Why the following code fails the test ?

public class BinString {
	
	public BinString() {}
	
	public String convert(String s) {
		return binarise(sum(s));
	}
	
	public int sum(String s){
		if(s=="") 
			return 0;
		if(s.length()==1) 
			return ((int) (s.charAt(0)));
		return ((int) (s.charAt(0)))+sum(s.substring(1));
	}
	
	public String binarise(int x) {
		if(x==0)
			return "";
		if(x%2==1) 
			return "1"+binarise(x/2);
		return "0"+binarise(x/2);
	}
}

import static org.junit.Assert.*;

import org.junit.Before;
import org.junit.Test;

public class BinStringTest {

	private BinString binString;
	
	@Before protected void setUp(){
		binString = new BinString();
	}
	
	@Test public void testSumFunction() {
		int expected =0;
		
		assertEquals(expected, binString.sum(""));
		expected = 100;
		assertEquals(expected, binString.sum("d"));
		expected = 256;
		assertEquals(expected, binString.sum("Add"));
		
		
	}
	
	@Test public void testBinariseFunction(){
		String expected = "101";
		assertEquals(expected, binString.binarise(5));
		expected = "11111100";
		assertEquals(expected, binString.binarise(252));
				
	}
	
	@Test public void testTotalConversion() {
		String expected = "10000001";
		assertEquals(expected, binString.convert("A"));
	}
	

}