StringBuilder的用法

java连接字符串的方法Java连接字符串的方法很多，下面介绍几种常用的：1. 使用加号 + 连接字符串可以使用加号 + 连接多个字符串，例如：String str1 = 'Hello';String str2 = 'World';String result = str1 + ' ' + str2;System.out.println(result);这段代码将输出结果为：Hello World。

2. 使用 StringBuilder 或 StringBuffer 连接字符串StringBuilder 和 StringBuffer 都可以用来构建字符串，它们的主要区别在于线程安全性和执行速度。

StringBuilder 是非线程安全的，执行速度较快；StringBuffer 是线程安全的，执行速度较慢。

使用 StringBuilder 或 StringBuffer 连接字符串的示例代码如下：StringBuilder sb = new StringBuilder();sb.append('Hello');sb.append(' ');sb.append('World');String result = sb.toString();System.out.println(result);这段代码将输出结果为：Hello World。

3. 使用 StringJoiner 连接字符串StringJoiner 是 Java 8 新增的一个类，可以用来连接多个字符串，并且可以指定分隔符和前缀后缀等信息。

示例代码如下：StringJoiner sj = new StringJoiner(' ');sj.add('Hello');sj.add('World');String result = sj.toString();System.out.println(result);这段代码将输出结果为：Hello World。

Java学习笔记关于StringBuilder方法总结：1.用法：new StringBuilder.方法名或创建一个新的对象对象.方法名灵活运用返回值StringBuilder append(参数)追加内容到当前stringbuilder对象的末尾StringBuilder insert(位置，参数)将内容插入到StringBuilder对象的指定位置String toString()将StringBuilder对象转换为String对象int length()获取字符串的长度关于字符串：1.字符串对象创建后是不可修改的，所谓的修改实际上是创建了新的对象，所指向的内存空间不同例：String s1=”a”;String s2=”a”;s1==s2true;多次出现的字符常量，Java编译只生成一个，所以返回值为true；String s1=new String(“a”);String s2=new String(“a”);s1==s2false;s1,s2是不同的对象，占用着不同的内存区域，所以返回值为false；==比较的是内存如果只是想要比较内容是否相同用equals（）即可2.StringBuilder StringBuffer是可以修改的3.感悟：这种对于其他数据类型同样适用int不是一个对象integer是一个对象两个new的对象代表着不同的内存区域用==不能去判断4.String的常用方法总结int length()返回当前字符串的长度int indexOf(int ch)查找ch字符在该字符串中第一次出现的位置int indexOf(String str)查找str字符串第一次出现的位置int lastindexOf(int ch)查找ch字符串最后一次出现的位置int lastindexOf(String str)查找str字符串最后一次出现的位置String substring(int began)获取从began位置开始到结束位置的字符串String substring(int began,int end)获取从began位置到end位置的字符串String trim()返回去除了前后空格的字符串Boolean equals()ignoe…String toLowerCase()转换为小写toUpperCasechar charAt(int index)获取字符串中指定位置的字符String[]split(String regex,int limit)讲字符串分割为子字符串，返回字符串数组Byte[]getBytes()将该字符串转换为byte数组关于包装类：1.基本数据类型是不具备对象特征的，不能引用方法，所以出现了包装类2.byte Byte short Short int Integer long Long float Float double Double char Charater Boolean Boolean3.构造方法：Integer(int value)Integer(String s)4.新建对象a a.XXXvalue将对象的数据类型转化为XXX型；5.Integer i=Integer.parseInt(string s)或者Integer i=new Integer(int x)i.ParseInt()将字符串转化为int类型6.valueOf(String s)将字符串转化为Integer类型返回值static Integer7.与字符串之间的转换int c=10;String str1=Integer.toString(c);str1=String.valueOf（c）;str1=c+””;8.字符串转换为基本类型String str=”8”;int d=Integer.parseInt(str);int d=String.valueOf(str)；转化为谁就用谁.valueOf(被转化);关于Date和SimpleDateFormat类表示时间（不建议使用Date）：1.Date d=new Date();使用的无参构造方法是默认是当前的时间；2.在使用Date类时必须先引入java.util.Date类在使用SImperDateFormat的时候要引用java.text.SimperDateFormat因为用parse()方法很容易出现ParseException错误所以引用java.text.ParseException3.有参的Date类即人工设置时间例：SimpleDateFormat sdf=new SimpDateFormat（“yyyy年MM月dd日HH时mm分ss秒”）;Date date=sdf.parse(”1996年09月12日12时12分12秒“)格式相同才可以格式化为String类String string=sdf.format(date);关于Calendar类：1.Date类最主要的作用就是获得当前时间，同时这个类里面也具有设置时间以及一些其他的功能，但是由于本身设计的问题，这些方法却遭到众多批评，不建议使用，更推荐使用Calendar类进行时间和日期的处理。

StringBuilder的⽤法总结在程序开发过程中，我们常常碰到字符串连接的情况，⽅便和直接的⽅式是通过"+"符号来实现，但是这种⽅式达到⽬的的效率⽐较低，且每执⾏⼀次都会创建⼀个String对象，即耗时，⼜浪费空间。

使⽤StringBuilder类就可以避免这种问题的发⽣，下⾯就Stringbuilder的使⽤做个简要的总结：⼀、创建Stringbuilder对象StringBuilder strB = new StringBuilder();1、append(String str)/append(Char c)：字符串连接System.out.println("StringBuilder:"+strB.append("ch").append("111").append('c'));//return "StringBuilder:ch111c"2、toString()：返回⼀个与构建起或缓冲器内容相同的字符串System.out.println("String:"+strB.toString());//return "String:ch111c"3、appendcodePoint(int cp)：追加⼀个代码点，并将其转换为⼀个或两个代码单元并返回thisSystem.out.println("StringBuilder.appendCodePoint:"+strB.appendCodePoint(2));//return "StringBuilder.appendCodePoint:ch111c"4、setCharAt(int i, char c)：将第 i 个代码单元设置为 c（可以理解为替换）strB.setCharAt(2, 'd');System.out.println("StringBuilder.setCharAt:" + strB);//return "StringBuilder.setCharAt:chd11c"5、insert(int offset, String str)/insert(int offset, Char c)：在指定位置之前插⼊字符(串)System.out.println("StringBuilder.insertString:"+ strB.insert(2, "LS"));//return "StringBuilder.insertString:chLSd11c"System.out.println("StringBuilder.insertChar:"+ strB.insert(2, 'L'));//return "StringBuilder.insertChar:chLLSd11c"6、delete(int startIndex,int endIndex)：删除起始位置（含）到结尾位置（不含）之间的字符串System.out.println("StringBuilder.delete:"+ strB.delete(2, 4));//return "StringBuilder.delete:chSd11c"。

StringBuilder详解（String系列之2）本章介绍StringBuilder以及它的API的详细使⽤⽅法。

StringBuilder 简介StringBuilder 是⼀个可变的字符序列。

它继承于AbstractStringBuilder，实现了CharSequence接⼝。

StringBuffer 也是继承于AbstractStringBuilder的⼦类；但是，StringBuilder和StringBuffer不同，前者是⾮线程安全的，后者是线程安全的。

StringBuilder 和 CharSequence之间的关系图如下：StringBuilder函数列表StringBuilder()StringBuilder(int capacity)StringBuilder(CharSequence seq)StringBuilder(String str)StringBuilder append(float f)StringBuilder append(double d)StringBuilder append(boolean b)StringBuilder append(int i)StringBuilder append(long l)StringBuilder append(char c)StringBuilder append(char[] chars)StringBuilder append(char[] str, int offset, int len)StringBuilder append(String str)StringBuilder append(Object obj)StringBuilder append(StringBuffer sb)StringBuilder append(CharSequence csq)StringBuilder append(CharSequence csq, int start, int end)StringBuilder appendCodePoint(int codePoint)int capacity()char charAt(int index)int codePointAt(int index)int codePointBefore(int index)int codePointCount(int start, int end)StringBuilder delete(int start, int end)StringBuilder deleteCharAt(int index)void ensureCapacity(int min)void getChars(int start, int end, char[] dst, int dstStart)int indexOf(String subString, int start)int indexOf(String string)StringBuilder insert(int offset, boolean b)StringBuilder insert(int offset, int i)StringBuilder insert(int offset, long l)StringBuilder insert(int offset, float f)StringBuilder insert(int offset, double d)StringBuilder insert(int offset, char c)StringBuilder insert(int offset, char[] ch)StringBuilder insert(int offset, char[] str, int strOffset, int strLen)StringBuilder insert(int offset, String str)StringBuilder insert(int offset, Object obj)StringBuilder insert(int offset, CharSequence s)StringBuilder insert(int offset, CharSequence s, int start, int end)int lastIndexOf(String string)int lastIndexOf(String subString, int start)int length()int offsetByCodePoints(int index, int codePointOffset)StringBuilder replace(int start, int end, String string)StringBuilder reverse()void setCharAt(int index, char ch)void setLength(int length)CharSequence subSequence(int start, int end)String substring(int start)String substring(int start, int end)String toString()void trimToSize()AbstractStringBuilder 和 StringBuilder源码AbstractStringBuilder源码(基于jdk1.7.40)1package ng;23import sun.misc.FloatingDecimal;4import java.util.Arrays;56abstract class AbstractStringBuilder implements Appendable, CharSequence { 7char[] value;89int count;1011 AbstractStringBuilder() {12 }1314 AbstractStringBuilder(int capacity) {15 value = new char[capacity];16 }1718public int length() {19return count;20 }2122public int capacity() {23return value.length;24 }2526public void ensureCapacity(int minimumCapacity) {27if (minimumCapacity > 0)28 ensureCapacityInternal(minimumCapacity);29 }3031private void ensureCapacityInternal(int minimumCapacity) {32// overflow-conscious code33if (minimumCapacity - value.length > 0)34 expandCapacity(minimumCapacity);35 }3637void expandCapacity(int minimumCapacity) {38int newCapacity = value.length * 2 + 2;39if (newCapacity - minimumCapacity < 0)40 newCapacity = minimumCapacity;41if (newCapacity < 0) {42if (minimumCapacity < 0) // overflow43throw new OutOfMemoryError();44 newCapacity = Integer.MAX_VALUE;45 }46 value = Arrays.copyOf(value, newCapacity);47 }4849public void trimToSize() {50if (count < value.length) {51 value = Arrays.copyOf(value, count);52 }53 }5455public void setLength(int newLength) {56if (newLength < 0)57throw new StringIndexOutOfBoundsException(newLength);58 ensureCapacityInternal(newLength);5960if (count < newLength) {61for (; count < newLength; count++)62 value[count] = '\0';63 } else {64 count = newLength;65 }66 }6768public char charAt(int index) {69if ((index < 0) || (index >= count))70throw new StringIndexOutOfBoundsException(index);71return value[index];72 }7374public int codePointAt(int index) {75if ((index < 0) || (index >= count)) {76throw new StringIndexOutOfBoundsException(index);77 }78return Character.codePointAt(value, index);79 }8081public int codePointBefore(int index) {82int i = index - 1;86return Character.codePointBefore(value, index);87 }8889public int codePointCount(int beginIndex, int endIndex) {90if (beginIndex < 0 || endIndex > count || beginIndex > endIndex) {91throw new IndexOutOfBoundsException();92 }93return Character.codePointCountImpl(value, beginIndex, endIndex-beginIndex);94 }9596public int offsetByCodePoints(int index, int codePointOffset) {97if (index < 0 || index > count) {98throw new IndexOutOfBoundsException();99 }100return Character.offsetByCodePointsImpl(value, 0, count,101 index, codePointOffset);102 }103104public void getChars(int srcBegin, int srcEnd, char[] dst, int dstBegin)105 {106if (srcBegin < 0)107throw new StringIndexOutOfBoundsException(srcBegin);108if ((srcEnd < 0) || (srcEnd > count))109throw new StringIndexOutOfBoundsException(srcEnd);110if (srcBegin > srcEnd)111throw new StringIndexOutOfBoundsException("srcBegin > srcEnd");112 System.arraycopy(value, srcBegin, dst, dstBegin, srcEnd - srcBegin);113 }114115public void setCharAt(int index, char ch) {116if ((index < 0) || (index >= count))117throw new StringIndexOutOfBoundsException(index);118 value[index] = ch;119 }120121public AbstractStringBuilder append(Object obj) {122return append(String.valueOf(obj));123 }124125public AbstractStringBuilder append(String str) {126if (str == null) str = "null";127int len = str.length();128 ensureCapacityInternal(count + len);129 str.getChars(0, len, value, count);130 count += len;131return this;132 }133134// Documentation in subclasses because of synchro difference135public AbstractStringBuilder append(StringBuffer sb) {136if (sb == null)137return append("null");138int len = sb.length();139 ensureCapacityInternal(count + len);140 sb.getChars(0, len, value, count);141 count += len;142return this;143 }144145// Documentation in subclasses because of synchro difference146public AbstractStringBuilder append(CharSequence s) {147if (s == null)148 s = "null";149if (s instanceof String)150return this.append((String)s);151if (s instanceof StringBuffer)152return this.append((StringBuffer)s);153return this.append(s, 0, s.length());154 }155156public AbstractStringBuilder append(CharSequence s, int start, int end) {157if (s == null)158 s = "null";159if ((start < 0) || (start > end) || (end > s.length()))160throw new IndexOutOfBoundsException(161 "start " + start + ", end " + end + ", s.length() "162 + s.length());163int len = end - start;164 ensureCapacityInternal(count + len);165for (int i = start, j = count; i < end; i++, j++)166 value[j] = s.charAt(i);170171public AbstractStringBuilder append(char[] str) {172int len = str.length;173 ensureCapacityInternal(count + len);174 System.arraycopy(str, 0, value, count, len);175 count += len;176return this;177 }178179public AbstractStringBuilder append(char str[], int offset, int len) { 180if (len > 0) // let arraycopy report AIOOBE for len < 0 181 ensureCapacityInternal(count + len);182 System.arraycopy(str, offset, value, count, len);183 count += len;184return this;185 }186187public AbstractStringBuilder append(boolean b) {188if (b) {189 ensureCapacityInternal(count + 4);190 value[count++] = 't';191 value[count++] = 'r';192 value[count++] = 'u';193 value[count++] = 'e';194 } else {195 ensureCapacityInternal(count + 5);196 value[count++] = 'f';197 value[count++] = 'a';198 value[count++] = 'l';199 value[count++] = 's';200 value[count++] = 'e';201 }202return this;203 }204205public AbstractStringBuilder append(char c) {206 ensureCapacityInternal(count + 1);207 value[count++] = c;208return this;209 }210211public AbstractStringBuilder append(int i) {212if (i == Integer.MIN_VALUE) {213 append("-2147483648");214return this;215 }216int appendedLength = (i < 0) ? Integer.stringSize(-i) + 1217 : Integer.stringSize(i);218int spaceNeeded = count + appendedLength;219 ensureCapacityInternal(spaceNeeded);220 Integer.getChars(i, spaceNeeded, value);221 count = spaceNeeded;222return this;223 }224225public AbstractStringBuilder append(long l) {226if (l == Long.MIN_VALUE) {227 append("-9223372036854775808");228return this;229 }230int appendedLength = (l < 0) ? Long.stringSize(-l) + 1231 : Long.stringSize(l);232int spaceNeeded = count + appendedLength;233 ensureCapacityInternal(spaceNeeded);234 Long.getChars(l, spaceNeeded, value);235 count = spaceNeeded;236return this;237 }238239public AbstractStringBuilder append(float f) {240new FloatingDecimal(f).appendTo(this);241return this;242 }243244public AbstractStringBuilder append(double d) {245new FloatingDecimal(d).appendTo(this);246return this;247 }248249public AbstractStringBuilder delete(int start, int end) {250if (start < 0)253 end = count;254if (start > end)255throw new StringIndexOutOfBoundsException();256int len = end - start;257if (len > 0) {258 System.arraycopy(value, start+len, value, start, count-end); 259 count -= len;260 }261return this;262 }263264public AbstractStringBuilder appendCodePoint(int codePoint) {265final int count = this.count;266267if (Character.isBmpCodePoint(codePoint)) {268 ensureCapacityInternal(count + 1);269 value[count] = (char) codePoint;270this.count = count + 1;271 } else if (Character.isValidCodePoint(codePoint)) {272 ensureCapacityInternal(count + 2);273 Character.toSurrogates(codePoint, value, count);274this.count = count + 2;275 } else {276throw new IllegalArgumentException();277 }278return this;279 }280281public AbstractStringBuilder deleteCharAt(int index) {282if ((index < 0) || (index >= count))283throw new StringIndexOutOfBoundsException(index);284 System.arraycopy(value, index+1, value, index, count-index-1); 285 count--;286return this;287 }288289public AbstractStringBuilder replace(int start, int end, String str) { 290if (start < 0)291throw new StringIndexOutOfBoundsException(start);292if (start > count)293throw new StringIndexOutOfBoundsException("start > length()"); 294if (start > end)295throw new StringIndexOutOfBoundsException("start > end"); 296297if (end > count)298 end = count;299int len = str.length();300int newCount = count + len - (end - start);301 ensureCapacityInternal(newCount);302303 System.arraycopy(value, end, value, start + len, count - end); 304 str.getChars(value, start);305 count = newCount;306return this;307 }308309public String substring(int start) {310return substring(start, count);311 }312313public CharSequence subSequence(int start, int end) {314return substring(start, end);315 }316317public String substring(int start, int end) {318if (start < 0)319throw new StringIndexOutOfBoundsException(start);320if (end > count)321throw new StringIndexOutOfBoundsException(end);322if (start > end)323throw new StringIndexOutOfBoundsException(end - start);324return new String(value, start, end - start);325 }326327public AbstractStringBuilder insert(int index, char[] str, int offset, 328int len)329 {330if ((index < 0) || (index > length()))331throw new StringIndexOutOfBoundsException(index);332if ((offset < 0) || (len < 0) || (offset > str.length - len))333throw new StringIndexOutOfBoundsException(334 "offset " + offset + ", len " + len + ", str.length "337 System.arraycopy(value, index, value, index + len, count - index); 338 System.arraycopy(str, offset, value, index, len);339 count += len;340return this;341 }342343public AbstractStringBuilder insert(int offset, Object obj) {344return insert(offset, String.valueOf(obj));345 }346347public AbstractStringBuilder insert(int offset, String str) {348if ((offset < 0) || (offset > length()))349throw new StringIndexOutOfBoundsException(offset);350if (str == null)351 str = "null";352int len = str.length();353 ensureCapacityInternal(count + len);354 System.arraycopy(value, offset, value, offset + len, count - offset); 355 str.getChars(value, offset);356 count += len;357return this;358 }359360public AbstractStringBuilder insert(int offset, char[] str) {361if ((offset < 0) || (offset > length()))362throw new StringIndexOutOfBoundsException(offset);363int len = str.length;364 ensureCapacityInternal(count + len);365 System.arraycopy(value, offset, value, offset + len, count - offset); 366 System.arraycopy(str, 0, value, offset, len);367 count += len;368return this;369 }370371public AbstractStringBuilder insert(int dstOffset, CharSequence s) { 372if (s == null)373 s = "null";374if (s instanceof String)375return this.insert(dstOffset, (String)s);376return this.insert(dstOffset, s, 0, s.length());377 }378379public AbstractStringBuilder insert(int dstOffset, CharSequence s, 380int start, int end) {381if (s == null)382 s = "null";383if ((dstOffset < 0) || (dstOffset > this.length()))384throw new IndexOutOfBoundsException("dstOffset "+dstOffset); 385if ((start < 0) || (end < 0) || (start > end) || (end > s.length()))386throw new IndexOutOfBoundsException(387 "start " + start + ", end " + end + ", s.length() "388 + s.length());389int len = end - start;390 ensureCapacityInternal(count + len);391 System.arraycopy(value, dstOffset, value, dstOffset + len,392 count - dstOffset);393for (int i=start; i<end; i++)394 value[dstOffset++] = s.charAt(i);395 count += len;396return this;397 }398399public AbstractStringBuilder insert(int offset, boolean b) {400return insert(offset, String.valueOf(b));401 }402403public AbstractStringBuilder insert(int offset, char c) {404 ensureCapacityInternal(count + 1);405 System.arraycopy(value, offset, value, offset + 1, count - offset); 406 value[offset] = c;407 count += 1;408return this;409 }410411public AbstractStringBuilder insert(int offset, int i) {412return insert(offset, String.valueOf(i));413 }414415public AbstractStringBuilder insert(int offset, long l) {416return insert(offset, String.valueOf(l));417 }418421 }422423public AbstractStringBuilder insert(int offset, double d) {424return insert(offset, String.valueOf(d));425 }426427public int indexOf(String str) {428return indexOf(str, 0);429 }430431public int indexOf(String str, int fromIndex) {432return String.indexOf(value, 0, count,433 str.toCharArray(), 0, str.length(), fromIndex);434 }435436public int lastIndexOf(String str) {437return lastIndexOf(str, count);438 }439440public int lastIndexOf(String str, int fromIndex) {441return stIndexOf(value, 0, count,442 str.toCharArray(), 0, str.length(), fromIndex);443 }444445public AbstractStringBuilder reverse() {446boolean hasSurrogate = false;447int n = count - 1;448for (int j = (n-1) >> 1; j >= 0; --j) {449char temp = value[j];450char temp2 = value[n - j];451if (!hasSurrogate) {452 hasSurrogate = (temp >= Character.MIN_SURROGATE && temp <= Character.MAX_SURROGATE) 453 || (temp2 >= Character.MIN_SURROGATE && temp2 <= Character.MAX_SURROGATE);454 }455 value[j] = temp2;456 value[n - j] = temp;457 }458if (hasSurrogate) {459// Reverse back all valid surrogate pairs460for (int i = 0; i < count - 1; i++) {461char c2 = value[i];462if (Character.isLowSurrogate(c2)) {463char c1 = value[i + 1];464if (Character.isHighSurrogate(c1)) {465 value[i++] = c1;466 value[i] = c2;467 }468 }469 }470 }471return this;472 }473474public abstract String toString();475476final char[] getValue() {477return value;478 }479 }View CodeStringBuilder源码(基于jdk1.7.40)1package ng;23public final class StringBuilder4extends AbstractStringBuilder5implements java.io.Serializable, CharSequence {67static final long serialVersionUID = 4383685877147921099L;89// 构造函数。