java - NullPointerException 导致应用程序立即崩溃

Question

我对错误的问题标题表示歉意。

我的问题是，一旦执行操作，我的应用程序就会立即崩溃。当我读回错误时，我看到一个 NullPointerException 这就是崩溃的原因。

该错误表明它发生在 Main.java 的第 48 行。这是我的完整 Main.java:

 package com.fuzzyapplications.tutorial.youtube.app;

 import android.content.Intent;
 import android.support.v7.app.ActionBarActivity;
 import android.os.Bundle;
 import android.view.Menu;
 import android.view.MenuItem;
 import android.view.View;
 import android.widget.EditText;

public class Main extends ActionBarActivity {

public static final String EXTRA_MESSAGE = "com.fuzzyapplications.practice.app.MESSAGE";


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

}


@Override
public boolean onCreateOptionsMenu(Menu menu) {

    // Inflate the menu; this adds items to the action bar if it is present.
    getMenuInflater().inflate(R.menu.main, menu);
    return true;
}

@Override
public boolean onOptionsItemSelected(MenuItem item) {
    // Handle action bar item clicks here. The action bar will
    // automatically handle clicks on the Home/Up button, so long
    // as you specify a parent activity in AndroidManifest.xml.
    int id = item.getItemId();
    if (id == R.id.action_settings) {
        return true;
    }
    return super.onOptionsItemSelected(item);
}

public void sendMessage(View view){
    Intent intent = new Intent(this, DisplayMessageActivity.class);

    EditText editText = (EditText) findViewById(R.string.edit_message);

    String message = editText.getText().toString();

    intent.putExtra(EXTRA_MESSAGE, message);

    startActivity(intent);


 }

}

此外，这里是应该激活的 Activity (一旦按下按钮

package com.fuzzyapplications.tutorial.youtube.app;

  import android.content.Intent;
  import android.support.v7.app.ActionBarActivity;
  import android.os.Bundle;
  import android.view.Menu;
  import android.view.MenuItem;
  import android.widget.TextView;

  public class DisplayMessageActivity extends ActionBarActivity {

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);

    Intent intent = getIntent();
    String message = intent.getStringExtra(Main.EXTRA_MESSAGE);
    TextView textView = new TextView(this);
    textView.setTextSize(40);
    textView.setText(message);
    setContentView(textView);
}


@Override
public boolean onCreateOptionsMenu(Menu menu) {

    // Inflate the menu; this adds items to the action bar if it is present.
    getMenuInflater().inflate(R.menu.display_message, menu);
    return true;
}

@Override
public boolean onOptionsItemSelected(MenuItem item) {
    // Handle action bar item clicks here. The action bar will
    // automatically handle clicks on the Home/Up button, so long
    // as you specify a parent activity in AndroidManifest.xml.
    int id = item.getItemId();
    if (id == R.id.action_settings) {
        return true;
    }
    return super.onOptionsItemSelected(item);
}

 }

希望这对您来说是足够的信息!我是 android 新手，所以我不确定应该为这个问题提供多少信息，所以如果我没有足够的信息，请告诉我!再次感谢。

编辑:添加了 Logcat 信息和布局信息!

Logcat:

   FATAL EXCEPTION: main
    java.lang.IllegalStateException: Could not execute method of the activity
            at android.view.View$1.onClick(View.java:3802)
            at android.view.View.performClick(View.java:4437)
            at android.view.View$PerformClick.run(View.java:18365)
            at android.os.Handler.handleCallback(Handler.java:725)
            at android.os.Handler.dispatchMessage(Handler.java:92)
            at android.os.Looper.loop(Looper.java:137)
            at android.app.ActivityThread.main(ActivityThread.java:5285)
            at java.lang.reflect.Method.invokeNative(Native Method)
            at java.lang.reflect.Method.invoke(Method.java:511)
            at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1102)
            at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:869)
            at dalvik.system.NativeStart.main(Native Method)
     Caused by: java.lang.reflect.InvocationTargetException
            at java.lang.reflect.Method.invokeNative(Native Method)
            at java.lang.reflect.Method.invoke(Method.java:511)
            at android.view.View$1.onClick(View.java:3797)
            at android.view.View.performClick(View.java:4437)
            at android.view.View$PerformClick.run(View.java:18365)
            at android.os.Handler.handleCallback(Handler.java:725)
            at android.os.Handler.dispatchMessage(Handler.java:92)
            at android.os.Looper.loop(Looper.java:137)
            at android.app.ActivityThread.main(ActivityThread.java:5285)
            at java.lang.reflect.Method.invokeNative(Native Method)
            at java.lang.reflect.Method.invoke(Method.java:511)
            at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1102)
            at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:869)
            at dalvik.system.NativeStart.main(Native Method)
     Caused by: java.lang.NullPointerException
            at com.fuzzyapplications.tutorial.youtube.app.Main.sendMessage(Main.java:49)
            at java.lang.reflect.Method.invokeNative(Native Method)
            at java.lang.reflect.Method.invoke(Method.java:511)
            at android.view.View$1.onClick(View.java:3797)
            at android.view.View.performClick(View.java:4437)
            at android.view.View$PerformClick.run(View.java:18365)
            at android.os.Handler.handleCallback(Handler.java:725)
            at android.os.Handler.dispatchMessage(Handler.java:92)
            at android.os.Looper.loop(Looper.java:137)
            at android.app.ActivityThread.main(ActivityThread.java:5285)
            at java.lang.reflect.Method.invokeNative(Native Method)
            at java.lang.reflect.Method.invoke(Method.java:511)
            at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1102)
            at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:869)
            at dalvik.system.NativeStart.main(Native Method)

布局文件:

主要:

<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:tools="http://schemas.android.com/tools"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:orientation="horizontal"
>

<EditText
    android:id="@+id/edit_message"
    android:layout_weight="1"
    android:layout_width="0dp"
    android:layout_height="wrap_content"
    android:hint="@string/edit_message"/>

<Button
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:text="@string/button_send"
    android:onClick="sendMessage"
    />


  </LinearLayout>

MessageSendActivity 布局:

<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:paddingLeft="@dimen/activity_horizontal_margin"
    android:paddingRight="@dimen/activity_horizontal_margin"
    android:paddingTop="@dimen/activity_vertical_margin"
    android:paddingBottom="@dimen/activity_vertical_margin"
    tools:context="com.fuzzyapplications.practice.app.DisplayMessageActivity">

<TextView
    android:text="@string/hello_world"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content" />

 </RelativeLayout>

Answer 1

您正在尝试引用对 string 的引用(R.java 中可能不存在该引用)，而不是对您声明的 id 的引用在 xml 布局中android:id="@+id/edit_message"。它会导致 NullPointerException。使用:

EditText editText = (EditText) findViewById(R.id.edit_message);

而不是:

EditText editText = (EditText) findViewById(R.string.edit_message);