Generate a random alphabet

How to generate a random number in android? In my activity , it has one button, textView and editText. The user firstly need to type an alphabet between A-D. When button is clicked, it will display an alphabet. The alphabet should not same with the input.

The alphabet will then deleted from the list.

Assume the list is {A,B,C,D} If user type A, the system will display any alphabet except A. Let said the output is B, then B will deleted from the list. So if the button clicked again, it will not display B again.

@Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        final Random r = new Random();
        input=(EditText)findViewById(R.id.editText);
        a=input.getText().toString();
        btnNext = (Button) findViewById(R.id.button);
        Name=(TextView)findViewById(R.id.textView);
        btnNext.setOnClickListener(new View.OnClickListener() {  //if next button is clicked
            @Override
            public void onClick(View v) {
                if(a=="A") 
                {
                    //what should write here ? 
                }
                Name.setText(Integer.toString(r.nextInt(100) + 1));
            }
        });
    }

This is what I've tried, but it is used to generate a random number. Any hints ? Thanks