Thread: Beginner Java Method

Beginner warm up problem that I can't figure out why isn't working. Thanks for any assistance in advance. (Note: Trying to use substring method not charAt() method.)

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Given a string, return a string made of the first 2 chars (if present), however include first char only if it is 'o' and include the second only if it is 'z', so "ozymandias" yields "oz".

public String startOz(String str) {
 
String r = ""; 
 
  if (str.substring(0).equals("o")) 
  r = str.substring(0);  
 
  if (str.substring(1).equals("z")) 
  r = str.substring(1); 
 
  return r;
}

Expected Run
startOz("ozymandias") → "oz" "" X
startOz("bzoo") → "z" "" X
startOz("oxx") → "o" "" X
startOz("oz") → "oz" "z" X
startOz("ounce") → "o" "" X
startOz("o") → "o" "o" OK
startOz("abc") → "" "" OK
startOz("") → "" "Exception:java.lang.StringIndexOutOfBoundsExcepti on: String index out of range: -1 (line number:8)" X
startOz("zoo") → "" "" OK
startOz("aztec") → "z" "" X
startOz("zzzz") → "z" "" X
startOz("oznic") → "oz" "" X

CodingBat Java Warmup-1 startOz

If it possible to not use the length method? I figured I could get it working with the substring method, but I am having problems.