应用程序崩溃并"OnClick"侦听器上的消息"Unfortunately, Project has stopped."

Question

当我在 Eclipse 中的 AVD 中运行该应用程序时，它工作正常，直到我单击添加按钮：该应用程序已停止并显示消息。不幸的是，项目已经停止。我对安卓很陌生，所以，我可能会犯一些愚蠢的错误。请查看此代码，并将我从这场危机中解救出来。

主要活动.java

package com.example.firstproject;
import android.app.Activity;
import android.os.Bundle;
import android.view.Menu;
import android.widget.Button;
import android.widget.EditText;

public class MainActivity extends Activity {
     Button button,btnAdd,btnSub;
     EditText val1,val2;
    double num1,num2,res;
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        val1 = (EditText) findViewById(R.id.txtVal1);
        val2 = (EditText) findViewById(R.id.txtVal2);
        btnAdd = (Button) findViewById(R.id.btnSum);
        btnSub = (Button) findViewById(R.id.btnDiff);
        btnAdd.setOnClickListener(new MyOnClickListener(this));
    }

    @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;
    }
}

MyOnClickListener.java

package com.example.firstproject;
import android.view.View;
import android.view.View.OnClickListener;
import android.widget.Toast;
public class MyOnClickListener extends MainActivity implements OnClickListener{
    MainActivity caller;
    public MyOnClickListener(MainActivity activity) {
    this.caller = activity;
    }
    public void onClick(View view) {
        num1 = Double.parseDouble(val1.getText().toString());
        num2 = Double.parseDouble(val2.getText().toString());
        res = num1 + num2;
        Toast.makeText(this, String.valueOf(res), Toast.LENGTH_SHORT).show();
    }
}

activity_main.xml

<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:id="@+id/LinearLayout1"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:orientation="vertical"
    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=".MainActivity" >
    <EditText
        android:id="@+id/txtVal1"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:ems="10"
        android:inputType="number" >
        <requestFocus />
    </EditText>
    <EditText
        android:id="@+id/txtVal2"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:ems="10"
        android:inputType="number" />
    <Button
        android:id="@+id/btnSum"
        android:layout_width="127dp"
        android:layout_height="wrap_content"
        android:text="Add" />
    <Button
        android:id="@+id/btnDiff"
        android:layout_width="127dp"
        android:layout_height="wrap_content"
        android:text="Subtract" />
</LinearLayout>

Error_log

07-13 01:02:24.800: D/(972): HostConnection::get() New Host Connection established 0xb8b52560, tid 972
07-13 01:02:24.930: W/EGL_emulation(972): eglSurfaceAttrib not implemented
07-13 01:02:24.960: D/OpenGLRenderer(972): Enabling debug mode 0
07-13 01:02:42.092: D/AndroidRuntime(972): Shutting down VM
07-13 01:02:42.092: W/dalvikvm(972): threadid=1: thread exiting with uncaught exception (group=0xb3aa3b90)
07-13 01:02:42.152: E/AndroidRuntime(972): FATAL EXCEPTION: main
07-13 01:02:42.152: E/AndroidRuntime(972): Process: com.example.firstproject, PID: 972
07-13 01:02:42.152: E/AndroidRuntime(972): java.lang.NullPointerException
07-13 01:02:42.152: E/AndroidRuntime(972):  at com.example.firstproject.MyOnClickListener.onClick(MyOnClickListener.java:14)
07-13 01:02:42.152: E/AndroidRuntime(972):  at android.view.View.performClick(View.java:4424)
07-13 01:02:42.152: E/AndroidRuntime(972):  at android.view.View$PerformClick.run(View.java:18383)
07-13 01:02:42.152: E/AndroidRuntime(972):  at android.os.Handler.handleCallback(Handler.java:733)
07-13 01:02:42.152: E/AndroidRuntime(972):  at android.os.Handler.dispatchMessage(Handler.java:95)
07-13 01:02:42.152: E/AndroidRuntime(972):  at android.os.Looper.loop(Looper.java:137)
07-13 01:02:42.152: E/AndroidRuntime(972):  at android.app.ActivityThread.main(ActivityThread.java:4998)
07-13 01:02:42.152: E/AndroidRuntime(972):  at java.lang.reflect.Method.invokeNative(Native Method)
07-13 01:02:42.152: E/AndroidRuntime(972):  at java.lang.reflect.Method.invoke(Method.java:515)
07-13 01:02:42.152: E/AndroidRuntime(972):  at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:777)
07-13 01:02:42.152: E/AndroidRuntime(972):  at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:593)
07-13 01:02:42.152: E/AndroidRuntime(972):  at dalvik.system.NativeStart.main(Native Method)

Answer 1

您可以获取val1和val2值，并且可以像这样实现button侦听器是同一个类。

  public class MainActivity extends Activity {
     Button button,btnAdd,btnSub;
     EditText val1,val2;
    double num1,num2,res;
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        val1 = (EditText) findViewById(R.id.txtVal1);
        val2 = (EditText) findViewById(R.id.txtVal2);
        btnAdd = (Button) findViewById(R.id.btnSum);
        btnSub = (Button) findViewById(R.id.btnDiff);
        btnAdd.setOnClickListener(new OnClickListener(){
          public void onClick(View view) {
              num1 = Double.parseDouble(val1.getText().toString());
              num2 = Double.parseDouble(val2.getText().toString());
              res = num1 + num2;
              Toast.makeText(MainActivity.this, String.valueOf(res), Toast.LENGTH_SHORT).show();
              }
          });
       }

    @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;
    }
}

Answer 2

从变量EditText获取num1值时，您已经获得了null object reference of EditText val1。因此，请使用 caller 访问此变量。

public void onClick(View view) {
        num1 = Double.parseDouble(caller.val1.getText().toString());
        num2 = Double.parseDouble(caller.val2.getText().toString());
        res = num1 + num2;
        Toast.makeText(caller, String.valueOf(res), Toast.LENGTH_SHORT).show();
}

Answer 3

您需要

一个Context引用。 您可以在创建Class时显式传递为参数。 所以你的代码应该是，

公共类 MyOnClickListener 扩展 MainActivity 实现 OnClickListener{

MainActivity caller;
public MyOnClickListener(MainActivity activity) {
this.caller = activity;
}
public void onClick(View view) {
    num1 = Double.parseDouble(val1.getText().toString());
    num2 = Double.parseDouble(val2.getText().toString());
    res = num1 + num2;
    Toast.makeText(caller, String.valueOf(res), Toast.LENGTH_SHORT).show();
}
}