Here is a method which gives you all combinations of specified size from a random length string. Similar to quinmars' solution, but works for varied input and k.

The code can be changed to wrap around, ie 'dab' from input 'abcd' w k=3.

public void run(String data, int howMany){
    choose(data, howMany, new StringBuffer(), 0);
}


//n choose k
private void choose(String data, int k, StringBuffer result, int startIndex){
    if (result.length()==k){
        System.out.println(result.toString());
        return;
    }
    
    for (int i=startIndex; i<data.length(); i++){
        result.append(data.charAt(i));
        choose(data,k,result, i+1);
        result.setLength(result.length()-1);
    }
}

Output for "abcde":

abc abd abe acd ace ade bcd bce bde cde

Here is a method which gives you all combinations of specified size from a random length string. Similar to quinmars' solution, but works for varied input and k.

The code can be changed to wrap around, ie 'dab' from input 'abcd' w k=3.

public void run(String data, int howMany){
    choose(data, howMany, new StringBuffer(), 0);
}


//n choose k
private void choose(String data, int k, StringBuffer result, int startIndex){
    if (result.length()==k){
        System.out.println(result.toString());
        return;
    }
    
    for (int i=startIndex; i<data.length(); i++){
        result.append(data.charAt(i));
        choose(data,k,result, i+1);
        result.setLength(result.length()-1);
    }
}

Output for "abcde":

abc abd abe acd ace ade bcd bce bde cde