How to pass user input from one activity to another in Android

Question

I'm creating a basic conversion app which converts the users input from km-knots or vice versa. At present it displays the result in the same place as where the user initially type in their figure. I'm wondering how is it possible to display the result in a new activity? Currently I've only managed to make it appear, as I said, in the same place as where the user initially type in their figure and a second afterwards the second activity pops up. What am I missing here? Please look at my code, thanks:

MainActivity.java

package winfield.joe.wind.v1;

import android.os.Bundle;
import android.app.Activity;
import android.content.Intent;
import android.view.View;
import android.widget.EditText;
import android.widget.RadioButton;
import android.widget.Toast;

public class MainActivity extends Activity {

    // public var
    private EditText text;

    // default func
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        Toast.makeText(this, "onCreate!", Toast.LENGTH_LONG).show();
        setContentView(R.layout.activity_main);
        // findViewById = Finds a view that was identified by the id attribute
        // from the XML that was processed in onCreate(Bundle).
        // (EditText) = typecast
        text = (EditText) findViewById(R.id.userInput);
    }

    //Will be executed by clicking on the calculate button because we assigned "calc" to the "onClick" Property

    public void calc(View view) {

        RadioButton toKilometers = (RadioButton) findViewById(R.id.toKilometers);
        RadioButton toKnots = (RadioButton) findViewById(R.id.toKnots);

        if (text.getText().length() == 0) {
            // if the text field is empty show the message "enter a valid number" via toast message
            Toast.makeText(this, "enter a valid number", Toast.LENGTH_LONG).show();
        } else {

            int result = R.string.userInput;
            Intent i = new Intent(MainActivity.this, SecondActivity.class);
            putExtra("userInput", result); 
            startActivity(i);

            // parse input Value from Text Field
            double inputValue = Double.parseDouble(text.getText().toString());
            // convert to...
            if (toKilometers.isChecked()) {
                text.setText(String.valueOf(convertToKM(inputValue)));
                // uncheck "to km" Button
                toKilometers.setChecked(false);
                // check "to knots" Button
                toKnots.setChecked(true);
            } else { /* if toKnots button isChecked() */
                text.setText(String.valueOf(convertToKnots(inputValue)));
                // uncheck "to knots" Button
                toKnots.setChecked(false);
                // check "to km" Button
                toKilometers.setChecked(true);
            }
        }

    }


    private void putExtra(String string, int result) {
        // TODO Auto-generated method stub
    }

    private double convertToKM(double inputValue) {
        // convert knots to km
        return (inputValue * 1.8);
    }

    private double convertToKnots(double inputValue) {
        // convert km to knots
        return (inputValue * 0.539956803);
    }

}

SecondActivity.java

package winfield.joe.wind.v1;

import android.app.Activity;
import android.app.AlertDialog;
import android.content.DialogInterface;
import android.content.Intent;
import android.os.Bundle;
import android.view.View;

public class SecondActivity extends Activity {

    Bundle extras = getIntent().getExtras();
    int result = extras.getInt("results"); 

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

    //onClick GoBack method assigned to the Go Back? button which returns the user to main activity from the second activity
    public void GoBack(View view) {

        AlertDialog.Builder builder = new AlertDialog.Builder(this);
        builder.setTitle("Are you sure?");

            // set dialog message
            builder .setCancelable(false)

                    .setPositiveButton("Convert Again",new DialogInterface.OnClickListener() {
                        public void onClick(DialogInterface dialog,int id) {
                            //You return to Main Activity  
                            Intent intent = new Intent(SecondActivity.this, MainActivity.class);
                            startActivity(intent);
                        }
                    })

                    .setNeutralButton("Back to Home",new DialogInterface.OnClickListener() {
                        public void onClick(DialogInterface dialog,int id) {
                            //You return to the home page
                            Intent intent = new Intent(Intent.ACTION_MAIN);
                            intent.addCategory(Intent.CATEGORY_HOME);
                            startActivity(intent);
                        }
                    })

                    .setNegativeButton("View Results",new DialogInterface.OnClickListener() {
                        public void onClick(DialogInterface dialog,int id) {
                            // if this button is clicked, just close the dialog box and do nothing
                            dialog.cancel();
                        }
                    });

        AlertDialog alert = builder.create();
        alert.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="fill_parent"
    android:layout_height="fill_parent"
    android:layout_alignParentBottom="true"
    android:layout_alignParentEnd="true"
    android:background="@color/bgColor"
    android:orientation="vertical">

    <!--TITLE-->

    <TextView
        android:id="@+id/titleMain"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_gravity="center"
        android:layout_marginTop="22dp"
        android:gravity="center"
        android:text="@string/titleMain"
        android:textColor="@color/textColor"
        android:textColorHint="@color/textColor"
        android:textColorLink="#ffffff"
        android:textSize="30sp" />

    <!--USER-INPUT-->

    <EditText
        android:id="@+id/userInput"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:layout_marginLeft="20dp"
        android:layout_marginRight="20dp"
        android:ems="10"
        android:hint="@+string/userInput"
        android:inputType="numberDecimal"
        android:paddingEnd="40dp"
        android:paddingStart="20dp"
        android:paddingTop="30dp" >

        <requestFocus />
    </EditText>

      <!--TWO RADIO BUTTONS-->
      <LinearLayout

          android:layout_width="match_parent"
          android:layout_height="wrap_content"
          android:paddingLeft="10dp"
          android:paddingRight="10dp">

          <!--toKNOTS-->

          <RadioButton
              android:id="@+id/toKnots"
              android:layout_width="0dp"
              android:layout_height="wrap_content"
              android:layout_gravity="center"
              android:layout_marginStart="10dp"
              android:layout_marginTop="10dp"
              android:layout_weight="0.04"
              android:checked="true"
              android:text="@string/toKnots" />

          <!--toKM-->

          <RadioButton
              android:id="@+id/toKilometers"
              android:layout_width="wrap_content"
              android:layout_height="wrap_content"
              android:layout_gravity="center"
              android:layout_marginEnd="20dp"
              android:layout_marginTop="10dp"
              android:text="@string/toKilometers" />

      </LinearLayout>

      <!--CALCULATE-->

      <Button
          android:id="@+id/valueCalc"
          android:layout_width="wrap_content"
          android:layout_height="wrap_content"
          android:layout_gravity="center"
          android:layout_marginEnd="10dp"
          android:layout_marginStart="10dp"
          android:layout_marginTop="30dp"
          android:onClick="calc"
          android:text="@string/valueCalc" />

</LinearLayout>

activity_second.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="fill_parent"
    android:layout_height="fill_parent"
    android:layout_alignParentBottom="true"
    android:layout_alignParentEnd="true"
    android:background="@color/bgColor"
    android:orientation="vertical"
    tools:context="${relativePackage}.${activityClass}" >

    <!-- TITLE -->

    <TextView
        android:id="@+id/titleResults"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_gravity="center"
        android:layout_marginTop="24dp"
        android:gravity="center"
        android:text="@string/titleResults"
        android:textColor="@color/textColor"
        android:textColorHint="@color/textColor"
        android:textColorLink="#ffffff"
        android:textSize="25sp" />

    <EditText
        android:id="@+id/results"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:layout_gravity="center"
        android:layout_margin="20dp"
        android:layout_marginTop="30dp"
        android:ems="10"
        android:hint="@string/results"
        android:inputType="numberDecimal"
        android:textColorHint="@color/textColor" >

        <requestFocus />
    </EditText>

    <Button
        android:id="@+id/GoBack"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_gravity="center"
        android:layout_marginTop="40dp"
        android:text="@string/GoBack"
        android:onClick="GoBack" />

</LinearLayout>

LOGCAT

03-09 12:27:35.692: E/AndroidRuntime(2911): java.lang.RuntimeException: Unable to start activity ComponentInfo{winfield.joe.wind.v1/winfield.joe.wind.v1.SecondActivity}: java.lang.NullPointerException
03-09 12:27:35.692: E/AndroidRuntime(2911):     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:779)
03-09 12:27:45.262: E/AndroidRuntime(2934): java.lang.RuntimeException: Unable to start activity ComponentInfo{winfield.joe.wind.v1/winfield.joe.wind.v1.SecondActivity}: java.lang.NullPointerException
03-09 12:27:45.262: E/AndroidRuntime(2934):     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:779)
03-09 12:29:52.172: E/AndroidRuntime(2989): java.lang.RuntimeException: Unable to instantiate activity ComponentInfo{winfield.joe.wind.v1/winfield.joe.wind.v1.SecondActivity}: java.lang.NullPointerException
03-09 12:29:52.172: E/AndroidRuntime(2989):     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:779)
03-09 12:32:29.322: E/AndroidRuntime(3056): java.lang.RuntimeException: Unable to instantiate activity ComponentInfo{winfield.joe.wind.v1/winfield.joe.wind.v1.SecondActivity}: java.lang.NullPointerException
03-09 12:32:29.322: E/AndroidRuntime(3056):     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:779)
03-09 12:36:29.872: E/AndroidRuntime(3112): java.lang.RuntimeException: Unable to instantiate activity ComponentInfo{winfield.joe.wind.v1/winfield.joe.wind.v1.SecondActivity}: java.lang.NullPointerException
03-09 12:36:29.872: E/AndroidRuntime(3112):     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:779)
03-09 12:44:25.022: E/AndroidRuntime(3172): java.lang.RuntimeException: Unable to instantiate activity ComponentInfo{winfield.joe.wind.v1/winfield.joe.wind.v1.SecondActivity}: java.lang.NullPointerException
03-09 12:44:25.022: E/AndroidRuntime(3172):     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:779)
03-09 12:53:38.802: E/AndroidRuntime(3242): java.lang.RuntimeException: Unable to instantiate activity ComponentInfo{winfield.joe.wind.v1/winfield.joe.wind.v1.SecondActivity}: java.lang.NullPointerException
03-09 12:53:38.802: E/AndroidRuntime(3242):     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:779)
03-09 13:19:58.122: E/AndroidRuntime(3299): java.lang.RuntimeException: Unable to instantiate activity ComponentInfo{winfield.joe.wind.v1/winfield.joe.wind.v1.SecondActivity}: java.lang.NullPointerException
03-09 13:19:58.122: E/AndroidRuntime(3299):     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:779)

Answer 1

Use below code to pass values from one Activity to Another

Intent launchResult = new Intent(MainActivity.this, SecondActivity.class);
launchResult.putExtra("output",result);

Answer 2

If you want to pass any content from one activity to another, you can do it by using putExtra with intent, it will send data from one activity to another, and in another activity get that data by using getExtras.

To send data,

String str = "value";
Intent launchResult = new Intent(MainActivity.this, SecondActivity.class);
putExtra("key", str);
startActivity(launchResult);

To receive data in another activity,

Bundle extras = getIntent().getExtras();
String str = extras.getStr("key");

Edit:

In my answer key is any unique value that must remain same in both Sender Activity and Receiver Activity.

You are getting the error because you have passed a string value where you had to pass key, and the key in receiver activity in not matching either.

See the change below, it should work

MainActivity.class

int result = R.string.userInput;
Intent i = new Intent(MainActivity.this, SecondActivity.class);
putExtra("userInput", result); 
startActivity(i);

SecondActivity.class

Bundle extras = getIntent().getExtras();
int result = extras.getInt("userInput");