尝试在空对象引用上调用虚拟方法'void android.widget.TextView.append（java.lang.CharSequence）'

Question

I'm unable to get data(text) from an EditText to a TextView via a Button. Logic should be fine and I can't really find the mistake.

I get the error as soon as I click the button which triggeres "sendMessage(View v)". I can't append text to "chat" but I don't know why.

MainActivity.java:

package com.chattr.neonardo.chattr;

import android.os.Bundle;
import android.support.v7.app.AppCompatActivity;
import android.util.Log;
import android.view.View;
import android.widget.Button;
import android.widget.EditText;
import android.widget.TextView;
import android.widget.Toast;

public class MainActivity extends AppCompatActivity {

    TextView chat;
    EditText message;
    Button send;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        chat = (TextView) findViewById(R.id.chat);
        message = (EditText) findViewById(R.id.message);
        send = (Button) findViewById(R.id.send);

        Log.d("test", "test2");

/*        send.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                chat.append("\n" + message.getText().toString());
            }
        });*/
    }

    public void onMatchTap(View v) {
        Toast myToast = Toast.makeText(getApplicationContext(), R.string.searching, Toast.LENGTH_LONG);
        myToast.show();
        setContentView(R.layout.chat_main);
        Log.d("test", "test4");
    }

    public void sendMessage(View v) {
        Log.d("test", "test6");
        chat.append("\n" + message.getText().toString());
        Log.d("test", "test7");
    }

}

I've tried something with the onClickListener but it didn't work so I commented it out.

main_activity.xml:

<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:app="http://schemas.android.com/apk/res-auto"
xmlns:tools="http://schemas.android.com/tools"
android:id="@+id/activity_main"
android:layout_width="match_parent"
android:layout_height="match_parent"
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="com.chattr.neonardo.chattr.MainActivity">

<ImageView
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    app:srcCompat="@drawable/chattr"
    android:id="@+id/imageView"
    android:layout_alignParentTop="true"
    android:layout_alignParentLeft="true"
    android:layout_alignParentStart="true" />

<Button
    android:id="@+id/button"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:layout_alignParentBottom="true"
    android:layout_centerHorizontal="true"
    android:layout_marginBottom="91dp"
    android:onClick="onMatchTap"
    android:text="@string/start_chat" />

I'm working with two different xml's because I'm changing scenes.

chat_main.xml:

<?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:orientation="vertical" android:layout_width="match_parent"
android:layout_height="match_parent"
android:weightSum="1">

<TextView
    android:id="@+id/chat"
    android:layout_width="match_parent"
    android:layout_height="wrap_content"
    android:layout_weight="1.00"
    android:autoLink="web"
    android:text="@string/chat_connection"
    android:visibility="visible" />

<LinearLayout
    android:layout_width="match_parent"
    android:layout_height="wrap_content"
    android:orientation="horizontal">

    <EditText
        android:id="@+id/message"
        android:layout_width="301dp"
        android:layout_height="wrap_content"
        android:ems="10"
        android:inputType="text"
        android:text="@string/defaultMessage" />

    <Button
        android:id="@+id/send"
        android:layout_width="wrap_content"
        android:layout_height="match_parent"
        android:elevation="0dp"
        android:onClick="sendMessage"
        android:text="Send" />
</LinearLayout>

That's the error:

    Process: com.chattr.neonardo.chattr, PID: 24340
java.lang.IllegalStateException: Could not execute method for android:onClick
    at android.support.v7.app.AppCompatViewInflater$DeclaredOnClickListener.onClick(AppCompatViewInflater.java:293)
    at android.view.View.performClick(View.java:5637)
    at android.view.View$PerformClick.run(View.java:22429)
    at android.os.Handler.handleCallback(Handler.java:751)
    at android.os.Handler.dispatchMessage(Handler.java:95)
    at android.os.Looper.loop(Looper.java:154)
    at android.app.ActivityThread.main(ActivityThread.java:6119)
    at java.lang.reflect.Method.invoke(Native Method)
    at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:886)
    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:776)
Caused by: java.lang.reflect.InvocationTargetException
    at java.lang.reflect.Method.invoke(Native Method)
    at android.support.v7.app.AppCompatViewInflater$DeclaredOnClickListener.onClick(AppCompatViewInflater.java:288)
    at android.view.View.performClick(View.java:5637) 
    at android.view.View$PerformClick.run(View.java:22429) 
    at android.os.Handler.handleCallback(Handler.java:751) 
    at android.os.Handler.dispatchMessage(Handler.java:95) 
    at android.os.Looper.loop(Looper.java:154) 
    at android.app.ActivityThread.main(ActivityThread.java:6119) 
    at java.lang.reflect.Method.invoke(Native Method) 
    at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:886) 
    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:776) 
Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.TextView.append(java.lang.CharSequence)' on a null object reference
    at com.chattr.neonardo.chattr.MainActivity.sendMessage(MainActivity.java:47)
    at java.lang.reflect.Method.invoke(Native Method) 
    at android.support.v7.app.AppCompatViewInflater$DeclaredOnClickListener.onClick(AppCompatViewInflater.java:288) 
    at android.view.View.performClick(View.java:5637) 
    at android.view.View$PerformClick.run(View.java:22429) 
    at android.os.Handler.handleCallback(Handler.java:751) 
    at android.os.Handler.dispatchMessage(Handler.java:95) 
    at android.os.Looper.loop(Looper.java:154) 
    at android.app.ActivityThread.main(ActivityThread.java:6119) 
    at java.lang.reflect.Method.invoke(Native Method) 
    at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:886) 
    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:776)

Answer 1

Remove setContentView(R.layout.chat_main);

You should use Intent to open a new activity inside onClick method.

Intent intent = new Intent(CurrentActivity.this,NextActivity.class);
startActivity(intent);

Should be

public void onMatchTap(View v) {
        Toast myToast = Toast.makeText(getApplicationContext(), R.string.searching, Toast.LENGTH_LONG).show();
        Intent intent = new Intent(MainActivity.this,ChatMain.class);
        startActivity(intent);
        Log.d("test", "test4");
    }

Answer 2

chat = (TextView) findViewById(R.id.chat);

此TextView是在chat_main.xml文件中定义的，然后尝试将其绑定到Main Activity中。这就是为什么它将提供NullPointerException的原因。