Android 应用程序未在 android studio 上运行并强制关闭

Question

我是 Android 应用程序开发的新手。我制作了一个简单的应用程序，只要单击按钮，文本就会发生变化，但每当我运行它时，应用程序就会强制关闭。请帮忙。

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代码

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

你必须在 onCreate 中分配 View ，试试这个

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!");
                }
            }
    );
}
}