Android calculator program crashing

Question

I made a basic android program as a writing exercise. But it crashes whenever it opens. I tried it on a phone, but sadly my android studio emulator doesn't work on my computer so my debugging abilities are not quite a lot. It builds fine so I guess its not a syntax error or something.

AndroidManifest.xml:

<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
    package="com.net.crackerjacksystems.talkies">

    <application
        android:allowBackup="true"
        android:icon="@mipmap/ic_launcher"
        android:label="@string/app_name"
        android:supportsRtl="true"
        android:theme="@style/AppTheme">
        <activity android:name=".MainActivity">
            <intent-filter>
                <action android:name="android.intent.action.MAIN" />

                <category android:name="android.intent.category.LAUNCHER" />
            </intent-filter>
        </activity>
    </application>

</manifest>

activity_main.xml:

<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout
    xmlns:android="http://schemas.android.com/apk/res/android"
    android:theme="@style/AppTheme"
    android:layout_width="fill_parent"
    android:layout_height="fill_parent"
    android:orientation="vertical">
<Button
    android:id="@+id/Zero"
    android:layout_width="260dp"
    android:layout_height="wrap_content"
    android:layout_alignParentBottom="true"
    android:text="@string/Button0"
    android:textSize="40sp"/>
<Button
    android:id="@+id/One"
    android:layout_width="130dp"
    android:layout_height="wrap_content"
    android:layout_above="@id/Zero"
    android:text="@string/Button1"
    android:textSize="40sp"/>
<Button
    android:id="@+id/Two"
    android:layout_width="130dp"
    android:layout_height="wrap_content"
    android:layout_above="@id/Zero"
    android:layout_toRightOf="@+id/One"
    android:layout_toEndOf="@+id/One"
    android:text="@string/Button2"
    android:textSize="40sp"/>
<Button
    android:id="@+id/Three"
    android:layout_width="130dp"
    android:layout_height="wrap_content"
    android:layout_above="@id/Zero"
    android:layout_toRightOf="@+id/One"
    android:layout_toEndOf="@+id/Two"
    android:text="@string/Button3"
    android:textSize="40sp"/>
<Button
    android:id="@+id/Four"
    android:layout_width="130dp"
    android:layout_height="wrap_content"
    android:layout_above="@+id/One"
    android:text="@string/Button4"
    android:textSize="40sp"/>
<Button
    android:id="@+id/Five"
    android:layout_width="130dp"
    android:layout_height="wrap_content"
    android:layout_above="@+id/One"
    android:layout_toRightOf="@id/Four"
    android:layout_toEndOf="@id/Four"
    android:text="@string/Button5"
    android:textSize="40sp"/>
<Button
    android:id="@+id/Six"
    android:layout_width="130dp"
    android:layout_height="wrap_content"
    android:layout_above="@+id/One"
    android:layout_toRightOf="@id/Five"
    android:layout_toEndOf="@id/Five"
    android:text="@string/Button6"
    android:textSize="40sp"/>
<Button
    android:id="@+id/Seven"
    android:layout_width="130dp"
    android:layout_height="wrap_content"
    android:layout_above="@+id/Four"
    android:text="@string/Button7"
    android:textSize="40sp"/>
<Button
    android:id="@+id/Eight"
    android:layout_width="130dp"
    android:layout_height="wrap_content"
    android:layout_above="@+id/Five"
    android:layout_toRightOf="@id/Seven"
    android:layout_toEndOf="@id/Seven"
    android:text="@string/Button8"
    android:textSize="40sp"/>
<Button
    android:id="@+id/Nine"
    android:layout_width="130dp"
    android:layout_height="wrap_content"
    android:layout_above="@+id/Six"
    android:layout_toRightOf="@id/Eight"
    android:layout_toEndOf="@id/Eight"
    android:text="@string/Button9"
    android:textSize="40sp"/>
<Button
    android:id="@+id/Add"
    android:layout_width="96dp"
    android:layout_height="wrap_content"
    android:layout_above="@+id/Seven"
    android:text="@string/ButtonAdd"
    android:textSize="40sp"/>
<Button
    android:id="@+id/Subtract"
    android:layout_width="96dp"
    android:layout_height="wrap_content"
    android:layout_above="@+id/Eight"
    android:layout_toRightOf="@id/Add"
    android:layout_toEndOf="@id/Add"
    android:text="@string/ButtonSubtract"
    android:textSize="40sp"/>
<Button
    android:id="@+id/Multiply"
    android:layout_width="96dp"
    android:layout_height="wrap_content"
    android:layout_above="@+id/Eight"
    android:layout_toRightOf="@id/Subtract"
    android:layout_toEndOf="@id/Subtract"
    android:text="@string/ButtonMultiply"
    android:textSize="40sp"/>
<Button
    android:id="@+id/Divide"
    android:layout_width="98dp"
    android:layout_height="wrap_content"
    android:layout_above="@+id/Eight"
    android:layout_toRightOf="@id/Multiply"
    android:layout_toEndOf="@id/Multiply"
    android:text="@string/ButtonDivide"
    android:textSize="40sp"/>
<Button
    android:id="@+id/Clear"
    android:layout_width="192dp"
    android:layout_height="wrap_content"
    android:layout_above="@+id/Add"
    android:text="@string/ButtonClear"
    android:textSize="40sp"/>
<Button
    android:id="@+id/AllClear"
    android:layout_width="192dp"
    android:layout_height="wrap_content"
    android:layout_above="@+id/Add"
    android:layout_toRightOf="@id/Clear"
    android:layout_toEndOf="@id/Clear"
    android:text="@string/ButtonAllClear"
    android:textSize="40sp"/>
<Button
    android:id="@+id/Equals"
    android:layout_width="192dp"
    android:layout_height="wrap_content"
    android:layout_toRightOf="@id/Zero"
    android:layout_toEndOf="@id/Zero"
    android:layout_alignParentBottom="true"
    android:text="@string/ButtonEqual"
    android:textSize="40sp"/>
<TextView
    android:id="@+id/Screen"
    android:layout_width="match_parent"
    android:layout_height="wrap_content"
    android:layout_above="@+id/Clear"
    android:textSize="82sp"
    />
</RelativeLayout>

MainActivity.java:

package com.net.crackerjacksystems.talkies;
import android.app.Activity;
import android.os.Bundle;
import android.view.View;
import android.widget.Button;
import android.view.View.OnClickListener;
import android.widget.TextView;

public class MainActivity extends Activity {
int first = 0, second = 0;
boolean onFirst = true;
char op;
Button zero = (Button)findViewById(R.id.Zero);
Button one = (Button)findViewById(R.id.One);
Button two = (Button)findViewById(R.id.Two);
Button three = (Button)findViewById(R.id.Three);
Button four = (Button)findViewById(R.id.Four);
Button five = (Button)findViewById(R.id.Five);
Button six = (Button)findViewById(R.id.Six);
Button seven = (Button)findViewById(R.id.Seven);
Button eight = (Button)findViewById(R.id.Eight);
Button nine = (Button)findViewById(R.id.Nine);
Button add = (Button)findViewById(R.id.Add);
Button sub = (Button)findViewById(R.id.Subtract);
Button mul = (Button)findViewById(R.id.Multiply);
Button div = (Button)findViewById(R.id.Divide);
Button cl = (Button)findViewById(R.id.Clear);
Button acl = (Button)findViewById(R.id.AllClear);
Button eql = (Button)findViewById(R.id.Equals);
TextView screen = (TextView)findViewById(R.id.Screen);
@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);
    main();
}
void main(){
    screen.setText(first);
    zero.setOnClickListener(new OnClickListener() {
        @Override
        public void onClick(View v) {
            if(onFirst){first = (first*10);screen.setText(first);}
            else{second = (second*10);screen.setText(second);}
        }
    });
    one.setOnClickListener(new OnClickListener() {
        @Override
        public void onClick(View v) {
            if(onFirst){first = (first*10)+1;screen.setText(first);}
            else{second = (second*10);screen.setText(second);}
        }
    });
    two.setOnClickListener(new OnClickListener() {
        @Override
        public void onClick(View v) {
            if(onFirst){first = (first*10)+2;screen.setText(first);}
            else{second = (second*10);screen.setText(second);}
        }
    });
    three.setOnClickListener(new OnClickListener() {
        @Override
        public void onClick(View v) {
            if(onFirst){first = (first*10)+3;screen.setText(first);}
            else{second = (second*10);screen.setText(second);}
        }
    });
    four.setOnClickListener(new OnClickListener() {
        @Override
        public void onClick(View v) {
            if(onFirst){first = (first*10)+4;screen.setText(first);}
            else{second = (second*10);screen.setText(second);}
        }
    });
    five.setOnClickListener(new OnClickListener() {
        @Override
        public void onClick(View v) {
            if(onFirst){first = (first*10)+5;screen.setText(first);}
            else{second = (second*10);screen.setText(second);}
        }
    });
    six.setOnClickListener(new OnClickListener() {
        @Override
        public void onClick(View v) {
            if(onFirst){first = (first*10)+6;screen.setText(first);}
            else{second = (second*10);screen.setText(second);}
        }
    });
    seven.setOnClickListener(new OnClickListener() {
        @Override
        public void onClick(View v) {
            if(onFirst){first = (first*10)+7;screen.setText(first);}
            else{second = (second*10);screen.setText(second);}
        }
    });
    eight.setOnClickListener(new OnClickListener() {
        @Override
        public void onClick(View v) {
            if(onFirst){first = (first*10)+8;screen.setText(first);}
            else{second = (second*10);screen.setText(second);}
        }
    });
    nine.setOnClickListener(new OnClickListener() {
        @Override
        public void onClick(View v) {
            if(onFirst){first = (first*10)+9;screen.setText(first);}
            else{second = (second*10);screen.setText(second);}
        }
    });
    add.setOnClickListener(new OnClickListener() {
        @Override
        public void onClick(View v) {
            screen.setText(null);
            op = '+';
            onFirst = false;
        }
    });
    sub.setOnClickListener(new OnClickListener() {
        @Override
        public void onClick(View v) {
            screen.setText(null);
            op = '-';
            onFirst = false;
        }
    });
    mul.setOnClickListener(new OnClickListener() {
        @Override
        public void onClick(View v) {
            screen.setText(null);
            op = '*';
            onFirst = false;
        }
    });
    div.setOnClickListener(new OnClickListener() {
        @Override
        public void onClick(View v) {
            screen.setText(null);
            op = '/';
            onFirst = false;
        }
    });
    cl.setOnClickListener(new OnClickListener() {
        @Override
        public void onClick(View v) {
            screen.setText(null);
            if(onFirst){first = 0;}
            else{second = 0;}
        }
    });
    acl.setOnClickListener(new OnClickListener() {
        @Override
        public void onClick(View v) {
            screen.setText(null);
            first = 0;
            second = 0;
        }
    });
    eql.setOnClickListener(new OnClickListener() {
        @Override
        public void onClick(View v) {
            if(op == '+'){screen.setText(first+second);}
            if(op == '-'){screen.setText(first-second);}
            if(op == '*'){screen.setText(String.valueOf(first*second));}
            if(op == '/'){screen.setText(String.valueOf(first/second));}
        }
    });
}
}

strings.xml:

<resources>
<string name="app_name">Calculator</string>
<string name="Button1">1</string>
<string name="Button2">2</string>
<string name="Button3">3</string>
<string name="Button4">4</string>
<string name="Button5">5</string>
<string name="Button6">6</string>
<string name="Button7">7</string>
<string name="Button8">8</string>
<string name="Button9">9</string>
<string name="Button0">0</string>
<string name="ButtonAdd">+</string>
<string name="ButtonSubtract">—</string>
<string name="ButtonMultiply">✕</string>
<string name="ButtonDivide">÷</string>
<string name="ButtonClear">C</string>
<string name="ButtonAllClear">AC</string>
<string name="ButtonEqual">=</string>
</resources>

Just to clarify, its a really simple calculator.
Would be nice if anyone could help over why this crashes.

Possible duplicate of [What is a NullPointerException, and how do I fix it?](http://stackoverflow.com/questions/218384/what-is-a-nullpointerexception-and-how-do-i-fix-it) — Sagar Jogadia, Jul 27 '16 at 14:55

Bill · Accepted Answer · 2016-07-27T15:44:52.170

Move all your findViewById() calls inside your onCreate() as it won't be able to "find" before you call setContentView().

private Button zero, one, two, three, four, five, six, seven, eight, nine, add, sub, mul, div, cl, acl, eql;
private TextView screen;

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

    zero = (Button)findViewById(R.id.Zero);
    one = (Button)findViewById(R.id.One);
    two = (Button)findViewById(R.id.Two);
    three = (Button)findViewById(R.id.Three);
    four = (Button)findViewById(R.id.Four);
    five = (Button)findViewById(R.id.Five);
    six = (Button)findViewById(R.id.Six);
    seven = (Button)findViewById(R.id.Seven);
    eight = (Button)findViewById(R.id.Eight);
    nine = (Button)findViewById(R.id.Nine);
    add = (Button)findViewById(R.id.Add);
    sub = (Button)findViewById(R.id.Subtract);
    mul = (Button)findViewById(R.id.Multiply);
    div = (Button)findViewById(R.id.Divide);
    cl = (Button)findViewById(R.id.Clear);
    acl = (Button)findViewById(R.id.AllClear);
    eql = (Button)findViewById(R.id.Equals);
    screen = (TextView)findViewById(R.id.Screen);

    main();
}

But then won't it go out of scope when I go to the main? I think its better to declare all the objects in the start and then initialize it in the onCreate. — Udit Dey, Jul 27 '16 at 15:32

Android calculator program crashing

1 Answers1