java - ArrayList 已初始化，但在尝试 .add(string) 方法时仍返回 NullPointerException

Question

我在尝试使用 ArrayList 的 add 方法时遇到此错误。我可以看到很多人都遇到同样的问题，但他们在向数组列表添加元素之前没有初始化数组列表。但我正在尝试添加，并且，如果消息数组列表在该方法之前为空，我仍然要求控制台返回，并且它返回 false。

public class ShowTalkActivity extends Activity implements OnClickListener {

    private ListView listView;
    ArrayList<String> messages;
    ArrayAdapter<String> adapter;

    @Override
    protected void onCreate(Bundle savedInstanceState) {

        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_show_talk);

        Intent intent = getIntent();
        String selected = intent.getStringExtra("selected");
        setTitle(selected);

        listView = (ListView) findViewById(R.id.messagesList);
        messages = new ArrayList<String>();
        adapter = new ArrayAdapter<String>(this,R.layout.message_list_row, R.id.messagesList, messages);
        listView.setAdapter(adapter);

        ((Button) findViewById(R.id.sendMessage)).setOnClickListener(this);
    }

...

    @Override
    public void onClick(View view) {
        System.out.println(messages==null);
        messages.add("Anything");
        adapter.notifyDataSetChanged();
    }
}

System.out.println(messages==null);打印错误。我还尝试在开始时初始化消息 ArrayList，而不是仅仅声明。有谁知道发生了什么事吗？谢谢!

更新

这是 logcat 跟踪:

java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.TextView.setText(java.lang.CharSequence)' on a null object reference
        at android.widget.ArrayAdapter.createViewFromResource(ArrayAdapter.java:392)
        at android.widget.ArrayAdapter.getView(ArrayAdapter.java:362)
        at android.widget.AbsListView.obtainView(AbsListView.java:2344)
        at android.widget.ListView.makeAndAddView(ListView.java:1864)
        at android.widget.ListView.fillDown(ListView.java:698)
        at android.widget.ListView.fillFromTop(ListView.java:759)
        at android.widget.ListView.layoutChildren(ListView.java:1673)
        at android.widget.AbsListView.onLayout(AbsListView.java:2148)
        at android.view.View.layout(View.java:15596)
        at android.view.ViewGroup.layout(ViewGroup.java:4966)
        at android.widget.RelativeLayout.onLayout(RelativeLayout.java:1076)
        at android.view.View.layout(View.java:15596)
        at android.view.ViewGroup.layout(ViewGroup.java:4966)
        at android.widget.FrameLayout.layoutChildren(FrameLayout.java:573)
        at android.widget.FrameLayout.onLayout(FrameLayout.java:508)
        at android.view.View.layout(View.java:15596)
        at android.view.ViewGroup.layout(ViewGroup.java:4966)
        at com.android.internal.widget.ActionBarOverlayLayout.onLayout(ActionBarOverlayLayout.java:494)
        at android.view.View.layout(View.java:15596)
        at android.view.ViewGroup.layout(ViewGroup.java:4966)
        at android.widget.FrameLayout.layoutChildren(FrameLayout.java:573)
        at android.widget.FrameLayout.onLayout(FrameLayout.java:508)
        at android.view.View.layout(View.java:15596)
        at android.view.ViewGroup.layout(ViewGroup.java:4966)
        at android.view.ViewRootImpl.performLayout(ViewRootImpl.java:2072)
        at android.view.ViewRootImpl.performTraversals(ViewRootImpl.java:1829)
        at android.view.ViewRootImpl.doTraversal(ViewRootImpl.java:1054)
        at android.view.ViewRootImpl$TraversalRunnable.run(ViewRootImpl.java:5779)
        at android.view.Choreographer$CallbackRecord.run(Choreographer.java:767)
        at android.view.Choreographer.doCallbacks(Choreographer.java:580)
        at android.view.Choreographer.doFrame(Choreographer.java:550)
        at android.view.Choreographer$FrameDisplayEventReceiver.run(Choreographer.java:753)
        at android.os.Handler.handleCallback(Handler.java:739)
        at android.os.Handler.dispatchMessage(Handler.java:95)
        at android.os.Looper.loop(Looper.java:135)
        at android.app.ActivityThread.main(ActivityThread.java:5221)
        at java.lang.reflect.Method.invoke(Native Method)
        at java.lang.reflect.Method.invoke(Method.java:372)
        at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:899)
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:694)

Answer 1

MainActivity.java

package com.example.example;

import java.util.ArrayList;

import android.app.Activity;
import android.content.Intent;
import android.os.Bundle;
import android.view.View;
import android.view.View.OnClickListener;
import android.widget.ArrayAdapter;
import android.widget.Button;
import android.widget.ListView;

public class MainActivity extends Activity implements OnClickListener {

	private ListView listView;
	ArrayList<String> messages;
	ArrayAdapter<String> adapter;

	@Override
	protected void onCreate(Bundle savedInstanceState) {

		super.onCreate(savedInstanceState);
		setContentView(R.layout.activity_main);

		setTitle("Setted title");

		listView = (ListView) findViewById(R.id.messagesList);
		messages = new ArrayList<String>();
		adapter = new ArrayAdapter(this, R.layout.message_list_row,
				R.id.messagesList, messages);

		listView.setAdapter(adapter);

		Button sendmessage = (Button) findViewById(R.id.sendMessage);
		sendmessage.setOnClickListener(this);

	}

	@Override
	public void onClick(View view) {
		System.out.println(messages == null);
		messages.add("Anything");
		adapter.notifyDataSetChanged();
	}
}

acitivity_main.xml

<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:id="@+id/LinearLayout1"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:orientation="vertical"
    android:paddingBottom="@dimen/activity_vertical_margin"
    android:paddingLeft="@dimen/activity_horizontal_margin"
    android:paddingRight="@dimen/activity_horizontal_margin"
    android:paddingTop="@dimen/activity_vertical_margin"
    tools:context=".MainActivity" >

    <ListView
        android:id="@+id/messagesList"
        android:layout_width="match_parent"
        android:layout_height="322dp" >
    </ListView>

    <Button
        android:id="@+id/sendMessage"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="Button" />

</LinearLayout>

message_list_row.xml

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

    <TextView
        android:id="@+id/messagesList"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="Large Text"
        android:textAppearance="?android:attr/textAppearanceLarge" />

</LinearLayout>

上面的代码产生了Result