Single thread program.

SingleThread.java

public class SingleThread {
    public static void main(String[] args) {
        StringBuilder sb = new StringBuilder();
        String str = "abc(cdef)gh(ijk)lmn";
        int i = 0;
        boolean inParenthesis = false;
        for(;;){
            if(i >= str.length()){
                break;
            }
            char ch = str.charAt(i);
            switch(ch){
                case '(':
                    inParenthesis = true;
                    break;
                case ')':
                    inParenthesis = false;
                    break;
                default:
                    if(inParenthesis){
                        sb.append(ch);
                    }
            }
            ++i;
        }
        System.out.println(sb);
    }
}

Output only in a parenthesis.

cdefijk