java - Android Studio-应用已停止

请问哪里有问题？
这是我的第一个应用程序，但我真的不知道，哪里有问题，有人可以帮助我吗？
如果我启动它，它将显示消息：不幸的是helloworld_01已停止。
非常感谢您的答复和帮助！

码

package com.example.martin.helloworld_01;

import android.app.AlertDialog;
import android.support.v7.app.ActionBarActivity;
import android.os.Bundle;
import android.view.Gravity;
import android.view.Menu;
import android.view.MenuItem;
import android.view.View;
import android.widget.EditText;
import android.widget.TextView;




public class MainActivity extends ActionBarActivity {
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
    }

    public  int GetNumber1() {
        EditText text1 = (EditText)findViewById(R.id.editText);
        int num1 = Integer.parseInt(text1.getText().toString());
        return num1;
    }
    public int GetNumber2(){
        EditText text2 = (EditText)findViewById(R.id.editText2);
        int num2 = Integer.parseInt(text2.getText().toString());
        return num2;
    }


    public int num1 = GetNumber1();
    public int num2 = GetNumber2();
    public TextView result = (TextView)findViewById(R.id.textView);

    public void Sum(View v) {
        int sum = num1+num2;
        result.setText(Integer.toString(sum));
    }

    public void Subtraction(View v){
        int subtr= num1-num2;
        result.setText(Integer.toString(subtr));
    }

    public void Multiplication (View v){
        int mult = num1*num2;
        result.setText(Integer.toString(mult));
    }

    public void Division(View v){
        if (num2==0)
        {
            AlertDialog.Builder popupBuilder = new AlertDialog.Builder(this);
            TextView myMsg = new TextView(this);
            myMsg.setText("You can not division with 0!");
            myMsg.setGravity(Gravity.CENTER_HORIZONTAL);
            popupBuilder.setView(myMsg);
        }
        else {
        float divis = num1/num2;
            result.setText(Float.toString(divis));
        }
    }



    @Override
    public boolean onCreateOptionsMenu(Menu menu) {
        // Inflate the menu; this adds items to the action bar if it is present.
        getMenuInflater().inflate(R.menu.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();

        //noinspection SimplifiableIfStatement
        if (id == R.id.action_settings) {
            return true;
        }

        return super.onOptionsItemSelected(item);
    }
}

LogCat

03-30 12:11:48.554    1925-1925/com.example.martin.helloworld_01 I/art﹕ Not late-enabling -Xcheck:jni (already on)
03-30 12:11:48.805    1925-1925/com.example.martin.helloworld_01 D/AndroidRuntime﹕ Shutting down VM
    --------- beginning of crash
03-30 12:11:48.806    1925-1925/com.example.martin.helloworld_01 E/AndroidRuntime﹕ FATAL EXCEPTION: main
    Process: com.example.martin.helloworld_01, PID: 1925
    java.lang.RuntimeException: Unable to instantiate activity ComponentInfo{com.example.martin.helloworld_01/com.example.martin.helloworld_01.MainActivity}: java.lang.NullPointerException: Attempt to invoke virtual method 'android.view.View android.view.Window.findViewById(int)' on a null object reference
            at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2209)
            at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2360)
            at android.app.ActivityThread.access$800(ActivityThread.java:144)
            at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1278)
            at android.os.Handler.dispatchMessage(Handler.java:102)
            at android.os.Looper.loop(Looper.java:135)
            at android.app.ActivityThread.main(ActivityThread.java:5221)
            at java.lang.reflect.Method.invoke(Native Method)
            at java.lang.reflect.Method.invoke(Method.java:372)
            at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:899)
            at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:694)
     Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'android.view.View android.view.Window.findViewById(int)' on a null object reference
            at android.app.Activity.findViewById(Activity.java:2071)
            at com.example.martin.helloworld_01.MainActivity.GetNumber1(MainActivity.java:24)
            at com.example.martin.helloworld_01.MainActivity.<init>(MainActivity.java:35)
            at java.lang.reflect.Constructor.newInstance(Native Method)
            at java.lang.Class.newInstance(Class.java:1572)
            at android.app.Instrumentation.newActivity(Instrumentation.java:1065)
            at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2199)
            at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2360)
            at android.app.ActivityThread.access$800(ActivityThread.java:144)
            at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1278)
            at android.os.Handler.dispatchMessage(Handler.java:102)
            at android.os.Looper.loop(Looper.java:135)
            at android.app.ActivityThread.main(ActivityThread.java:5221)
            at java.lang.reflect.Method.invoke(Native Method)
            at java.lang.reflect.Method.invoke(Method.java:372)
            at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:899)
            at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:694)

XML格式

<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=".MainActivity">

    <EditText
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:inputType="number"
        android:ems="10"
        android:id="@+id/editText"
        android:layout_alignParentTop="true"
        android:layout_alignParentLeft="true"
        android:layout_alignParentStart="true"
        android:layout_marginTop="73dp"
        android:hint="Number1" />

    <EditText
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:inputType="number"
        android:ems="10"
        android:id="@+id/editText2"
        android:layout_below="@+id/editText"
        android:layout_alignParentLeft="true"
        android:layout_alignParentStart="true"
        android:layout_marginTop="49dp"
        android:hint="Number2" />

    <TextView
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:textAppearance="?android:attr/textAppearanceLarge"
        android:text="Result"
        android:id="@+id/textView"
        android:textIsSelectable="false"
        android:textSize="50dp"
        android:layout_marginBottom="43dp"
        android:layout_alignParentBottom="true"
        android:layout_centerHorizontal="true" />

    <Button
        android:layout_width="50dp"
        android:layout_height="50dp"
        android:text="+"
        android:id="@+id/button"
        android:layout_below="@+id/editText2"
        android:layout_alignParentLeft="true"
        android:layout_alignParentStart="true"
        android:layout_marginTop="42dp"
        android:onClick="Sum" />

    <Button
        android:layout_width="50dp"
        android:layout_height="50dp"
        android:text="-"
        android:id="@+id/button2"
        android:layout_alignTop="@+id/button"
        android:layout_alignLeft="@+id/textView"
        android:layout_alignStart="@+id/textView"
        android:onClick="Subtraction" />

    <Button
        android:layout_width="50dp"
        android:layout_height="50dp"
        android:text="*"
        android:id="@+id/button3"
        android:layout_alignTop="@+id/button2"
        android:layout_alignRight="@+id/textView"
        android:layout_alignEnd="@+id/textView"
        android:onClick="Multiplication" />

    <Button
        android:layout_width="50dp"
        android:layout_height="50dp"
        android:text="/"
        android:id="@+id/button4"
        android:layout_alignTop="@+id/button3"
        android:layout_alignParentRight="true"
        android:layout_alignParentEnd="true"
        android:onClick="Division" />

</RelativeLayout>

最佳答案

您需要在onCreate()中实例化view元素，然后再在类中的其他任何地方使用它们。我们将它们声明为类成员，因此其他函数也可以访问它们并在onCreate()中实例化它们。
您需要在按钮按下时执行的函数中调用GetNumber()方法，因为用户在此之前已经在文本框中输入了一些内容。

您可以尝试以下实现：

public class MainActivity extends ActionBarActivity
{
    EditText text1, text2;
    TextView result

    @Override
    protected void onCreate(Bundle savedInstanceState)
    {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        text1 = (EditText)findViewById(R.id.editText);
        text2 = (EditText)findViewById(R.id.editText2);
        result = (TextView)findViewById(R.id.textView);
    }

    public  int GetNumber1()
    {
        int num1 = Integer.parseInt(text1.getText().toString());
        return num1;
    }
    public int GetNumber2()
    {
        int num2 = Integer.parseInt(text2.getText().toString());
        return num2;
    }

    public void Sum(View v)
    {
        int num1 = GetNumber1();
        int num2 = GetNumber2();

        int sum = num1+num2;
        result.setText(Integer.toString(sum));
    }

    public void Subtraction(View v)
    {
        int num1 = GetNumber1();
        int num2 = GetNumber2();

        int subtr= num1-num2;
        result.setText(Integer.toString(subtr));
    }

    public void Multiplication (View v)
    {
        int num1 = GetNumber1();
        int num2 = GetNumber2();

        int mult = num1*num2;
        result.setText(Integer.toString(mult));
    }

    public void Division(View v)
    {
        int num1 = GetNumber1();
        int num2 = GetNumber2();

        if (num2==0)
        {
            AlertDialog.Builder popupBuilder = new AlertDialog.Builder(this);
            TextView myMsg = new TextView(this);
            myMsg.setText("You can not division with 0!");
            myMsg.setGravity(Gravity.CENTER_HORIZONTAL);
            popupBuilder.setView(myMsg);
        }
        else
        {
            float divis = num1/num2;
            result.setText(Float.toString(divis));
        }
    }

    @Override
    public boolean onCreateOptionsMenu(Menu menu) {
        // Inflate the menu; this adds items to the action bar if it is present.
        getMenuInflater().inflate(R.menu.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();

        //noinspection SimplifiableIfStatement
        if (id == R.id.action_settings) {
            return true;
        }

        return super.onOptionsItemSelected(item);
    }
}