public class MainActivity extends AppCompatActivity {

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        try {
            String str="6b9edb9ad3dc87e36220809b8f5860698cccee7d807fd5cc5d008a7ab7fe75b5";
            String hexTestStr = hex2String(str);
            String encode = new String(Base64.encode(hexTestStr.getBytes(), Base64.DEFAULT ));
            Log.d("MainActivity", "encode = " + encode);
            String decode = new String(Base64.decode(encode.getBytes(), Base64.DEFAULT ));
            Log.d("MainActivity", "decode = " + decode);

        } catch (Exception err) {

        }
    }

    public String hex2String(String hexString) {
        StringBuilder str = new StringBuilder();
        for (int i=0 ; i<hexString.length() ; i+=2)
            str.append((char) Integer.parseInt(hexString.substring(i, i + 2), 16));
        return str.toString();
    }

}