在Kotlin中，这样做:

this.title = resources.getText(R.string.fast_learning)

如果你想要它一次，让系统处理剩下的(不是动态的)，那么在你的manifest文件中这样做:

<application
        android:allowBackup="true"
        android:icon="@drawable/ic_launcher"
        android:label="@string/app_name"
        android:theme="@style/AppTheme" >
        <activity
            android:name=".MainActivity"
            android:label="@string/app_name_full" > //This is my custom title name on activity. <- The question is about this one.
            <intent-filter android:label="@string/app_launcher_name" > //This is my custom Icon title name (launcher name that you see in android apps/homescreen)
                <action android:name="android.intent.action.MAIN" />

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

MainActivity内部:

public class act1 extends AppCompatActivity {
@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.act1);

    setTitle("First Activity");

}

}

在Kotlin中，这样做:

this.title = resources.getText(R.string.fast_learning)

setTitle(getResources().getText(R.string.MyTitle));

如果你想在Java文件中设置标题，那么写在你的活动onCreate

setTitle("Your Title");

如果你想在Manifest里写

    <activity
        android:name=".MainActivity"
        android:label="Your Title" >
        <intent-filter>
            <action android:name="android.intent.action.MAIN" />

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