public class MaxLengthWatcher implements TextWatcher{
    private int maxLen = 0;
    private EditText editText;

    public MaxLengthWatcher(int maxLen, EditText editText) {
        this.maxLen = maxLen;
        this.editText = editText;
    }

    @Override
    public void beforeTextChanged(CharSequence s, int start, int count, int after) {

    }

    @Override
    public void onTextChanged(CharSequence s, int start, int before, int count) {

    }

    @Override
    public void afterTextChanged(Editable s) {
        Editable editable = editText.getText();
        int len = editable.length();
        if (len > maxLen){
            int selEndIndex = Selection.getSelectionEnd(editable);
            String str = editable.toString();
            String newStr = str.substring(0, maxLen);
            editText.setText(newStr);
            editable = editText.getText();
            int newLen = editable.length();
            if (selEndIndex > newLen){
                selEndIndex = editable.length();
            }
            Selection.setSelection(editable, selEndIndex);
            Toast.makeText(mContext, "最多输入" + maxLen + "个字符", Toast.LENGTH_SHORT).show();
        }
    }
}  

一个中文算两个字符：

public class MaxLengthWatcher implements TextWatcher {
    private int maxLen = 0;
    private EditText editText;
    private Context mContext;

    public MaxLengthWatcher(int maxLen, EditText editText, Context context) {
        this.maxLen = maxLen;
        this.editText = editText;
        mContext = context;
    }

    @Override
    public void beforeTextChanged(CharSequence s, int start, int count, int after) {

    }

    @Override
    public void onTextChanged(CharSequence s, int start, int before, int count) {

    }

    @Override
    public void afterTextChanged(Editable s) {

        try {
            Editable editable = editText.getText();
            //转成bytes，getBytes("GBK")中一个汉字占两个字节
            int len = 0;
            len = editable.toString().getBytes("GBK").length;
            Timber.e("len: " + len);
            if (len > maxLen){
                int selEndIndex = Selection.getSelectionEnd(editable);
                //避免最后出现半个汉字处理（如果是汉字，byte < 0 (不完全正确，这里应该没问题)）
                byte[] bytes = editable.toString().getBytes("GBK");
                int length;
                int count = 0;
                for (int i = 0; i < maxLen; i++) {
                    if (bytes[i] < 0) count++;
                }
                if ((count % 2) != 0){
                    length = maxLen - 1;
                } else {
                    length = maxLen;
                }
                String newStr = new String(Arrays.copyOf(bytes, length), "GBK");

                editText.setText(newStr);
                editable = editText.getText();
                int newLen = editable.length();
                if (selEndIndex > newLen){
                    selEndIndex = editable.length();
                }
                Selection.setSelection(editable, selEndIndex);
                ToastUtils.show(mContext, String.format("最多输入%s个汉字或%s个字符", maxLen / 2, maxLen));
            }
        } catch (UnsupportedEncodingException e) {
            e.printStackTrace();
        } catch (Exception e){
            e.printStackTrace();
        }
    }
}