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

也可参考[CSDN: 记录一下Popmenu的使用以及一些坑：自定义Popupmenu、Menu、Popupmenu添加icon][16]

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 click
                openOptionsMenu();
            }
        });

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 29
    buildToolsVersion '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;

    @Override
    public 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 click
                openOptionsMenu();
            }
        });
    }



    @Override
    public 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);
    }


    @Override

    public 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