应用程序崩溃时开放，请帮助我

问题描述

我最近开始学习。我试图做出带有按钮的应用程序，会触发声音，但我的应用程序崩溃时开放，其与Java code的一个问题，因为当我删除code运行良好

i recently started learning. i'm trying to make a app with buttons that triggers sound but my app crashes when opening, its a problem with the java code because when i delete the code it runs fine

import android.app.Activity;
import android.media.MediaPlayer;
import android.os.Bundle;
import android.view.View;
import android.widget.Button;
import android.widget.LinearLayout;

public class MainActivity extends Activity {

LinearLayout hello;
Button btn;
MediaPlayer sound;

@Override
protected void onPause() {
    super.onPause();
    sound.release();
}

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);
    sound = MediaPlayer.create(this, R.raw.kick);
    btn = (Button) findViewById(R.id.btn);
    btn.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View v) {
            MediaPlayer mp = MediaPlayer.create(getApplicationContext(), R.raw.kick);
            mp.start();

        }
    });




}

}

logcat的：

java.lang.RuntimeException: Unable to start activity ComponentInfo{sp.umavibe.com.sp404/sp.umavibe.com.sp404.MainActivity}: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.Button.setOnClickListener(android.view.View$OnClickListener)' on a null object reference
        at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2411)
        at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2470)
        at android.app.ActivityThread.-wrap11(ActivityThread.java)
        at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1343)
        at android.os.Handler.dispatchMessage(Handler.java:102)
        at android.os.Looper.loop(Looper.java:148)
        at android.app.ActivityThread.main(ActivityThread.java:5401)
        at java.lang.reflect.Method.invoke(Native Method)
        at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:725)
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:615)
 Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.Button.setOnClickListener(android.view.View$OnClickListener)' on a null object reference
        at sp.umavibe.com.sp404.MainActivity.onCreate(MainActivity.java:28)
        at android.app.Activity.performCreate(Activity.java:6116)
        at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1106)
        at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2364)
at             android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2470)
 at android.app.ActivityThread.-wrap11(ActivityThread.java)
 at     android.app.ActivityThread$H.handleMessage(ActivityThread.java:1343)
 at android.os.Handler.dispatchMessage(Handler.java:102)
at android.os.Looper.loop(Looper.java:148)
at android.app.ActivityThread.main(ActivityThread.java:5401)
at java.lang.reflect.Method.invoke(Native Method)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:725)

在com.android.internal.os.ZygoteInit.main（ZygoteInit.java:615）

            at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:615)

推荐答案

该错误是在按钮初始化

btn = (Button) findViewById(R.id.btn);

请确认你已经创建了 activity_main.xml中按钮id为 BTN 。

Make sure you have created a button in the activity_main.xml with the id btn.

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