Question

所以我在我的l1上得到“基于空对象引用”，并且不知道在谷歌搜索后如何解决它。我曾尝试过多种编码不和，但他们说这是因为折旧的代码，但是当我用Google搜索时，它表明折旧的代码仍然有效，而我遵循的youtube教程也能正常工作。

MainActivity-

package com.getcontacts;

import android.database.Cursor;
import android.provider.ContactsContract;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.view.View;
import android.widget.ListView;
import android.widget.SimpleCursorAdapter;






public class MainActivity extends AppCompatActivity {

    ListView l1;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        l1 = findViewById(R.id.listView);
    }

    public void get(View v) {
        Cursor cursor = getContentResolver().query(ContactsContract.CommonDataKinds.Phone.CONTENT_URI,null,null,null,null);
        startManagingCursor(cursor);


        String[] from = {ContactsContract.CommonDataKinds.Phone.DISPLAY_NAME, ContactsContract.CommonDataKinds.Phone.NUMBER,
                ContactsContract.CommonDataKinds.Phone._ID};

        int[] to = {android.R.id.text1,android.R.id.text2};



        SimpleCursorAdapter simpleCursorAdapter = new SimpleCursorAdapter(this,android.R.layout.simple_list_item_2,cursor,from,to,1);
        l1.setAdapter(simpleCursorAdapter);
        l1.setChoiceMode(ListView.CHOICE_MODE_MULTIPLE);


    }
}

这是错误-

D/AndroidRuntime: Shutting down VM
E/AndroidRuntime: FATAL EXCEPTION: main
    Process: com.getcontacts, PID: 599
    java.lang.IllegalStateException: Could not execute method for android:onClick
        at android.support.v7.app.AppCompatViewInflater$DeclaredOnClickListener.onClick(AppCompatViewInflater.java:390)
        at android.view.View.performClick(View.java:5205)
        at android.view.View$PerformClick.run(View.java:21164)
        at android.os.Handler.handleCallback(Handler.java:739)
        at android.os.Handler.dispatchMessage(Handler.java:95)
        at android.os.Looper.loop(Looper.java:148)
        at android.app.ActivityThread.main(ActivityThread.java:5417)
        at java.lang.reflect.Method.invoke(Native Method)
        at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:730)
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:620)
     Caused by: java.lang.reflect.InvocationTargetException
        at java.lang.reflect.Method.invoke(Native Method)
        at android.support.v7.app.AppCompatViewInflater$DeclaredOnClickListener.onClick(AppCompatViewInflater.java:385)
        at android.view.View.performClick(View.java:5205) 
        at android.view.View$PerformClick.run(View.java:21164) 
        at android.os.Handler.handleCallback(Handler.java:739) 
        at android.os.Handler.dispatchMessage(Handler.java:95) 
        at android.os.Looper.loop(Looper.java:148) 
        at android.app.ActivityThread.main(ActivityThread.java:5417) 
        at java.lang.reflect.Method.invoke(Native Method) 
        at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:730) 
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:620) 
     Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.ListView.setAdapter(android.widget.ListAdapter)' on a null object reference
        at com.getcontacts.MainActivity.get(MainActivity.java:42)
        at java.lang.reflect.Method.invoke(Native Method) 
        at android.support.v7.app.AppCompatViewInflater$DeclaredOnClickListener.onClick(AppCompatViewInflater.java:385) 
        at android.view.View.performClick(View.java:5205) 
        at android.view.View$PerformClick.run(View.java:21164) 
        at android.os.Handler.handleCallback(Handler.java:739) 
        at android.os.Handler.dispatchMessage(Handler.java:95) 
        at android.os.Looper.loop(Looper.java:148) 
        at android.app.ActivityThread.main(ActivityThread.java:5417) 
        at java.lang.reflect.Method.invoke(Native Method) 
        at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:730) 
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:620)

Answer 1

显然是这样的：findViewById(R.id.listView);返回null，请仔细检查您的标签，并且该元素在调用时是否确实存在

Answer 2

检查您要在布局文件中尝试引用的l1，以确保您提供了ID并完全按照Java中的拼写对其进行了拼写。

获得“基于空对象的引用”

2 个答案: