android - why does my app always crashs? -

I'm a Nob, I used the search function to get the answer but I could not find anything, so I have My code in Android studio is to test my 7 Nexus as a real device, but every time I try to run it, a message appears on my alliance, which says it has ended straightaway : Adds text to number by clicking on the S button The origin of a calculator is here, my code is:

  import android.os.Bundle; Import android App Import android.view.View; Import android.widget.Button; Import android.widget.TextView; Public classroom enhances main activity activity {int actuel; Button BTN 0 = (button) Find VVBIID (RID.btn0); Button BTN1 = find (VBBID) (VBBID1); Button Find BTN2 = (button) VVBIID (RID. BTN 2); Button BTN3 = (button) Find VVBIID (RBTN3); Button BTN4 = (button) Find VVBIID (RID. BTN4); Button BTN5 = (button) Find VVBIID (RID. BTN5); Button BTN6 = (button) Find VVBIID (RID. BTN6); Button BTN7 = (button) Find VVBIID (RID. BTN7); Button BTN8 = (button) Find VVBIID (RID.btn8); Button Find BTN 9 = (button) VVBIID (RID. BTN 9); Find textview output = (text view) VVBIID (Raidteviewview); @ Override protected void saved create (bundled Instansstet) {Suprknkret (saved Instensstet); SetContentView (R.layout.activity_main); Btn0.setOnClickListener (New View.OnClickListener () {@Override Click Public Zero (see View) {addNumbers (0, View);}}); Btn1.setOnClickListener (New View.OnClickListener () {@Override Click Public Zero (see View) {addNumbers (1, see);}}); Btn2.setOnClickListener (New View.OnClickListener () {@Override Click Public Zero (see View) {addNumbers (2, see);}}); Btn3.setOnClickListener (View new. OnClickListener () {@ Override Public Zero (click View) {addNumbers (3, Views);}}) Btn4.setOnClickListener (New View.OnClickListener () {@Override Click Public Zero (see View) {addNumbers (4, Views);}}); Btn5.setOnClickListener (New View.OnClickListener () {@Override Click Public Zero (see View) {addNumbers (5, Views);}}); Btn6.setOnClickListener (New View.OnClickListener () {@Override Click Public Zero (see View) {addNumbers (6, see);}}); Btn7.setOnClickListener (New View.OnClickListener () {@Override Click Public Zero (see View) {addNumbers (7, see);}}); Btn8.setOnClickListener (New View.OnClickListener () {@Override Click Public Zero (see view) {addNumbers (8, see);}}); Btn9.setOnClickListener (New View.OnClickListener () {@Override Click Public Zero (see View) {addNumbers (9, Views);}}); } Private Final Zero addNumbers (integer number, see view) {actuel = actuel * 10+ num; Output.setText (String.valueOf (Actuel)); }}    
 
 
 Update your code to:  
  Public classroom increases main activity activity {int actuel; Button BTN 0 = Faucet; Button BTN1 = Faucet; Button BTN2 = Faucet; Button BTN3 = Faucet; Button BTN 4 = Faucet; Button BTN5 = Faucet; Button BTN 6 = Faucet; Button BT7 = Faucet; Button BTN8 = Faucet; Button BTN 9 = Faucet; Textview output = null; @ Override protected void saved create (bundled Instansstet) {Suprknkret (saved Instensstet); SetContentView (R.layout.activity_main); BTN 0 = (button) Find VVIBID (RID BTN 0); BTN1 = (Find Button) VVBIID (RBTN1); BTN2 = (Find Button) VVBIID (RID. BTN 2); BTN3 = Find (VBibIID) (RBTN3); Find BTN 4 = (VBibIID (RBTN4); BTN 5 = Find (VBIID) (BIDN 5); BTN6 = (button) Find VVBID (RID. BTN6); BTN7 = (Find Button) VBiID (RID BTN 7); BTN 8 = Find (VBBID) (RBTN8); BTN 9 = Find VBID (RID BTN 9); Output = (TextView) FindViewById (R.id.textView); ...   
 Your app crashes because all your BTNX and output variables are zero.