Android應用程序未在Android工作室上運行，並將強制關閉

Question

我是Android應用程序開發新手。我做了一個簡單的應用程序，其中文本隨着按鈕被點擊而改變，但是每當我運行它時應用程序將強制關閉。善意幫助。

以下是我的XML

<?xml version="1.0" encoding="utf-8"?> 
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android" 
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.progneers.www.app.MainActivity"> 

<TextView 
    android:text="@string/txtString" 
    android:layout_width="wrap_content" 
    android:layout_height="wrap_content" 
    android:layout_alignParentTop="true" 
    android:layout_centerHorizontal="true" 
    android:layout_marginTop="41dp" 
    android:id="@+id/myTextView" 
    android:textAppearance="@style/TextAppearance.AppCompat" /> 

<Button 
    android:text="@string/click_me" 
    android:layout_width="wrap_content" 
    android:layout_height="wrap_content" 
    android:layout_below="@+id/myTextView" 
    android:layout_centerHorizontal="true" 
    android:layout_marginTop="100dp" 
    android:id="@+id/myButton" /> 
</RelativeLayout> 

以下是我的Java代碼

package com.progneers.www.app; 

import android.support.v7.app.AppCompatActivity; 
import android.os.Bundle; 
import android.view.View; 
import android.widget.Button; 
import android.widget.TextView; 

public class MainActivity extends AppCompatActivity { 

    private TextView myTextView = (TextView) findViewById(R.id.myTextView); 
    private Button myButton = (Button) findViewById(R.id.myButton); 

    @Override 
    protected void onCreate(Bundle savedInstanceState) { 
     super.onCreate(savedInstanceState); 
     setContentView(R.layout.activity_main); 
     myButton.setOnClickListener(
       new Button.OnClickListener(){ 
        public void onClick(View v){ 
         myTextView.setText("Button Clicked! Thank You!"); 
        } 
       } 
     ); 
    } 
    } 

Error shows after app closed

Answer 1

您必須分配在OnCreate意見，試試這個

public class MainActivity extends AppCompatActivity { 

private TextView myTextView; 
private Button myButton; 

@Override 
protected void onCreate(Bundle savedInstanceState) { 
    super.onCreate(savedInstanceState); 
    setContentView(R.layout.activity_main); 
    myTextView = (TextView) findViewById(R.id.myTextView); 
    myButton = (Button) findViewById(R.id.myButton); 
    myButton.setOnClickListener(
      new Button.OnClickListener(){ 
       public void onClick(View v){ 
        myTextView.setText("Button Clicked! Thank You!"); 
       } 
      } 
    ); 
} 
}