/* package whatever; // don't place package name! */
 
import java.math.BigInteger;
import java.util.Iterator;
 
class Substrings{
 
    private class SubstringIterator implements Iterator<String> {
 
        String s;
        BigInteger cur = BigInteger.ZERO;
        BigInteger max;
 
        public SubstringIterator(String s) {
            this.s = s;
            max = BigInteger.ONE.shiftLeft(s.length()).subtract(BigInteger.ONE);
        }
 
        @Override
        public boolean hasNext() {
            return cur.compareTo(max) < 0;
        }
 
        @Override
        public String next() {
            cur = cur.add(BigInteger.ONE);
            StringBuilder sb = new StringBuilder();
            for(int i = 0x00; i < s.length(); i++) {
                if(cur.testBit(i)) {
                    sb.append(s.charAt(i));
                }
            }
            return sb.toString();
        }
 
        @Override
        public void remove() {
            throw new UnsupportedOperationException("This is not a collection iterator");
        }
 
 
    }
 
    public void run () {
        String s = "abcd";
        SubstringIterator ssi = new SubstringIterator(s);
        while(ssi.hasNext()) {
            System.out.println(ssi.next());
        }
    }
 
     public static void main(String []args){
        new Substrings().run();
     }
}

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