Question

我正在尝试构建一个简单的导航抽屉，但我不断收到这个恼人的错误。我对Android很新，所以我真的不知道如何修复/意味着什么。任何帮助表示赞赏。

爪哇：

package com.google.helpstl2;

import android.support.v4.widget.DrawerLayout;
import android.support.v7.app.ActionBarActivity;
import android.os.Bundle;
import android.view.Menu;
import android.view.MenuItem;
import android.view.View;
import android.widget.AdapterView;
import android.widget.ArrayAdapter;
import android.widget.ListView;
import android.widget.AdapterView.OnItemClickListener;
import android.widget.Toast;

import java.util.List;

public class Home extends ActionBarActivity implements OnItemClickListener {

    private DrawerLayout drawerLayout;
    private ListView listView;
    private String[] planets;
    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        drawerLayout = (DrawerLayout) findViewById(R.id.drawerLayout);
        planets = getResources().getStringArray(R.array.planets);
        listView = (ListView) findViewById(R.id.drawerList);
        listView.setAdapter(new ArrayAdapter<>(this, android.R.layout.simple_expandable_list_item_1, planets));
        listView.setOnItemClickListener(this);

    }



    @Override
    public boolean onCreateOptionsMenu(Menu menu) {
        // Inflate the menu; this adds items to the action bar if it is present.
        getMenuInflater().inflate(R.menu.menu_home, menu);
        return true;
    }

    @Override
    public boolean onOptionsItemSelected(MenuItem item) {
        // Handle action bar item clicks here. The action bar will
        // automatically handle clicks on the Home/Up button, so long
        // as you specify a parent activity in AndroidManifest.xml.
        int id = item.getItemId();

        //noinspection SimplifiableIfStatement
        if (id == R.id.action_settings) {
            return true;
        }

        return super.onOptionsItemSelected(item);
    }

    @Override
    public void onItemClick(AdapterView<?> parent, View view, int position, long id) {
        Toast.makeText(this,planets[position],Toast.LENGTH_LONG).show();
    }
}

相关XML？

<android.support.v4.widget.DrawerLayout
    xmlns:android="http://schemas.android.com/apk/res/android"
    android:id="@+id/drawerLayout"
    android:layout_width="match_parent"
    android:layout_height="match_parent">
    <!-- The main content view -->
    <FrameLayout
        android:id="@+id/mainContent"
        android:layout_width="match_parent"
        android:layout_height="match_parent" />

    <ListView android:id="@+id/drawerList"
        android:layout_width="240dp"

        android:layout_height="match_parent"
        android:layout_gravity="left"
        />
</android.support.v4.widget.DrawerLayout>

Logcat错误输出是：

Process: com.google.helpstl2, PID: 2205 java.lang.RuntimeException: Unable to start activity ComponentInfo{com.google.helpstl2/com.google.helpstl2.Home}: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.ListView.setAdapter(android.widget.ListAdapter)' on a null object reference

Answer 1

可能是包含内容的

<android.support.v4.widget.DrawerLayout
    xmlns:android="http://schemas.android.com/apk/res/android"
    android:id="@+id/drawerLayout"
    android:layout_width="match_parent"
    android:layout_height="match_parent">
    <!-- The main content view -->
    <FrameLayout
        android:id="@+id/mainContent"
        android:layout_width="match_parent"
        android:layout_height="match_parent" />

    <ListView android:id="@+id/drawerList"
        android:layout_width="240dp"

        android:layout_height="match_parent"
        android:layout_gravity="left"
        />
</android.support.v4.widget.DrawerLayout>

不是activity_main.xml，因此您在找到ID时会获得NPE：drawerList

一次检查这件事。

您可以在错误行添加空检查：

if(listView != null ) {    
listView.setAdapter(new ArrayAdapter<>(this, android.R.layout.simple_expandable_list_item_1, planets));
            listView.setOnItemClickListener(this);
}

Android上的错误尝试调用虚方法;我不明白

1 个答案: