atFirst java challenge


I am working on below challenge
I wrote my code as below

public String atFirst(String str) {
  return str.substring(0,2);
  return str.substring(0)+"@";
  return "@";

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I am failing below tests.
Expected	Run		
atFirst("hello") → "he"	"he"	OK	    
atFirst("hi") → "hi"	"hi"	OK	    
atFirst("h") → "h@"	"h@"	OK	    
atFirst("") → "@@"	"@"	X	    
atFirst("kitten") → "ki"	"ki"	OK	    
atFirst("java") → "ja"	"ja"	OK	    
atFirst("j") → "j@"	"j@"	OK	    
other tests

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How to fix and improve my code. Thanks in advance
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Zsolt PribuszCommented:
Insert one more @ in line 10:

return "@@";
Zsolt PribuszCommented:
Or you can do this challenge with only one line:

public String atFirst(String str) {

  return (str+"@@").substring(0,2);

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gudii9Author Commented:
return (str+"@@").substring(0,2);

wHAT IS MEANING OF ABOVE return statement. i never used this kind of return statement. please advise
Breaking it down:
 String temp1= str+"@@";
 String temp2=temp1.substring(0,2);
 return temp2;
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