Why does my android app crash on clicking the button?

Question

I created this simple app using android studio, which is supposed to calculate x percent of y and it shows no errors in the code in the android studio window however when I click the button in the app it crashes????? I am new to coding, I would appreciate any help.

package com.sabri.myapplication;

import android.os.Bundle;
import android.support.v7.app.AppCompatActivity;
import android.support.v7.widget.Toolbar;
import android.view.Menu;
import android.view.MenuItem;
import android.view.View; 
import android.widget.Button;
import android.widget.EditText;
import android.widget.TextView;

public class MainActivity extends AppCompatActivity {

TextView textView;
EditText percentText;
EditText numberText;


@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);
    Toolbar toolbar = (Toolbar) findViewById(R.id.toolbar);
    setSupportActionBar(toolbar);


     Button button = (Button) findViewById(R.id.button);
     if (button == null) throw new AssertionError();
    button.setOnClickListener(new View.OnClickListener() {
         @Override
        public void onClick(View view) {
             float percentage = Float.parseFloat(percentText.getText().toString());
             float dec = percentage / 100;
             float total = dec * Float.parseFloat(numberText.getText().toString());
             textView.setText(Float.toString(total));

         }
     });

}

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

@Override
public boolean onOptionsItemSelected(MenuItem item) {
    // Handle action bar item clicks here. The action bar will
    // automatically handle clicks on the Home/Up button, so long
    // as you specify a parent activity in AndroidManifest.xml.
    int id = item.getItemId();

    //noinspection SimplifiableIfStatement
    if (id == R.id.action_settings) {
        return true;
    }

    return super.onOptionsItemSelected(item);
}
}

Answer 1

I'm assuming that you're getting a NullPointerException error. You never initialized the percentText and numberText variables.

Try putting this in your onCreate() method where you assign your Button variable

textView = (TextView)findViewById(R.id.textView);
numberText = (EditText)findviewbyid(R.id.numberText);
percentText = (EditText)findviewbyid(R.id.percentText);

Always make sure that your variables are initialized before you ever try doing anything to/with them. Might be a good idea to create a new method intializeViewVariables() in which you initialize all of your View variables you plan on working with and just calling that method in your onCreate() .

And please, if you're asking for help with a crash/compiler errors, include the error message in your post!