirs = ['src']res.srcDirs = ['res']assets.srcDirs = ['assets']}// Move the tests to tests/java, tests/res, etc...//instrumentTest.setRoot('tests')// Move the build types to build-types/<type>// For instance, build-types/debug/java, build-types/debug/AndroidManifest.xml, ...// This moves them out of them default location under src/<type>/... which would// conflict with src/ being used by the main source set.// Adding new build types or product flavors should be accompanied// by a similar customization.debug.setRoot('build-types/debug')release.setRoot('build-types/release')}
}

整个的源代码如下：

package example.menu;import android.app.Activity;
import android.graphics.Color;
import android.os.Bundle;
import android.util.Log;
import android.view.Menu;
import android.view.MenuItem;
import android.view.SubMenu;
import android.view.MenuItem.OnMenuItemClickListener;
import android.widget.EditText;
import android.widget.Toast;import android.app.Fragment;
//import androidx.fragment.app.Fragment;
//import android.support.v7.widget.Toolbar;
//import androidx.appcompat.app.AppCompatActivity;
//import androidx.appcompat.app.AppCompatActivity;
//import androidx.appcompat.widget.Toolbar;import androidx.appcompat.widget.Toolbar;
import androidx.appcompat.app.AppCompatActivity;
//import android.support.v7.app.ActionBarActivity;
//import android.support.v7.app.AppCompatActivity;import android.view.View;
import android.view.View.OnClickListener;
import android.widget.Button;public class MenuTest extends Activity
//public class MenuTest extends AppCompatActivity
{final int FONT_10 = 0x111;final int FONT_12 = 0x112;final int FONT_14 = 0x113;final int FONT_16 = 0x114;final int FONT_18 = 0x115;final int PLAIN_ITEM = 0x11b;final int FONT_RED = 0x116;final int FONT_BLUE = 0x117;final int FONT_GREEN = 0x118;private EditText edit;@Overridepublic void onCreate(Bundle savedInstanceState){super.onCreate(savedInstanceState);setContentView(R.layout.main);edit = (EditText) findViewById(R.id.txt);//		Toolbar toolbar = (Toolbar) findViewById(R.id.toolbar);
//		setSupportActionBar(toolbar);//setHasOptionsMenu(true);
//		Toolbar toolbar = findViewById(R.id.toolbar);
//		toolbar.inflateMenu(R.menu.menu_main);//setHasOptionsMenu(true);//call it from onCreate(); or onViewCreated();//Toolbar myToolbar = (Toolbar) findViewById(R.id.toolbar);//android.widget.Toolbar myToolbar = (android.widget.Toolbar) findViewById(R.id.toolbar);//setSupportActionBar(myToolbar);//setSupportActionBar(myToolbar);//((MainActivity) getActivity()).setSupportActionBar(myToolbar);Button button = (Button) findViewById(R.id.button_send);button.setOnClickListener(new View.OnClickListener() {public void onClick(View v) {// Do something in response to button clickopenOptionsMenu();}});}@Overridepublic boolean onCreateOptionsMenu(Menu menu){SubMenu fontMenu = menu.addSubMenu("Font Size");fontMenu.setIcon(R.drawable.font);fontMenu.setHeaderIcon(R.drawable.font);fontMenu.setHeaderTitle("Select font size");fontMenu.add(0, FONT_10, 0, "10 font size");fontMenu.add(0, FONT_12, 0, "12 font size");fontMenu.add(0, FONT_14, 0, "14 font size");fontMenu.add(0, FONT_16, 0, "16 font size");fontMenu.add(0, FONT_18, 0, "18 font size");menu.add(0, PLAIN_ITEM, 0, "Normal Menu Selection (Toast)");SubMenu colorMenu = menu.addSubMenu("Text Color");colorMenu.setIcon(R.drawable.color);colorMenu.setHeaderIcon(R.drawable.color);colorMenu.setHeaderTitle("Select Text Color");colorMenu.add(0, FONT_RED, 0, "Red");colorMenu.add(0, FONT_GREEN, 0, "Green");colorMenu.add(0, FONT_BLUE, 0, "Blue");// Inflate the menu; this adds items to the action bar if it is present.//getMenuInflater().inflate(R.menu.job_status_option_menu, menu);return super.onCreateOptionsMenu(menu);}@Overridepublic boolean onOptionsItemSelected(MenuItem mi){switch (mi.getItemId()){case FONT_10:edit.setTextSize(10 * 2);break;case FONT_12:edit.setTextSize(12 * 2);break;case FONT_14:edit.setTextSize(14 * 2);break;case FONT_16:edit.setTextSize(16 * 2);break;case FONT_18:edit.setTextSize(18 * 2);break;case FONT_RED:edit.setTextColor(Color.RED);break;case FONT_GREEN:edit.setTextColor(Color.GREEN);break;case FONT_BLUE:edit.setTextColor(Color.BLUE);break;case PLAIN_ITEM:Toast toast = Toast.makeText(MenuTest.this, "This is a toast message!" , Toast.LENGTH_SHORT);toast.show();break;}return true;}}

main.xml的内容如下：

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"android:orientation="vertical"android:layout_width="fill_parent"android:layout_height="fill_parent">
<EditText android:id="@+id/txt" android:layout_width="fill_parent" android:layout_height="wrap_content" android:text="For Test Purpose! "android:editable="false"/><Button xmlns:android="http://schemas.android.com/apk/res/android"android:id="@+id/button_send"android:layout_width="wrap_content"android:layout_height="wrap_content"android:text="Pop Up Menu"android:onClick="popUpMenu" />
</LinearLayout>

其它办法

参考Android: Set up the app bar
可以使用setSupportActionBar(myToolbar);的Toolbar来触发onCreateOptionsMenu(Menu menu)这个方法，这个没试过，不清楚效果怎么样。

@Override
protected void onCreate(Bundle savedInstanceState) {super.onCreate(savedInstanceState);setContentView(R.layout.activity_my);Toolbar myToolbar = (Toolbar) findViewById(R.id.my_toolbar);setSupportActionBar(myToolbar);
}

参考

CSDN: Android Studio 没有 gradle.properties 文件
CSDN: onCreateOptionsMenu和onPrepareOptionsMenu用法
关于在Fragment中设置Menu无效问题
CSDN: 关于 fragment重写onCreateOptionsMenu不执行问题
Stackoverflow: Handling the missing MENU button in new versions of Android (3.x and up)
Stackoverlfow: Manifest merger failed : uses-sdk:minSdkVersion 1 cannot be smaller than version 14 declared in library
Android: Progamatically Open and Close an Activity’s Option Menu
Android: App Manifest Overview
Android: Set up the app bar
Android studio: cannot find symbol variable toolbar
Stackoverflow: Error “package android.support.v7.app does not exist”
Stackoverflow: Can’t use android.support.v7.widget.Toolbar with android.useAndroidX=true
Stackoverflow: setsupportactionbar() throws error
Stackoverflow: Android error: cannot find symbol class ActionBarActivity
Stackoverflow: You need to use a Theme.AppCompat theme (or descendant) with this activity