xyzThere java challenge

Hi,
I am working on below challenge
http://codingbat.com/prob/p136594
I tried my code as below

public boolean xyzThere(String str) {
 int xPos= str.indexOf('x');
 int len=str.length();
 
 String sub=str.substring(xPos,len);
 if(sub.equals("xyz")){
 return true;
 }
 else{
 return false;
 }
}

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I am getting below result

Expected      Run            
xyzThere("abcxyz") → true      true      OK         
xyzThere("abc.xyz") → false      true      X         
xyzThere("xyz.abc") → true      false      X         
xyzThere("abcxy") → false      false      OK         
xyzThere("xyz") → true      true      OK         
xyzThere("xy") → false      false      OK         
xyzThere("x") → false      false      OK         
xyzThere("") → false      Exception:java.lang.StringIndexOutOfBoundsException: String index out of range: -1 (line number:5)      X         
xyzThere("abc.xyzxyz") → true      false      X         
xyzThere("abc.xxyz") → true      false      X         
xyzThere(".xyz") → false      true      X         
xyzThere("12.xyz") → false      true      X         
xyzThere("12xyz") → true      true      OK         
xyzThere("1.xyz.xyz2.xyz") → false      false      OK         
other tests
X

how to  improve my approach, results and design of this challenge. How do i make a graphical venn or flow chart or some other relevant diagram to design it before writing code to decide best strategy?
 Please advise