<?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"
    >
<EditText android:id="@+id/edittext"
    android:layout_width="fill_parent" 
    android:layout_height="wrap_content" 
    />
<Button android:id="@+id/button"
    android:layout_width="fill_parent"
    android:layout_height="fill_parent"
    android:text="Ich bin ein Button"
    android:visibility="invisible"
    />
</LinearLayout>

import android.os.Bundle;
import android.widget.EditText;
import android.widget.Button;
import android.view.View;
import android.view.KeyEvent;

public class MainActivity extends Activity implements View.OnKeyListener
{
    /** Called when the activity is first created. */
    @Override
    public void onCreate(Bundle savedInstanceState)
    {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.main);
    
        EditText edit = (EditText) findViewById(R.id.edittext);
        edit.setOnKeyListener(this);
    }
    public boolean onKey(View v, int keyCode, KeyEvent event)
    {
        Button button = (Button) findViewById(R.id.button);
        button.setVisibility(0);
        return false;
    }
}

final EditText Stand_scan = (EditText) findViewById(R.id.stand_scannen_edit);
Stand_scan_var = Stand_scan.getText().toString();
        
Stand_scan.addTextChangedListener(new TextWatcher() {

            public void beforeTextChanged(CharSequence arg0, int arg1, int arg2, int arg3) {
                throw new UnsupportedOperationException("Not supported yet.");
            }

            public void onTextChanged(CharSequence arg0, int arg1, int arg2, int arg3) {
                throw new UnsupportedOperationException("Not supported yet.");
            }

            public void afterTextChanged(Editable arg0) {
                OK_Button.setVisibility(View.VISIBLE); // Hier fliegt mir die APP um die Ohren!
                throw new UnsupportedOperationException("Not supported yet.");
            }
        });

final EditText Stand_scan = (EditText) findViewById(R.id.stand_scannen_edit);
        Stand_scan_var = Stand_scan.getText().toString();

        TextWatcher textWatcher = new TextWatcher() {

            public void onTextChanged(CharSequence s, int start, int before, int count) {

                

            }
            public void afterTextChanged(Editable arg0) {

                OK_Button.setVisibility(View.VISIBLE);

            }
            public void beforeTextChanged(CharSequence s, int start, int count, int after) {

            }
            };


        Stand_scan.addTextChangedListener(textWatcher);