<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:orientation="vertical"
    android:layout_width="fill_parent"
    android:layout_height="fill_parent"

    >
<TextView  
    android:layout_width="wrap_content" 
    android:layout_height="wrap_content" 
    android:text="@string/ref"
    android:textSize="44dp"
    android:layout_gravity="center"
    android:gravity="center"
    android:id="@+id/tvDisplay"
    />
<Button
	android:layout_width="250dp"
	android:layout_height="wrap_content"
	android:text="Füg 1 hinzu!"
	android:layout_gravity="center" 
	android:textSize="20dp"
	android:id="@+id/addB"

/> 
<Button
	android:layout_width="250dp"
	android:layout_height="wrap_content"
	android:text="Zieh 1 ab!"
	android:layout_gravity="center" 
	android:textSize="20dp"
	android:id="@+id/subB"
/>       
    
</LinearLayout>

package test.jacob;

import android.R;
import android.R.id;
import android.R.layout;
import android.app.Activity;
import android.os.Bundle;
import android.view.View;
import android.widget.Button;
import android.widget.TextView;

public class TestActivity extends Activity {
    /** Called when the activity is first created. */
	int counter;
	Button add, sub;
	TextView display;
	
	
    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState); 
        counter = 0;
        setContentView(R.layout.main); <---- Fehler
        add=(Button)findViewById(R.id.addB);<---- Fehler
        sub=(Button)findViewById(R.id.subB);<---- Fehler
        display =(TextView) findViewById(R.id.tvDisplay);<---- Fehler
        add.setOnClickListener(new View.OnClickListener() {
			
			public void onClick(View v) {
				counter = counter + 1;
				display.setText("Deine Zahl ist "+ counter);
				
			}
		});
        sub.setOnClickListener(new View.OnClickListener() {
			
			public void onClick(View v) {
				counter = counter - 1;
				display.setText("Deine Zahl ist "+ counter);
				
			}
		});
    }
}

import android.R;
import android.R.id;
import android.R.layout;