Android : must write something before save

i have a edit text that i hopefully will be able to save soon, but to be able to save i have you would have to write something, i just need the code for checking if there is anything written in the edit text.

when i press the goback button i want to go back if i have not written anything, and i want to save if it has something written in it.

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

        Button button = (Button)findViewById(R.id.GoBack);
        button.setOnClickListener(new OnClickListener() {
            public void onClick(View v) {
                save();
                Intent i = new Intent(anteckningsblock.this, Notelist.class);
                startActivity(i);
            }
        });
    }

    void save() {
        try {
            File file = new File(getApplicationContext().getFilesDir(), "anteckning.txt");
            FileOutputStream fos = new FileOutputStream(file);
            DataOutputStream dos = new DataOutputStream(fos);
            dos.writeUTF("Namnet på anteckningen");
            dos.writeUTF("Innehållet");
            dos.close();
        } catch (Exception e) {
            e.printStackTrace();
        }
    }