StringBuffer 详解[通俗易懂]

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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

StringBuffer源码(基于jdk1.7.40)

package java.lang;

public final class StringBuffer
    extends AbstractStringBuilder
    implements 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 invocations
        if (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 invocations
        if (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 invocations
        return 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

源码如下(StringBufferTest.java):

/**
 * 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<carr.length; i++)
            System.out.printf("carr[%d]=%c ", i, carr[i]);
        System.out.println();

        System.out.println();
    }

    /**
     * StringBuffer 中index相关API演示
     */
    private static void testIndexAPIs() {
        System.out.println("-------------------------------- testIndexAPIs --------------------------------");

        StringBuffer sbuilder = new StringBuffer("abcAbcABCabCaBcAbCaBCabc");
        System.out.printf("sbuilder=%s\n", sbuilder);

        // 1. 从前往后，找出"bc"第一次出现的位置
        System.out.printf("%-30s = %d\n", "sbuilder.indexOf(\"bc\")", sbuilder.indexOf("bc"));

        // 2. 从位置5开始，从前往后，找出"bc"第一次出现的位置
        System.out.printf("%-30s = %d\n", "sbuilder.indexOf(\"bc\", 5)", sbuilder.indexOf("bc", 5));

        // 3. 从后往前，找出"bc"第一次出现的位置
        System.out.printf("%-30s = %d\n", "sbuilder.lastIndexOf(\"bc\")", sbuilder.lastIndexOf("bc"));

        // 4. 从位置4开始，从后往前，找出"bc"第一次出现的位置
        System.out.printf("%-30s = %d\n", "sbuilder.lastIndexOf(\"bc\", 4)", sbuilder.lastIndexOf("bc", 4));

        System.out.println();
    }

    /**
     * StringBuffer 的replace()示例
     */
    private static void testReplaceAPIs() {

        System.out.println("-------------------------------- testReplaceAPIs ------------------------------");

        StringBuffer sbuilder;

        sbuilder = new StringBuffer("0123456789");
        sbuilder.replace(0, 3, "ABCDE");
        System.out.printf("sbuilder=%s\n", sbuilder);

        sbuilder = new StringBuffer("0123456789");
        sbuilder.reverse();
        System.out.printf("sbuilder=%s\n", sbuilder);

        sbuilder = new StringBuffer("0123456789");
        sbuilder.setCharAt(0, 'M');
        System.out.printf("sbuilder=%s\n", sbuilder);

        System.out.println();
    }

    /**
     * StringBuffer 的delete()示例
     */
    private static void testDeleteAPIs() {

        System.out.println("-------------------------------- testDeleteAPIs -------------------------------");

        StringBuffer sbuilder = new StringBuffer("0123456789");
        
        // 删除位置0的字符，剩余字符是“123456789”。
        sbuilder.deleteCharAt(0);
        // 删除位置3(包括)到位置6(不包括)之间的字符，剩余字符是“123789”。
        sbuilder.delete(3,6);

        // 获取sb中从位置1开始的字符串
        String str1 = sbuilder.substring(1);
        // 获取sb中从位置3(包括)到位置5(不包括)之间的字符串
        String str2 = sbuilder.substring(3, 5);
        // 获取sb中从位置3(包括)到位置5(不包括)之间的字符串，获取的对象是CharSequence对象，此处转型为String
        String str3 = (String)sbuilder.subSequence(3, 5);

        System.out.printf("sbuilder=%s\nstr1=%s\nstr2=%s\nstr3=%s\n", 
                sbuilder, str1, str2, str3);

        System.out.println();
    }

    /**
     * StringBuffer 的insert()示例
     */
    private static void testInsertAPIs() {

        System.out.println("-------------------------------- testInsertAPIs -------------------------------");

        StringBuffer sbuilder = new StringBuffer();

        // 在位置0处插入字符数组
        sbuilder.insert(0, new char[]{'a','b','c','d','e'});
        // 在位置0处插入字符数组。0表示字符数组起始位置，3表示长度
        sbuilder.insert(0, new char[]{'A','B','C','D','E'}, 0, 3);
        // 在位置0处插入float
        sbuilder.insert(0, 1.414f);
        // 在位置0处插入double
        sbuilder.insert(0, 3.14159d);
        // 在位置0处插入boolean
        sbuilder.insert(0, true);
        // 在位置0处插入char
        sbuilder.insert(0, '\n');
        // 在位置0处插入int
        sbuilder.insert(0, 100);
        // 在位置0处插入long
        sbuilder.insert(0, 12345L);
        // 在位置0处插入StringBuilder对象
        sbuilder.insert(0, new StringBuffer("StringBuilder"));
        // 在位置0处插入StringBuilder对象。6表示被在位置0处插入对象的起始位置(包括)，13是结束位置(不包括)
        sbuilder.insert(0, new StringBuffer("STRINGBUILDER"), 6, 13);
        // 在位置0处插入StringBuffer对象。
        sbuilder.insert(0, new StringBuffer("StringBuffer"));
        // 在位置0处插入StringBuffer对象。6表示被在位置0处插入对象的起始位置(包括)，12是结束位置(不包括)
        sbuilder.insert(0, new StringBuffer("STRINGBUFFER"), 6, 12);
        // 在位置0处插入String对象。
        sbuilder.insert(0, "String");
        // 在位置0处插入String对象。1表示被在位置0处插入对象的起始位置(包括)，6是结束位置(不包括)
        sbuilder.insert(0, "0123456789", 1, 6);
        sbuilder.insert(0, '\n');

        // 在位置0处插入Object对象。此处以HashMap为例
        HashMap map = new HashMap();
        map.put("1", "one");
        map.put("2", "two");
        map.put("3", "three");
        sbuilder.insert(0, map);

        System.out.printf("%s\n\n", sbuilder);
    }

    /**
     * StringBuffer 的append()示例
     */
    private static void testAppendAPIs() {

        System.out.println("-------------------------------- testAppendAPIs -------------------------------");

        StringBuffer sbuilder = new StringBuffer();

        // 追加字符数组
        sbuilder.append(new char[]{'a','b','c','d','e'});
        // 追加字符数组。0表示字符数组起始位置，3表示长度
        sbuilder.append(new char[]{'A','B','C','D','E'}, 0, 3);
        // 追加float
        sbuilder.append(1.414f);
        // 追加double
        sbuilder.append(3.14159d);
        // 追加boolean
        sbuilder.append(true);
        // 追加char
        sbuilder.append('\n');
        // 追加int
        sbuilder.append(100);
        // 追加long
        sbuilder.append(12345L);
        // 追加StringBuilder对象
        sbuilder.append(new StringBuffer("StringBuilder"));
        // 追加StringBuilder对象。6表示被追加对象的起始位置(包括)，13是结束位置(不包括)
        sbuilder.append(new StringBuffer("STRINGBUILDER"), 6, 13);
        // 追加StringBuffer对象。
        sbuilder.append(new StringBuffer("StringBuffer"));
        // 追加StringBuffer对象。6表示被追加对象的起始位置(包括)，12是结束位置(不包括)
        sbuilder.append(new StringBuffer("STRINGBUFFER"), 6, 12);
        // 追加String对象。
        sbuilder.append("String");
        // 追加String对象。1表示被追加对象的起始位置(包括)，6是结束位置(不包括)
        sbuilder.append("0123456789", 1, 6);
        sbuilder.append('\n');

        // 追加Object对象。此处以HashMap为例
        HashMap map = new HashMap();
        map.put("1", "one");
        map.put("2", "two");
        map.put("3", "three");
        sbuilder.append(map);
        sbuilder.append('\n');

        // 追加unicode编码
        sbuilder.appendCodePoint(0x5b57);    // 0x5b57是“字”的unicode编码
        sbuilder.appendCodePoint(0x7b26);    // 0x7b26是“符”的unicode编码
        sbuilder.appendCodePoint(0x7f16);    // 0x7f16是“编”的unicode编码
        sbuilder.appendCodePoint(0x7801);    // 0x7801是“码”的unicode编码

        System.out.printf("%s\n\n", sbuilder);
    }
}

运行结果
：

-------------------------------- 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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