StringBuffer 详解

StringBuffer 简介

StringBuffer 是一个线程安全的可变的字符序列。它继承于AbstractStringBuilder，实现了CharSequence接口。
StringBuilder 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。

StringBuffer 和 CharSequence之间的关系图如下：

StringBuffer源码(基于jdk1.7.40)

package java.lang;public final class StringBufferextends AbstractStringBuilderimplements java.io.Serializable, CharSequence
{static final long serialVersionUID = 3388685877147921107L;public StringBuffer() {super(16);}public StringBuffer(int capacity) {super(capacity);}public StringBuffer(String str) {super(str.length() + 16);append(str);}public StringBuffer(CharSequence seq) {this(seq.length() + 16);append(seq);}public synchronized int length() {return count;}public synchronized int capacity() {return value.length;}public synchronized void ensureCapacity(int minimumCapacity) {if (minimumCapacity > value.length) {expandCapacity(minimumCapacity);}}public synchronized void trimToSize() {super.trimToSize();}public synchronized void setLength(int newLength) {super.setLength(newLength);}public synchronized char charAt(int index) {if ((index < 0) || (index >= count))throw new StringIndexOutOfBoundsException(index);return value[index];}public synchronized int codePointAt(int index) {return super.codePointAt(index);}public synchronized int codePointBefore(int index) {return super.codePointBefore(index);}public synchronized int codePointCount(int beginIndex, int endIndex) {return super.codePointCount(beginIndex, endIndex);}public synchronized int offsetByCodePoints(int index, int codePointOffset) {return super.offsetByCodePoints(index, codePointOffset);}public synchronized void getChars(int srcBegin, int srcEnd, char[] dst,int dstBegin){super.getChars(srcBegin, srcEnd, dst, dstBegin);}public synchronized void setCharAt(int index, char ch) {if ((index < 0) || (index >= count))throw new StringIndexOutOfBoundsException(index);value[index] = ch;}public synchronized StringBuffer append(Object obj) {super.append(String.valueOf(obj));return this;}public synchronized StringBuffer append(String str) {super.append(str);return this;}public synchronized StringBuffer append(StringBuffer sb) {super.append(sb);return this;}public StringBuffer append(CharSequence s) {// Note, synchronization achieved via other invocationsif (s == null)s = "null";if (s instanceof String)return this.append((String)s);if (s instanceof StringBuffer)return this.append((StringBuffer)s);return this.append(s, 0, s.length());}public synchronized StringBuffer append(CharSequence s, int start, int end){super.append(s, start, end);return this;}public synchronized StringBuffer append(char[] str) {super.append(str);return this;}public synchronized StringBuffer append(char[] str, int offset, int len) {super.append(str, offset, len);return this;}public synchronized StringBuffer append(boolean b) {super.append(b);return this;}public synchronized StringBuffer append(char c) {super.append(c);return this;}public synchronized StringBuffer append(int i) {super.append(i);return this;}public synchronized StringBuffer appendCodePoint(int codePoint) {super.appendCodePoint(codePoint);return this;}public synchronized StringBuffer append(long lng) {super.append(lng);return this;}public synchronized StringBuffer append(float f) {super.append(f);return this;}public synchronized StringBuffer append(double d) {super.append(d);return this;}public synchronized StringBuffer delete(int start, int end) {super.delete(start, end);return this;}public synchronized StringBuffer deleteCharAt(int index) {super.deleteCharAt(index);return this;}public synchronized StringBuffer replace(int start, int end, String str) {super.replace(start, end, str);return this;}public synchronized String substring(int start) {return substring(start, count);}public synchronized CharSequence subSequence(int start, int end) {return super.substring(start, end);}public synchronized String substring(int start, int end) {return super.substring(start, end);}public synchronized StringBuffer insert(int index, char[] str, int offset,int len){super.insert(index, str, offset, len);return this;}public synchronized StringBuffer insert(int offset, Object obj) {super.insert(offset, String.valueOf(obj));return this;}public synchronized StringBuffer insert(int offset, String str) {super.insert(offset, str);return this;}public synchronized StringBuffer insert(int offset, char[] str) {super.insert(offset, str);return this;}public StringBuffer insert(int dstOffset, CharSequence s) {// Note, synchronization achieved via other invocationsif (s == null)s = "null";if (s instanceof String)return this.insert(dstOffset, (String)s);return this.insert(dstOffset, s, 0, s.length());}public synchronized StringBuffer insert(int dstOffset, CharSequence s,int start, int end){super.insert(dstOffset, s, start, end);return this;}public StringBuffer insert(int offset, boolean b) {return insert(offset, String.valueOf(b));}public synchronized StringBuffer insert(int offset, char c) {super.insert(offset, c);return this;}public StringBuffer insert(int offset, int i) {return insert(offset, String.valueOf(i));}public StringBuffer insert(int offset, long l) {return insert(offset, String.valueOf(l));}public StringBuffer insert(int offset, float f) {return insert(offset, String.valueOf(f));}public StringBuffer insert(int offset, double d) {return insert(offset, String.valueOf(d));}public int indexOf(String str) {return indexOf(str, 0);}public synchronized int indexOf(String str, int fromIndex) {return String.indexOf(value, 0, count,str.toCharArray(), 0, str.length(), fromIndex);}public int lastIndexOf(String str) {// Note, synchronization achieved via other invocationsreturn lastIndexOf(str, count);}public synchronized int lastIndexOf(String str, int fromIndex) {return String.lastIndexOf(value, 0, count,str.toCharArray(), 0, str.length(), fromIndex);}public synchronized StringBuffer reverse() {super.reverse();return this;}public synchronized String toString() {return new String(value, 0, count);}private static final java.io.ObjectStreamField[] serialPersistentFields ={new java.io.ObjectStreamField("value", char[].class),new java.io.ObjectStreamField("count", Integer.TYPE),new java.io.ObjectStreamField("shared", Boolean.TYPE),};private synchronized void writeObject(java.io.ObjectOutputStream s)throws java.io.IOException {java.io.ObjectOutputStream.PutField fields = s.putFields();fields.put("value", value);fields.put("count", count);fields.put("shared", false);s.writeFields();}private void readObject(java.io.ObjectInputStream s)throws java.io.IOException, ClassNotFoundException {java.io.ObjectInputStream.GetField fields = s.readFields();value = (char[])fields.get("value", null);count = fields.get("count", 0);}
}

StringBuffer 示例

/*** StringBuffer 演示程序** @author skywang*/
import java.util.HashMap;public class StringBufferTest {public static void main(String[] args) {testInsertAPIs() ;testAppendAPIs() ;testReplaceAPIs() ;testDeleteAPIs() ;testIndexAPIs() ;testOtherAPIs() ;}/*** StringBuffer 的其它API示例*/private static void testOtherAPIs() {System.out.println("-------------------------------- testOtherAPIs --------------------------------");StringBuffer sbuilder = new StringBuffer("0123456789");int cap = sbuilder.capacity();System.out.printf("cap=%d\n", cap);char c = sbuilder.charAt(6);System.out.printf("c=%c\n", c);char[] carr = new char[4];sbuilder.getChars(3, 7, carr, 0);for (int i=0; i

-------------------------------- testInsertAPIs -------------------------------
{3=three, 2=two, 1=one}
12345StringBUFFERStringBufferBUILDERStringBuilder12345100
true3.141591.414ABCabcde-------------------------------- testAppendAPIs -------------------------------
abcdeABC1.4143.14159true
10012345StringBuilderBUILDERStringBufferBUFFERString12345
{3=three, 2=two, 1=one}
字符编码-------------------------------- testReplaceAPIs ------------------------------
sbuilder=ABCDE3456789
sbuilder=9876543210
sbuilder=M123456789-------------------------------- testDeleteAPIs -------------------------------
sbuilder=123789
str1=23789
str2=78
str3=78-------------------------------- testIndexAPIs --------------------------------
sbuilder=abcAbcABCabCaBcAbCaBCabc
sbuilder.indexOf("bc")         = 1
sbuilder.indexOf("bc", 5)      = 22
sbuilder.lastIndexOf("bc")     = 22
sbuilder.lastIndexOf("bc", 4)  = 4-------------------------------- testOtherAPIs --------------------------------
cap=26
c=6
carr[0]=3 carr[1]=4 carr[2]=5 carr[3]=6

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