difference among String,StringBuilder,StringBuffer

String常用构造函数

 1 String(byte[] bytes)
 2 
 3 String(byte[] bytes, int offset, int length)
 4 
 5 String(char[] value)
 6 
 7 String(char[] value, int offset, int count)
 8 
 9 String(int[] codePoints, int offset, int count)
10 
11 String(String original)
12 
13 String(StringBuffer buffer)
14 
15 String(StringBuilder builder)

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 this `String`.
`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)` Equivalent to `valueOf(char[])`.
`static String`	`copyValueOf(char[] data, int offset, int count)` Equivalent to `valueOf(char[], int, int)`.
`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.

`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`.
`static String`	`join(CharSequence delimiter, CharSequence... elements)` Returns a new String composed of copies of the `CharSequence elements` joined together with a copy of the specified `delimiter`.
`static String`	`join(CharSequence delimiter, Iterable<? extends CharSequence> elements)` Returns a new `String` composed of copies of the `CharSequence elements` joined together with a copy of the specified `delimiter`.
`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` by `codePointOffset` 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 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 character sequence that is a subsequence of this sequence.
`String`	`substring(int beginIndex)` Returns a string that is a substring of this string.
`String`	`substring(int beginIndex, int endIndex)` Returns a 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()` 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 string whose value is this string, with any leading and trailing whitespace removed.
`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.

StringBuffer和StringBuilder的对象是变量，所以效率要比String要高，java虚拟机不需要回收。

序号	方法描述
1	public StringBuffer append(String s) 将指定的字符串追加到此字符序列。
2	public StringBuffer reverse() 将此字符序列用其反转形式取代。
3	public delete(int start, int end) 移除此序列的子字符串中的字符。
4	public insert(int offset, int i) 将 `int` 参数的字符串表示形式插入此序列中。
5	replace(int start, int end, String str) 使用给定 `String` 中的字符替换此序列的子字符串中的字符。

序号	方法描述
1	int capacity() 返回当前容量。
2	char charAt(int index) 返回此序列中指定索引处的 `char` 值。
3	void ensureCapacity(int minimumCapacity) 确保容量至少等于指定的最小值。
4	void getChars(int srcBegin, int srcEnd, char[] dst, int dstBegin) 将字符从此序列复制到目标字符数组 `dst`。
5	int indexOf(String str) 返回第一次出现的指定子字符串在该字符串中的索引。
6	int indexOf(String str, int fromIndex) 从指定的索引处开始，返回第一次出现的指定子字符串在该字符串中的索引。
7	int lastIndexOf(String str) 返回最右边出现的指定子字符串在此字符串中的索引。
8	int lastIndexOf(String str, int fromIndex) 返回最后一次出现的指定子字符串在此字符串中的索引。
9	int length() 返回长度（字符数）。
10	void setCharAt(int index, char ch) 将给定索引处的字符设置为 `ch`。
11	void setLength(int newLength) 设置字符序列的长度。
12	CharSequence subSequence(int start, int end) 返回一个新的字符序列，该字符序列是此序列的子序列。
13	String substring(int start) 返回一个新的 `String`，它包含此字符序列当前所包含的字符子序列。
14	String substring(int start, int end) 返回一个新的 `String`，它包含此序列当前所包含的字符子序列。
15	String toString() 返回此序列中数据的字符串表示形式。

public StringBuffer deleteCharAt(int index)

public void trimToSize()