prefix.java

//Compressed, 342 characters
public static String p(String s){String[]w=s.toLowerCase().split(" ");String e="";for(int j=0,i=0;j<w.length;j++){for(;i<w.length;i++){String x=w[i],y=w[j];String r="";for(int k=0;k<Math.min(y.length(),x.length());k++){char a=y.charAt(k);char b=x.charAt(k);r=a==b?r.concat(String.valueOf(a)):r;}e=r.length()>e.length()&&j!=i?r:e;}}return e;}

//Uncompressed
    public static String p(String s) {
        String[] w = s.toLowerCase().split(" ");
        String e = "";
        for (int j = 0, i = 0, m = w.length; j < m; j++) {
            for (; i < m; i++) {
                String x = w[i], y = w[j], r = "";
                for (int k = 0; k < Math.min(y.length(), x.length()); k++) {
                    char a = y.charAt(k), b = x.charAt(k);
                    r = a == b ? r.concat(String.valueOf(a)) : r;
                }
                e = r.length() > e.length() && j != i ? r : e;
            }
        }
        return e;
    }
    
    
//How I wrote it before minimizing character count:
    public static String prefix(String sentence) {
        String[] words = sentence.toLowerCase().split(" ");
        return isolate(words);
    }


    public static String isolate(String... strings) {
        String result = "";
        for (int j = 0; j < strings.length; j++) {
            for (int i = 0; i < strings.length; i++) {
                String string = strings[i], string1 = strings[j];
                String result2 = "";
                for (int k = 0; k < Math.min(string1.length(), string.length()); k++) {
                    char a = string1.charAt(k);
                    char b = string.charAt(k);
                    result2 = a == b ? result2.concat(String.valueOf(a)) : result2;
                }
                result = result2.length() > result.length() && j != i ? result2 : result;
            }
        }

        return result;
    }