如何解決NullPointerException異常：試圖調用虛擬方法無效

Question

我剛剛創建了一個菜單使用旋轉器和工具欄的一些圖標，而我開始這個活動它給了我這個錯誤

Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.support.v7.app.ActionBar.setDisplayShowTitleEnabled(boolean)' on a null object reference 

這個類是擴展AppCompatActivity 如何過我的錯誤類此

package abtech.waiteriano.com.waitrer; 

import android.os.Build; 
import android.os.Bundle; 
import android.support.annotation.RequiresApi; 
import android.support.v7.app.AppCompatActivity; 
import android.view.Menu; 
import android.view.MenuInflater; 
import android.view.View; 
import android.widget.AdapterView; 
import android.widget.ArrayAdapter; 
import android.widget.Spinner; 
import android.widget.Toast; 

public class MenuActivity extends AppCompatActivity { 
    private android.support.v7.widget.Toolbar toolbar; 
    private String[] category = {"hello", "this", "is", "me"}; 

    @RequiresApi(api = Build.VERSION_CODES.LOLLIPOP) 
    @Override 
    protected void onCreate(Bundle savedInstanceState) { 
     super.onCreate(savedInstanceState); 
     setContentView(R.layout.activity_main); 
     toolbar = (android.support.v7.widget.Toolbar) findViewById(R.id.toolbar); 
     setSupportActionBar(toolbar); 
     getSupportActionBar().setDisplayShowTitleEnabled(false); 
     Spinner navigationSpinner = new Spinner(getSupportActionBar().getThemedContext()); 
     ArrayAdapter<String> adapter = new ArrayAdapter<String>(this,android.R.layout.simple_spinner_item, category); 
     navigationSpinner.setAdapter(adapter); 
     toolbar.addView(navigationSpinner, 0); 
     navigationSpinner.setOnItemSelectedListener(new AdapterView.OnItemSelectedListener() { 
      @Override 
      public void onItemSelected(AdapterView<?> parent, View view, int position, long id) { 
       Toast.makeText(MenuActivity.this, 
         "you selected: " + category[position], 
         Toast.LENGTH_SHORT).show(); 
      } 

      @Override 
      public void onNothingSelected(AdapterView<?> parent) { 

      } 
     }); 
    } 
    @Override 
    public boolean onCreateOptionsMenu(Menu menu) { 
     MenuInflater inflater = getMenuInflater(); 
     inflater.inflate(R.menu.iconmenu, menu); 
     return true; 
    } 
} 

，並在此行

getSupportActionBar().setDisplayShowTitleEnabled(false); 

錯誤

，這是我的xml文件

<LinearLayout 
    xmlns:android="http://schemas.android.com/apk/res/android" 
    xmlns:tools="http://schemas.android.com/tools" 
    android:layout_width="match_parent" 
    android:layout_height="match_parent" 
    tools:context=".MainActivity"> 

    <android.support.v7.widget.Toolbar 
     android:id="@+id/toolbar" 
     android:layout_width="match_parent" 
     android:layout_height="wrap_content" 
     android:background="?attr/colorPrimary" 
     android:minHeight="?attr/actionBarSize" /> 

</LinearLayout> 

如果任何人能告訴我，我做錯了什麼這會是很好的

Answer 1

確保您activity_main佈局文件包含「工具欄」的一個工具欄和id 。事情是這樣的：

<?xml version="1.0" encoding="utf-8"?> 
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android" 
    xmlns:tools="http://schemas.android.com/tools" 
    android:layout_width="match_parent" 
    android:layout_height="match_parent" 
    xmlns:app="http://schemas.android.com/apk/res-auto" 
    android:orientation="vertical"> 

    <android.support.v7.widget.Toolbar 
      android:id="@+id/toolbar" 
      android:layout_width="match_parent" 
      android:layout_height="?attr/actionBarSize" 
      android:background="?attr/colorPrimary" 
      app:popupTheme="@style/AppTheme.PopupOverlay" /> 

</LinearLayout>