Question

我刚创建了一个带有微调器的菜单和工具栏中的一些图标，当我开始此活动时，它给了我这个错误

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

此类扩展了AppCompatActivity 我的错误类怎么样呢

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;
    }
}

以及此行中的错误

getSupportActionBar().setDisplayShowTitleEnabled(false);

这是我的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>

如果有人能告诉我我做错了什么，那就好了

Answer 1

确保您的activity_main布局文件包含一个工具栏，其ID为“toolbar”。像这样：

<?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>

如何修复NullPointerException：尝试调用虚方法＆＃39; void

1 个答案: