str1: frist string
str2: second string
str3: JSP 프로그래밍!

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str1.length(): 12
str2.length(): 13
str3.length(): 10

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str1.charAt(4): t
str1.indexOf("ing"): 9
str1.indexOf("st", 4): 6
str2.indexOf('s'): 0
str2.indexOf('s', 4): 7
str3.indexOf("프로그래밍"): 4

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str1.substring(6): string
str2.substring(4, 9): nd st
str3.substring(4): 프로그래밍!

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str1.equals(str2): false
str1.compareTo(str2): -13