一旦在物理設備中運行，它就會迅速關閉

Question

java android
主要活動

這里在物理設備上運行時應用程序快速關閉，只能看到應用程序打開和關閉的 flash 用於添加 2 個數字，沒有顯示錯誤，應用程序運行時沒有錯誤，但在 logcat 中顯示“無法實例化活動 ComponentInfo{com .example.addingtwonumbers/com.example.addingtwonumbers.MainActivity}: java.lang.NullPointerException: Attempt to invoke virtual method 'android.content.pm.ApplicationInfo android.content.Context.getApplicationInfo()' on a null object reference"

package com.example.twonumbersaddition;

import androidx.appcompat.app.AppCompatActivity;

import android.os.Bundle;
import android.view.View;
import android.widget.EditText;

public class MainActivity extends AppCompatActivity
{

    EditText n1 = (EditText)findViewById(R.id.INPUT1);
    EditText n2 = (EditText)findViewById(R.id.INPUT2);
    
    @Override
    protected void onCreate(Bundle savedInstanceState)
    {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

    }
    public void add(View v)  //add method for adding 2 numbers
    {
        int i = Integer.parseInt(n1.getText().toString());
        int j = Integer.parseInt(n2.getText().toString());
        int k= i+j;

    }
}

清單文件

清單文件

<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
    package="com.example.addingtwonumbers">

    <application
        android:allowBackup="true"
        android:icon="@mipmap/ic_launcher"
        android:label="@string/app_name"
        android:roundIcon="@mipmap/ic_launcher_round"
        android:supportsRtl="true"
        android:theme="@style/Theme.AddingTwoNumbers">
        <activity android:name=".MainActivity"> //  activity is defined as 
            <intent-filter>//it is automatically ,have not modified it
                <action android:name="android.intent.action.MAIN" />
                <category android:name="android.intent.category.LAUNCHER" />
            </intent-filter>
        </activity>
    </application>

</manifest>

Answer 1

調用 setContentView 后，將 findViewById 調用移至 onCreate 方法。

public class MainActivity extends AppCompatActivity
{

    EditText n1;
    EditText n2;
    
    @Override
    protected void onCreate(Bundle savedInstanceState)
    {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        n1 = (EditText)findViewById(R.id.INPUT1);
        n2 = (EditText)findViewById(R.id.INPUT2);
    }
    public void add(View v)  //add method for adding 2 numbers
    {
        int i = Integer.parseInt(n1.getText().toString());
        int j = Integer.parseInt(n2.getText().toString());
        int k= i+j;

    }
}

當你在 onCreate 之前調用 findViewById 時，在 Activity 實例化時它會嘗試綁定你的視圖，因為 Activity 組件還不存在，你會得到一些錯誤。