String s = "1010110111111";
    int oneCount=0;
    // count the 1s
    for (int pos=0; pos<s.length(); pos++)
    {
        if (s.charAt(pos)=='1') oneCount++;
    }
    int zeroCount=s.length()-oneCount; // assumes string is 1s and 0s only
    // start result
    StringBuilder sb=new StringBuilder();
    // append 1s
    for (int counter=0; counter<oneCount; counter++)
        sb.append('1');
    //append 0s
    for (int counter=0; counter<zeroCount; counter++)
        sb.append('0');
    s=new String(sb); // back to String
    System.out.println(s);