Question

So whenever the these two lines are commented out its fine, but with them it crashes.

EditText convertValue = (EditText) findViewById(R.id.et_value_convert);
TextView convertResult = (TextView) findViewById(R.id.txt_result);

here’s my full class code, xml is below.

package com.doubleelite.maxtest;

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

public class ConverterActivity extends Activity {

EditText convertValue = (EditText) findViewById(R.id.et_value_convert);
TextView convertResult = (TextView) findViewById(R.id.txt_result);

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

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

// Custom method to handle button clicks.
public void onClick(View v) {
    switch(v.getId()) {

    case R.id.btn_to_cel:
        convertResult.setText(String.valueOf(convertFahrenheitToCelsius(Integer.valueOf(convertValue.getText().toString()))));
    }
}

private float convertFahrenheitToCelsius(float f) {
    return (f-32) * (5/9);
}

private float convertCelsiusToFahrenheit(float c) {
    return c * (9/5) + 32;
}

}

here is the 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="match_parent"
android:layout_height="match_parent"
android:orientation="vertical"
tools:context=".ConverterActivity" >

<EditText
    android:id="@+id/et_value_convert"
    android:layout_width="match_parent"
    android:layout_height="wrap_content"
    android:ems="10"
    android:hint="@string/text_value_convert"
    android:inputType="number"
    />

<Button
    android:id="@+id/btn_to_cel"
    android:layout_width="match_parent"
    android:layout_height="wrap_content"
    android:text="@string/button_celsius"
    android:onClick="onClick"
    />

<Button
    android:id="@+id/btn_to_fahr"
    android:layout_width="match_parent"
    android:layout_height="wrap_content"
    android:text="@string/button_fahr"
    android:onClick="onClick"
    />

<TextView
    android:id="@+id/txt_result"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:layout_gravity="center"
    android:text="Medium Text"
    android:textAppearance="?android:attr/textAppearanceMedium"
    tools:ignore="HardcodedText" />

</LinearLayout>

Answer 1

Currently you are trying to find view’s before setting layout for Activity.change your code as :

public class ConverterActivity extends Activity {

EditText convertValue ;
TextView convertResult;

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

 convertValue = (EditText) findViewById(R.id.et_value_convert);
 convertResult = (TextView) findViewById(R.id.txt_result);
}

 ///YOUR CODE HERE