to check for one character:
button12.setOnClickListener(new OnClickListener() {
            @Override
            public void onClick(View v) {
                if (!textView.getText().toString().contains("-")) {
                    textView.setText(textView.getText() + "-");

What i tried to check for two characters:
button12.setOnClickListener(new OnClickListener() {
            @Override
            public void onClick(View v) {
                if (!textView.getText().toString().contains("-","+")) {
                    textView.setText(textView.getText() + "-");