Braces and Brackets Autocomplete
Extract common code in each conditional branch

In the block:

    switch (c) {
        case R_BRACE:
            currString.insert(0, L_BRACE);
            i++;
            count++;
            break;
        case R_BRACKET:
            currString.insert(0, L_BRACKET);
            i++;
            count++;
            break;
        case R_PAREN:
            currString.insert(0, L_PAREN);
            i++;
            count++;
            break;
    }


You always execute:

        i++;
        count++;


So you can simplify and write just:

    switch (c) {
        case R_BRACE:
            currString.insert(0, L_BRACE);
            break;
        case R_BRACKET:
            currString.insert(0, L_BRACKET);
            break;
        case R_PAREN:
            currString.insert(0, L_PAREN);
            break;
    }
    i++;
    count++;