Question

Had a little idea for a personal app today. Never ever attempted Android so… any help is appreciated.

What I want is 2 input fields, take those values & * them against each other giving the result after tapping the calc button.

It looks OK to me, but like I said I am not all that good with Java and new to Android. My issue is when I tap the Calc button it force closes/stops working. Thanks in advance!

activity_main.xml

<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"
    tools:context=".MainActivity" 

    >

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

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

    <Button
        android:layout_weight="1"
        android:layout_width="0dp"
        android:layout_height="wrap_content" 
        android:id="@+id/calc"
        android:text="@string/calc"
        android:onClick="calc"/>

    <EditText
        android:layout_weight="1"
        android:layout_width="0dp" 
        android:layout_height="wrap_content" 
        android:id="@+id/output"
        android:inputType="number"
        />



</LinearLayout>

MainActivity.java

package com.example.iracingfuelcalc;

import android.app.Activity;
import android.os.Bundle;
import android.view.View;
import android.widget.Button;
import android.widget.EditText;

public class MainActivity extends Activity implements android.view.View.OnClickListener {


    Button calc;
    EditText numLaps,fuel, output;

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

        fuel = (EditText)findViewById(R.id.fuel);
        numLaps = (EditText)findViewById(R.id.numLaps);
        output = (EditText)findViewById(R.id.output);
        calc = (Button)findViewById(R.id.calc);


        calc.setOnClickListener(this);  

    }
public void onClick(View arg0){

    int fuelValue = -1;
    try {
    fuelValue = Integer.parseInt(fuel.getText().toString());
    }
    catch (NumberFormatException e)
    {
    //you don't have to do much here since fuelValue already has a default value (-1)
    }

    int lapsValue = -1;
    try {
        lapsValue = Integer.parseInt(numLaps.getText().toString());
    }
    catch (NumberFormatException e)
    {

    }

    int result = 0;



        result = fuelValue * lapsValue; 

    result += Integer.parseInt(output.getText().toString());
    output.setText("" + result);

}

}

strings.xml

<?xml version="1.0" encoding="utf-8"?>
<resources>

    <string name="app_name">iRacing Fuel Calc</string>
    <string name="fuel">Fuel per lap</string>
    <string name="numLaps">Number of laps</string>
    <string name="calc">Calculate</string>
    <string name="menu_settings">Settings</string>
    <string name="title_activity_display_message">TEST</string>
    <string name="output">0</string>

</resources>

Answer 1

Your first problem is that you don’t actually give a value to output. In onCreate() do

output = (EditText)findViewById(R.id.output);

You may also have a problem with the Integer parsing since anything that is not a number will throw a NumberFormatException. Make sure to only enter numbers or to set the EditText to accept or validate programmatically. One way to validate is by using the try/catch block. E.g.

int fuelValue = -1;
try {
fuelValue = Integer.parseInt(fuel.getText().toString());
}
catch (NumberFormatException e)
{
//you don't have to do much here since fuelValue already has a default value (-1)
}

If you want, you can set the EditText to accept in only numbers, via android:inputType. The docs have more info.

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

android:inputType = "number"/>