TabLayout获取空值/ NullPointerException

Question

我正在尝试制作三个制表符布局，但是我的代码不断返回NullPointerException！ 我为此专门创建了另一个项目（在本教程之后：https://examples.javacodegeeks.com/android/core/widget/android-tablayout-example/），并且一切正常。但是，当我对原始项目进行相同的更改时，“ TabLayout”继续收到“ null”，导致当我请求该特定活动时应用程序崩溃。我尝试调试，但是由于我不熟悉所有这些功能，因此无法解决任何问题。

这是活动中将“托管”标签和内容的代码：

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

        //Here is where the code gets the Null value:
        TabLayout tabLayout = ( TabLayout) findViewById(R.id.tab_layout);

        //Here is where the app crashes (on "First Tab", because of the same Null value):
        tabLayout.addTab(tabLayout.newTab().setText("First Tab"));
        tabLayout.addTab(tabLayout.newTab().setText("Second Tab"));
        tabLayout.addTab(tabLayout.newTab().setText("Third Tab"));
        tabLayout.setTabGravity(TabLayout.GRAVITY_FILL);

        final ViewPager viewPager = (ViewPager) findViewById(R.id.pager);
        final PagerAdapter adapter = new PagerAdapter
                (getSupportFragmentManager(), tabLayout.getTabCount());
        viewPager.setAdapter(adapter);
        viewPager.addOnPageChangeListener(new TabLayout.TabLayoutOnPageChangeListener(tabLayout));
        tabLayout.addOnTabSelectedListener(new TabLayout.OnTabSelectedListener() {

            @Override
            public void onTabSelected(TabLayout.Tab tab) {
                viewPager.setCurrentItem(tab.getPosition());
            }

            @Override
            public void onTabUnselected(TabLayout.Tab tab) {
            }

            @Override
            public void onTabReselected(TabLayout.Tab tab) {
            }
        });
    }
}

这是我收到的消息：

Process: festei.fun.festeisocialandroid, PID: 7285
    java.lang.RuntimeException: Unable to start activity ComponentInfo{festei.fun.festeisocialandroid/festei.fun.festeisocialandroid.Dashboard}: java.lang.NullPointerException: Attempt to invoke virtual method 'android.support.design.widget.TabLayout$Tab android.support.design.widget.TabLayout.newTab()' on a null object reference
        at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2830)
        at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2909)
        at android.app.ActivityThread.-wrap11(Unknown Source:0)
        at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1606)
        at android.os.Handler.dispatchMessage(Handler.java:105)
        at android.os.Looper.loop(Looper.java:164)
        at android.app.ActivityThread.main(ActivityThread.java:6592)
        at java.lang.reflect.Method.invoke(Native Method)
        at com.android.internal.os.Zygote$MethodAndArgsCaller.run(Zygote.java:240)
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:769)
     Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'android.support.design.widget.TabLayout$Tab android.support.design.widget.TabLayout.newTab()' on a null object reference
        at festei.fun.festeisocialandroid.Dashboard.onCreate(Dashboard.java:19)
        at android.app.Activity.performCreate(Activity.java:6984)
        at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1235)
        at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2783)
        at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2909) 
        at android.app.ActivityThread.-wrap11(Unknown Source:0) 
        at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1606) 
        at android.os.Handler.dispatchMessage(Handler.java:105) 
        at android.os.Looper.loop(Looper.java:164) 
        at android.app.ActivityThread.main(ActivityThread.java:6592) 
        at java.lang.reflect.Method.invoke(Native Method) 
        at com.android.internal.os.Zygote$MethodAndArgsCaller.run(Zygote.java:240) 
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:769)