String클래스 란?

String s = "겨울"; 암시적 객체 생성
String s = new String("겨울") 명시적 객체생성

암시적으로 내용이 같은 객체를 생성하면
공유한다 

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String 클래스

String s= new Strng("겨울");  //명시적 객채를 생성 //원형 

명시적으로 생성할경우 무조건 객체를 따로 만든다.
명시적 s1  =>   겨울    
명시적 s2  =>   겨울
명시적 s3  =>   겨울          
명시적 s4  =>   여름

String s = "겨울";   //암시적 객체를 생성   //편의를 위해서

암시적으로 생성할경우 문자열이 같을경우 객체를 공유한다.
암시적 s5  =>    
암시적 s6  =>   겨울
암시적 s7  =>          
암시적 s8  =>   여름

객체비교 : 객체가 있는지 없는지 확인 null 확인 // String s= null;  => if(s != null){ ... }
if (s1 == s2) false
if (s1 == s5) false
if (s5 == s6) true
if (s5 == s8) false

문자열비교 : ID 체크
if(s1.equals(s2)) true
if(s1.equals(s5)) true
if(s5.equals(s6)) true
if(s5.equals(s8)) false

Modifier and Type	Method and Description
char	charAt(int index) Returns the char value at the specified index.
int	codePointAt(int index) Returns the character (Unicode code point) at the specified index.
int	codePointBefore(int index) Returns the character (Unicode code point) before the specified index.
int	codePointCount(int beginIndex, int endIndex) Returns the number of Unicode code points in the specified text range of thisString.
int	compareTo(String anotherString) Compares two strings lexicographically.
int	compareToIgnoreCase(String str) Compares two strings lexicographically, ignoring case differences.
String	concat(String str) Concatenates the specified string to the end of this string.
boolean	contains(CharSequence s) Returns true if and only if this string contains the specified sequence of char values.
boolean	contentEquals(CharSequence cs) Compares this string to the specified CharSequence.
boolean	contentEquals(StringBuffer sb) Compares this string to the specified StringBuffer.
static String	copyValueOf(char[] data) Returns a String that represents the character sequence in the array specified.
static String	copyValueOf(char[] data, int offset, int count) Returns a String that represents the character sequence in the array specified.
boolean	endsWith(String suffix) Tests if this string ends with the specified suffix.
boolean	equals(Object anObject) Compares this string to the specified object.
boolean	equalsIgnoreCase(String anotherString) Compares this String to another String, ignoring case considerations.
static String	format(Locale l, String format, Object... args) Returns a formatted string using the specified locale, format string, and arguments.
static String	format(String format, Object... args) Returns a formatted string using the specified format string and arguments.
byte[]	getBytes() Encodes this String into a sequence of bytes using the platform's default charset, storing the result into a new byte array.
byte[]	getBytes(Charset charset) Encodes this String into a sequence of bytes using the given charset, storing the result into a new byte array.
void	getBytes(int srcBegin, int srcEnd, byte[] dst, int dstBegin) Deprecated.  This method does not properly convert characters into bytes. As of JDK 1.1, the preferred way to do this is via the getBytes() method, which uses the platform's default charset.
byte[]	getBytes(String charsetName) Encodes this String into a sequence of bytes using the named charset, storing the result into a new byte array.
void	getChars(int srcBegin, int srcEnd, char[] dst, int dstBegin) Copies characters from this string into the destination character array.
int	hashCode() Returns a hash code for this string.
int	indexOf(int ch) Returns the index within this string of the first occurrence of the specified character.
int	indexOf(int ch, int fromIndex) Returns the index within this string of the first occurrence of the specified character, starting the search at the specified index.
int	indexOf(String str) Returns the index within this string of the first occurrence of the specified substring.
int	indexOf(String str, int fromIndex) Returns the index within this string of the first occurrence of the specified substring, starting at the specified index.
String	intern() Returns a canonical representation for the string object.
boolean	isEmpty() Returns true if, and only if, length() is 0.
int	lastIndexOf(int ch) Returns the index within this string of the last occurrence of the specified character.
int	lastIndexOf(int ch, int fromIndex) Returns the index within this string of the last occurrence of the specified character, searching backward starting at the specified index.
int	lastIndexOf(String str) Returns the index within this string of the last occurrence of the specified substring.
int	lastIndexOf(String str, int fromIndex) Returns the index within this string of the last occurrence of the specified substring, searching backward starting at the specified index.
int	length() Returns the length of this string.
boolean	matches(String regex) Tells whether or not this string matches the given regular expression.
int	offsetByCodePoints(int index, int codePointOffset) Returns the index within this String that is offset from the given index bycodePointOffset code points.
boolean	regionMatches(boolean ignoreCase, int toffset,String other, int ooffset, int len) Tests if two string regions are equal.
boolean	regionMatches(int toffset, String other, int ooffset, int len) Tests if two string regions are equal.
String	replace(char oldChar, char newChar) Returns a new string resulting from replacing all occurrences of oldChar in this string with newChar.
String	replace(CharSequence target,CharSequence replacement) Replaces each substring of this string that matches the literal target sequence with the specified literal replacement sequence.
String	replaceAll(String regex, String replacement) Replaces each substring of this string that matches the given regular expression with the given replacement.
String	replaceFirst(String regex, String replacement) Replaces the first substring of this string that matches the given regular expression with the given replacement.
String[]	split(String regex) Splits this string around matches of the given regular expression.
String[]	split(String regex, int limit) Splits this string around matches of the given regular expression.
boolean	startsWith(String prefix) Tests if this string starts with the specified prefix.
boolean	startsWith(String prefix, int toffset) Tests if the substring of this string beginning at the specified index starts with the specified prefix.
CharSequence	subSequence(int beginIndex, int endIndex) Returns a new character sequence that is a subsequence of this sequence.
String	substring(int beginIndex) Returns a new string that is a substring of this string.
String	substring(int beginIndex, int endIndex) Returns a new string that is a substring of this string.
char[]	toCharArray() Converts this string to a new character array.
String	toLowerCase() Converts all of the characters in this String to lower case using the rules of the default locale.
String	toLowerCase(Locale locale) Converts all of the characters in this String to lower case using the rules of the given Locale.
String	toString() : 재정의 This object (which is already a string!) is itself returned.
String	toUpperCase() Converts all of the characters in this String to upper case using the rules of the default locale.
String	toUpperCase(Locale locale) Converts all of the characters in this String to upper case using the rules of the given Locale.
String	trim() Returns a copy of the string, with leading and trailing whitespace omitted.
static String	valueOf(boolean b) Returns the string representation of the boolean argument.
static String	valueOf(char c) Returns the string representation of the char argument.
static String	valueOf(char[] data) Returns the string representation of the char array argument.
static String	valueOf(char[] data, int offset, int count) Returns the string representation of a specific subarray of the char array argument.
static String	valueOf(double d) Returns the string representation of the double argument.
static String	valueOf(float f) Returns the string representation of the float argument.
static String	valueOf(int i) Returns the string representation of the int argument.
static String	valueOf(long l) Returns the string representation of the long argument.
static String	valueOf(Object obj) Returns the string representation of the Object argument.