public class Test {
    public static void main(String[] args) throws Throwable {
        String s = "login".toLowerCase();
        char[] c = s.toCharArray();
        int[] idx = new int[c.length];
        StringBuilder buf = new StringBuilder();
        while (idx[0] < 2) {
            buf.delete(0, buf.length());
            for (int i=0; i<idx.length; i++) {
                if (idx[i] == 1) {
                    buf.append(Character.toUpperCase(c[i]));
                } else {
                    buf.append(c[i]);
                }
            }
            System.out.println(buf);

            idx[idx.length-1]++;
            for (int i=idx.length-1; i>0; i--) {
                if (idx[i] == 2) {
                    idx[i] = 0;
                    idx[i-1]++;
                } else {
                    break;
                }
            }
        }
    }
}

------解决方案--------------------
Java code

public class Test26 {

    public static void main(String[] args) {
        String str = "lOgiN";
        String lowStr = str.toLowerCase();
        int length  = lowStr.length();
        for(int i=0; i<(int)Math.pow(2, lowStr.length());i++){
            char[] ch = lowStr.toCharArray();
            String s = Integer.toBinaryString(i);
            for(int j=0; j<s.length(); j++){
                if('1' == s.charAt(s.length() -j -1))
                {
                    ch[length - j -1] = (char)(ch[length - j -1] - 32);
                }
            }
            System.out.println(new String(ch));
        }
    }
}