Question

I would put in this code a control on EditText so that it only accepts hexadecimal numbers.
How do I go about doing this?

bin = (EditText)findViewById(R.id.editText02);
hex = (EditText)findViewById(R.id.editText3);
dec = (EditText)findViewById(R.id.editText1);
oct = (EditText)findViewById(R.id.editText04);

Answer 1

TextWatcher is also good option, however I prefer using custom filters.
thereby, simpler way is to use InputFilter and take control of every char on the fly, see example below, hope this helps

    import android.text.InputFilter;
    import android.text.InputType;

    EditText input_moodMsg; 
    // initialize this edittext etc etc
    //....
    // here comes the filter to control input on that component
    InputFilter inputFilter_moodMsg = new InputFilter() {
                @Override
                public CharSequence filter(CharSequence source, int start, int end,Spanned dest, int dstart, int dend) {

                    if (source.length()>44) return "";// max 44chars

// Here you can add more controls, e.g. allow only hex chars etc

                    for (int i = start; i < end; i++) { 
                         if (!Character.isLetterOrDigit(source.charAt(i)) && !Character.isSpaceChar(source.charAt(i))
                                 && source.charAt(i)!='-'
                                 && source.charAt(i)!='.'
                                 && source.charAt(i)!='!'
                                 ) { 
                             return "";     
                         }     
                    }
                    return null;   
                }
            };
            input_moodMsg.setFilters(new InputFilter[] { inputFilter_moodMsg });
            input_moodMsg.setInputType(InputType.TYPE_TEXT_FLAG_NO_SUGGESTIONS);