I have been spending a little time going over some of Google’s android tutorials, and decided to throw together my own simple calculator application. I know a good bit of JAVA, and have messed around a little bit of Android back a few SDKs ago. I think the code is fine, but I just can’t get the bastard to run. I have been trying to find an answer based on my LogCat stuff, but have had no luck. First, here is my MainActivity.java:
package com.AppliedArgonautics.calculator;
import android.app.Activity;
import android.os.Bundle;
import android.view.Menu;
import android.widget.EditText;
import android.widget.TextView;
public class MainActivity extends Activity {
EditText editText1 = (EditText)findViewById(R.id.editText1);
EditText editText2 = (EditText)findViewById(R.id.editText2);
TextView textView1 = (TextView)findViewById(R.id.textView1);
@Override
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
}
@Override
public boolean onCreateOptionsMenu(Menu menu) {
getMenuInflater().inflate(R.menu.activity_main, menu);
return true;
}
public void Add(){
float num1 = Float.parseFloat(editText1.getText().toString());
float num2 = Float.parseFloat(editText2.getText().toString());
float result = num1 + num2;
textView1.setText(Float.toString(result));
}
public void Subtract(){
float num1 = Float.parseFloat(editText1.getText().toString());
float num2 = Float.parseFloat(editText2.getText().toString());
float result = num1 - num2;
textView1.setText(Float.toString(result));
}
public void Multiply(){
float num1 = Float.parseFloat(editText1.getText().toString());
float num2 = Float.parseFloat(editText2.getText().toString());
float result = num1 * num2;
textView1.setText(Float.toString(result));
}
public void Divide(){
float num1 = Float.parseFloat(editText1.getText().toString());
float num2 = Float.parseFloat(editText2.getText().toString());
float result = num1 / num2;
textView1.setText(Float.toString(result));
}
public void Clear(){
editText1.setText("");
editText2.setText("");
textView1.setText("");
}
}
My manifest file:
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
package="com.AppliedArgonautics.calculator"
android:versionCode="1"
android:versionName="1.0" >
<uses-sdk
android:minSdkVersion="8"
android:targetSdkVersion="15" />
<application
android:icon="@drawable/ic_launcher"
android:label="@string/app_name"
android:theme="@style/AppTheme" >
<activity
android:name=".MainActivity"
android:label="@string/title_activity_main" >
<intent-filter>
<action android:name="android.intent.action.MAIN" />
<category android:name="android.intent.category.LAUNCHER" />
</intent-filter>
</activity>
</application>
</manifest>
then, the activity_main.xml
<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" >
<Button
android:id="@+id/button1"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_alignParentLeft="true"
android:layout_centerVertical="true"
android:layout_marginLeft="32dp"
android:onClick="Add"
android:text="Add" />
<Button
android:id="@+id/button3"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_alignLeft="@+id/button1"
android:layout_below="@+id/button1"
android:layout_marginTop="45dp"
android:onClick="Multiply"
android:text="Multiply" />
<Button
android:id="@+id/button2"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_above="@+id/button3"
android:layout_alignParentRight="true"
android:layout_marginRight="28dp"
android:onClick="Subtract"
android:text="Subtract" />
<Button
android:id="@+id/button4"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_alignBaseline="@+id/button3"
android:layout_alignBottom="@+id/button3"
android:layout_alignLeft="@+id/button2"
android:onClick="Divide"
android:text="Divide" />
<Button
android:id="@+id/button5"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_below="@+id/button1"
android:layout_centerHorizontal="true"
android:onClick="Clear"
android:text="Clear" />
<EditText
android:id="@+id/editText1"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_alignParentTop="true"
android:layout_alignRight="@+id/button1"
android:layout_marginTop="37dp"
android:ems="10"
android:inputType="phone" >
<requestFocus />
</EditText>
<EditText
android:id="@+id/editText2"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_alignBottom="@+id/editText1"
android:layout_alignLeft="@+id/button2"
android:layout_alignParentRight="true"
android:layout_alignTop="@+id/editText1"
android:ems="10"
android:inputType="phone" />
<TextView
android:id="@+id/textView1"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_alignLeft="@+id/button5"
android:layout_below="@+id/editText1"
android:layout_marginTop="56dp"
android:text="TextView" />
</RelativeLayout>
This is what LogCat shows when I try to run it:
/Trace ( 3186): error opening trace file: No such file or directory (2)
D/AndroidRuntime( 3186): Shutting down VM
W/dalvikvm( 3186): threadid=1: thread exiting with uncaught exception (group=0x40a13300)
E/AndroidRuntime( 3186): FATAL EXCEPTION: main
E/AndroidRuntime( 3186): java.lang.RuntimeException: Unable to instantiate activity ComponentInfo{com.AppliedArgonautics.calculator/com.AppliedArgonautics.calculator.MainActivi ty}: java.lang.NullPointerException
E/AndroidRuntime( 3186): at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:1983)
E/AndroidRuntime( 3186): at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2084)
E/AndroidRuntime( 3186): at android.app.ActivityThread.access$600(ActivityThread.java:130)
E/AndroidRuntime( 3186): at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1195)
E/AndroidRuntime( 3186): at android.os.Handler.dispatchMessage(Handler.java:99)
E/AndroidRuntime( 3186): at android.os.Looper.loop(Looper.java:137)
E/AndroidRuntime( 3186): at android.app.ActivityThread.main(ActivityThread.java:4745)
E/AndroidRuntime( 3186): at java.lang.reflect.Method.invokeNative(Native Method)
E/AndroidRuntime( 3186): at java.lang.reflect.Method.invoke(Method.java:511)
E/AndroidRuntime( 3186): at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:786)
E/AndroidRuntime( 3186): at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:553)
E/AndroidRuntime( 3186): at dalvik.system.NativeStart.main(Native Method)
E/AndroidRuntime( 3186): Caused by: java.lang.NullPointerException
E/AndroidRuntime( 3186): at android.app.Activity.findViewById(Activity.java:1825)
E/AndroidRuntime( 3186): at com.AppliedArgonautics.calculator.MainActivity.<init>(MainActivity.java:10)
E/AndroidRuntime( 3186): at java.lang.Class.newInstanceImpl(Native Method)
E/AndroidRuntime( 3186): at java.lang.Class.newInstance(Class.java:1319)
E/AndroidRuntime( 3186): at android.app.Instrumentation.newActivity(Instrumentation.java:1053)
E/AndroidRuntime( 3186): at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:1974)
E/AndroidRuntime( 3186): ... 11 more
W/ActivityManager( 160): Force finishing activity com.AppliedArgonautics.calculator/.MainActivity
I would really appreciate it if someone could have a look at this. I thought it would be cool to learn android, but am getting very frustrated with my inability to get even a simple program like this to launch. Thank you.
There may be lot of issues in your code, but first to get you started:
Create
EditTextandTextViewas instance variables. Assign views to those variables inonCreate.