目錄
從https://www.cnblogs.com/twzheng/p/5923642.html一文的結論進行驗證。
發現效率StringBuilder和StringBuffer的結果是相反的，故此本文加以論證。
實驗報告
StringBuffer>StringBuilder>StringJoin>StringContact>StringPlus+
測試代碼
用其原文測試代碼：
public class StringUtilsTests {
private static final int max = 1000000;
@Test
public void testPlus() {
System.out.println(">>> testPlus() <<<");
String str = "";
long start = System.currentTimeMillis();
for (int i = 0; i < max; i++) {
str = str + "a";
}
long end = System.currentTimeMillis();
long cost = end - start;
System.out.println(" {str + \"a\"} cost=" + cost + " ms");
}
@Test
public void testConcat() {
System.out.println(">>> testConcat() <<<");
String str = "";
long start = System.currentTimeMillis();
for (int i = 0; i < max; i++) {
str = str.concat("a");
}
long end = System.currentTimeMillis();
long cost = end - start;
System.out.println(" {str.concat(\"a\")} cost=" + cost + " ms");
}
@Test
public void testJoin() {
System.out.println(">>> testJoin() <<<");
long start = System.currentTimeMillis();
List<String> list = new ArrayList<String>();
for (int i = 0; i < max; i++) {
list.add("a");
}
long end1 = System.currentTimeMillis();
long cost1 = end1 - start;
StringUtils.join(list, "");
long end = System.currentTimeMillis();
long cost = end - end1;
System.out.println(" {list.add(\"a\")} cost1=" + cost1 + " ms");
System.out.println(" {StringUtils.join(list, \"\")} cost=" + cost
+ " ms");
}
@Test
public void testStringBuffer() {
System.out.println(">>> testStringBuffer() <<<");
long start = System.currentTimeMillis();
StringBuffer strBuffer = new StringBuffer();
for (int i = 0; i < max; i++) {
strBuffer.append("a");
}
strBuffer.toString();
long end = System.currentTimeMillis();
long cost = end - start;
System.out.println(" {strBuffer.append(\"a\")} cost=" + cost + " ms");
}
@Test
public void testStringBuilder() {
System.out.println(">>> testStringBuilder() <<<");
long start = System.currentTimeMillis();
StringBuilder strBuilder = new StringBuilder();
for (int i = 0; i < max; i++) {
strBuilder.append("a");
}
strBuilder.toString();
long end = System.currentTimeMillis();
long cost = end - start;
System.out
.println(" {strBuilder.append(\"a\")} cost=" + cost + " ms");
}
}
StringBuffer
我們重點看append的實現調用的是AbstractStringBuilder類方法：
@Override
public AbstractStringBuilder append(CharSequence s) {
if (s == null)
return appendNull();
if (s instanceof String)
return this.append((String)s);
if (s instanceof AbstractStringBuilder)
return this.append((AbstractStringBuilder)s);
return this.append(s, 0, s.length());
}
最終調用的是String的字符數組拷貝機制：
/**
* Copies characters from this string into the destination character
* array.
* <p>
* The first character to be copied is at index {@code srcBegin};
* the last character to be copied is at index {@code srcEnd-1}
* (thus the total number of characters to be copied is
* {@code srcEnd-srcBegin}). The characters are copied into the
* subarray of {@code dst} starting at index {@code dstBegin}
* and ending at index:
* <blockquote><pre>
* dstBegin + (srcEnd-srcBegin) - 1
* </pre></blockquote>
*
* @param srcBegin index of the first character in the string
* to copy.
* @param srcEnd index after the last character in the string
* to copy.
* @param dst the destination array.
* @param dstBegin the start offset in the destination array.
* @exception IndexOutOfBoundsException If any of the following
* is true:
* <ul><li>{@code srcBegin} is negative.
* <li>{@code srcBegin} is greater than {@code srcEnd}
* <li>{@code srcEnd} is greater than the length of this
* string
* <li>{@code dstBegin} is negative
* <li>{@code dstBegin+(srcEnd-srcBegin)} is larger than
* {@code dst.length}</ul>
*/
public void getChars(int srcBegin, int srcEnd, char dst[], int dstBegin) {
if (srcBegin < 0) {
throw new StringIndexOutOfBoundsException(srcBegin);
}
if (srcEnd > value.length) {
throw new StringIndexOutOfBoundsException(srcEnd);
}
if (srcBegin > srcEnd) {
throw new StringIndexOutOfBoundsException(srcEnd - srcBegin);
}
System.arraycopy(value, srcBegin, dst, dstBegin, srcEnd - srcBegin);
}
StringBuffer源碼：
public final class StringBuffer
extends AbstractStringBuilder
implements java.io.Serializable, CharSequence
{
/**
* A cache of the last value returned by toString. Cleared
* whenever the StringBuffer is modified.
*/
private transient char[] toStringCache;
/** use serialVersionUID from JDK 1.0.2 for interoperability */
static final long serialVersionUID = 3388685877147921107L;
/**
* Constructs a string buffer with no characters in it and an
* initial capacity of 16 characters.
*/
public StringBuffer() {
super(16);
}
/**
* Constructs a string buffer with no characters in it and
* the specified initial capacity.
*
* @param capacity the initial capacity.
* @exception NegativeArraySizeException if the {@code capacity}
* argument is less than {@code 0}.
*/
public StringBuffer(int capacity) {
super(capacity);
}
/**
* Constructs a string buffer initialized to the contents of the
* specified string. The initial capacity of the string buffer is
* {@code 16} plus the length of the string argument.
*
* @param str the initial contents of the buffer.
*/
public StringBuffer(String str) {
super(str.length() + 16);
append(str);
}
/**
* Constructs a string buffer that contains the same characters
* as the specified {@code CharSequence}. The initial capacity of
* the string buffer is {@code 16} plus the length of the
* {@code CharSequence} argument.
* <p>
* If the length of the specified {@code CharSequence} is
* less than or equal to zero, then an empty buffer of capacity
* {@code 16} is returned.
*
* @param seq the sequence to copy.
* @since 1.5
*/
public StringBuffer(CharSequence seq) {
this(seq.length() + 16);
append(seq);
}
@Override
public synchronized int length() {
return count;
}
@Override
public synchronized int capacity() {
return value.length;
}
@Override
public synchronized void ensureCapacity(int minimumCapacity) {
super.ensureCapacity(minimumCapacity);
}
/**
* @since 1.5
*/
@Override
public synchronized void trimToSize() {
super.trimToSize();
}
/**
* @throws IndexOutOfBoundsException {@inheritDoc}
* @see #length()
*/
@Override
public synchronized void setLength(int newLength) {
toStringCache = null;
super.setLength(newLength);
}
/**
* @throws IndexOutOfBoundsException {@inheritDoc}
* @see #length()
*/
@Override
public synchronized char charAt(int index) {
if ((index < 0) || (index >= count))
throw new StringIndexOutOfBoundsException(index);
return value[index];
}
/**
* @since 1.5
*/
@Override
public synchronized int codePointAt(int index) {
return super.codePointAt(index);
}
/**
* @since 1.5
*/
@Override
public synchronized int codePointBefore(int index) {
return super.codePointBefore(index);
}
/**
* @since 1.5
*/
@Override
public synchronized int codePointCount(int beginIndex, int endIndex) {
return super.codePointCount(beginIndex, endIndex);
}
/**
* @since 1.5
*/
@Override
public synchronized int offsetByCodePoints(int index, int codePointOffset) {
return super.offsetByCodePoints(index, codePointOffset);
}
/**
* @throws IndexOutOfBoundsException {@inheritDoc}
*/
@Override
public synchronized void getChars(int srcBegin, int srcEnd, char[] dst,
int dstBegin)
{
super.getChars(srcBegin, srcEnd, dst, dstBegin);
}
/**
* @throws IndexOutOfBoundsException {@inheritDoc}
* @see #length()
*/
@Override
public synchronized void setCharAt(int index, char ch) {
if ((index < 0) || (index >= count))
throw new StringIndexOutOfBoundsException(index);
toStringCache = null;
value[index] = ch;
}
@Override
public synchronized StringBuffer append(Object obj) {
toStringCache = null;
super.append(String.valueOf(obj));
return this;
}
@Override
public synchronized StringBuffer append(String str) {
toStringCache = null;
super.append(str);
return this;
}
/**
* Appends the specified {@code StringBuffer} to this sequence.
* <p>
* The characters of the {@code StringBuffer} argument are appended,
* in order, to the contents of this {@code StringBuffer}, increasing the
* length of this {@code StringBuffer} by the length of the argument.
* If {@code sb} is {@code null}, then the four characters
* {@code "null"} are appended to this {@code StringBuffer}.
* <p>
* Let <i>n</i> be the length of the old character sequence, the one
* contained in the {@code StringBuffer} just prior to execution of the
* {@code append} method. Then the character at index <i>k</i> in
* the new character sequence is equal to the character at index <i>k</i>
* in the old character sequence, if <i>k</i> is less than <i>n</i>;
* otherwise, it is equal to the character at index <i>k-n</i> in the
* argument {@code sb}.
* <p>
* This method synchronizes on {@code this}, the destination
* object, but does not synchronize on the source ({@code sb}).
*
* @param sb the {@code StringBuffer} to append.
* @return a reference to this object.
* @since 1.4
*/
public synchronized StringBuffer append(StringBuffer sb) {
toStringCache = null;
super.append(sb);
return this;
}
/**
* @since 1.8
*/
@Override
synchronized StringBuffer append(AbstractStringBuilder asb) {
toStringCache = null;
super.append(asb);
return this;
}
/**
* Appends the specified {@code CharSequence} to this
* sequence.
* <p>
* The characters of the {@code CharSequence} argument are appended,
* in order, increasing the length of this sequence by the length of the
* argument.
*
* <p>The result of this method is exactly the same as if it were an
* invocation of this.append(s, 0, s.length());
*
* <p>This method synchronizes on {@code this}, the destination
* object, but does not synchronize on the source ({@code s}).
*
* <p>If {@code s} is {@code null}, then the four characters
* {@code "null"} are appended.
*
* @param s the {@code CharSequence} to append.
* @return a reference to this object.
* @since 1.5
*/
@Override
public synchronized StringBuffer append(CharSequence s) {
toStringCache = null;
super.append(s);
return this;
}
/**
* @throws IndexOutOfBoundsException {@inheritDoc}
* @since 1.5
*/
@Override
public synchronized StringBuffer append(CharSequence s, int start, int end)
{
toStringCache = null;
super.append(s, start, end);
return this;
}
@Override
public synchronized StringBuffer append(char[] str) {
toStringCache = null;
super.append(str);
return this;
}
/**
* @throws IndexOutOfBoundsException {@inheritDoc}
*/
@Override
public synchronized StringBuffer append(char[] str, int offset, int len) {
toStringCache = null;
super.append(str, offset, len);
return this;
}
@Override
public synchronized StringBuffer append(boolean b) {
toStringCache = null;
super.append(b);
return this;
}
@Override
public synchronized StringBuffer append(char c) {
toStringCache = null;
super.append(c);
return this;
}
@Override
public synchronized StringBuffer append(int i) {
toStringCache = null;
super.append(i);
return this;
}
/**
* @since 1.5
*/
@Override
public synchronized StringBuffer appendCodePoint(int codePoint) {
toStringCache = null;
super.appendCodePoint(codePoint);
return this;
}
@Override
public synchronized StringBuffer append(long lng) {
toStringCache = null;
super.append(lng);
return this;
}
@Override
public synchronized StringBuffer append(float f) {
toStringCache = null;
super.append(f);
return this;
}
@Override
public synchronized StringBuffer append(double d) {
toStringCache = null;
super.append(d);
return this;
}
/**
* @throws StringIndexOutOfBoundsException {@inheritDoc}
* @since 1.2
*/
@Override
public synchronized StringBuffer delete(int start, int end) {
toStringCache = null;
super.delete(start, end);
return this;
}
/**
* @throws StringIndexOutOfBoundsException {@inheritDoc}
* @since 1.2
*/
@Override
public synchronized StringBuffer deleteCharAt(int index) {
toStringCache = null;
super.deleteCharAt(index);
return this;
}
/**
* @throws StringIndexOutOfBoundsException {@inheritDoc}
* @since 1.2
*/
@Override
public synchronized StringBuffer replace(int start, int end, String str) {
toStringCache = null;
super.replace(start, end, str);
return this;
}
/**
* @throws StringIndexOutOfBoundsException {@inheritDoc}
* @since 1.2
*/
@Override
public synchronized String substring(int start) {
return substring(start, count);
}
/**
* @throws IndexOutOfBoundsException {@inheritDoc}
* @since 1.4
*/
@Override
public synchronized CharSequence subSequence(int start, int end) {
return super.substring(start, end);
}
/**
* @throws StringIndexOutOfBoundsException {@inheritDoc}
* @since 1.2
*/
@Override
public synchronized String substring(int start, int end) {
return super.substring(start, end);
}
/**
* @throws StringIndexOutOfBoundsException {@inheritDoc}
* @since 1.2
*/
@Override
public synchronized StringBuffer insert(int index, char[] str, int offset,
int len)
{
toStringCache = null;
super.insert(index, str, offset, len);
return this;
}
/**
* @throws StringIndexOutOfBoundsException {@inheritDoc}
*/
@Override
public synchronized StringBuffer insert(int offset, Object obj) {
toStringCache = null;
super.insert(offset, String.valueOf(obj));
return this;
}
/**
* @throws StringIndexOutOfBoundsException {@inheritDoc}
*/
@Override
public synchronized StringBuffer insert(int offset, String str) {
toStringCache = null;
super.insert(offset, str);
return this;
}
/**
* @throws StringIndexOutOfBoundsException {@inheritDoc}
*/
@Override
public synchronized StringBuffer insert(int offset, char[] str) {
toStringCache = null;
super.insert(offset, str);
return this;
}
/**
* @throws IndexOutOfBoundsException {@inheritDoc}
* @since 1.5
*/
@Override
public StringBuffer insert(int dstOffset, CharSequence s) {
// Note, synchronization achieved via invocations of other StringBuffer methods
// after narrowing of s to specific type
// Ditto for toStringCache clearing
super.insert(dstOffset, s);
return this;
}
/**
* @throws IndexOutOfBoundsException {@inheritDoc}
* @since 1.5
*/
@Override
public synchronized StringBuffer insert(int dstOffset, CharSequence s,
int start, int end)
{
toStringCache = null;
super.insert(dstOffset, s, start, end);
return this;
}
/**
* @throws StringIndexOutOfBoundsException {@inheritDoc}
*/
@Override
public StringBuffer insert(int offset, boolean b) {
// Note, synchronization achieved via invocation of StringBuffer insert(int, String)
// after conversion of b to String by super class method
// Ditto for toStringCache clearing
super.insert(offset, b);
return this;
}
/**
* @throws IndexOutOfBoundsException {@inheritDoc}
*/
@Override
public synchronized StringBuffer insert(int offset, char c) {
toStringCache = null;
super.insert(offset, c);
return this;
}
/**
* @throws StringIndexOutOfBoundsException {@inheritDoc}
*/
@Override
public StringBuffer insert(int offset, int i) {
// Note, synchronization achieved via invocation of StringBuffer insert(int, String)
// after conversion of i to String by super class method
// Ditto for toStringCache clearing
super.insert(offset, i);
return this;
}
/**
* @throws StringIndexOutOfBoundsException {@inheritDoc}
*/
@Override
public StringBuffer insert(int offset, long l) {
// Note, synchronization achieved via invocation of StringBuffer insert(int, String)
// after conversion of l to String by super class method
// Ditto for toStringCache clearing
super.insert(offset, l);
return this;
}
/**
* @throws StringIndexOutOfBoundsException {@inheritDoc}
*/
@Override
public StringBuffer insert(int offset, float f) {
// Note, synchronization achieved via invocation of StringBuffer insert(int, String)
// after conversion of f to String by super class method
// Ditto for toStringCache clearing
super.insert(offset, f);
return this;
}
/**
* @throws StringIndexOutOfBoundsException {@inheritDoc}
*/
@Override
public StringBuffer insert(int offset, double d) {
// Note, synchronization achieved via invocation of StringBuffer insert(int, String)
// after conversion of d to String by super class method
// Ditto for toStringCache clearing
super.insert(offset, d);
return this;
}
/**
* @since 1.4
*/
@Override
public int indexOf(String str) {
// Note, synchronization achieved via invocations of other StringBuffer methods
return super.indexOf(str);
}
/**
* @since 1.4
*/
@Override
public synchronized int indexOf(String str, int fromIndex) {
return super.indexOf(str, fromIndex);
}
/**
* @since 1.4
*/
@Override
public int lastIndexOf(String str) {
// Note, synchronization achieved via invocations of other StringBuffer methods
return lastIndexOf(str, count);
}
/**
* @since 1.4
*/
@Override
public synchronized int lastIndexOf(String str, int fromIndex) {
return super.lastIndexOf(str, fromIndex);
}
/**
* @since JDK1.0.2
*/
@Override
public synchronized StringBuffer reverse() {
toStringCache = null;
super.reverse();
return this;
}
@Override
public synchronized String toString() {
if (toStringCache == null) {
toStringCache = Arrays.copyOfRange(value, 0, count);
}
return new String(toStringCache, true);
}
/**
* Serializable fields for StringBuffer.
*
* @serialField value char[]
* The backing character array of this StringBuffer.
* @serialField count int
* The number of characters in this StringBuffer.
* @serialField shared boolean
* A flag indicating whether the backing array is shared.
* The value is ignored upon deserialization.
*/
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),
};
/**
* readObject is called to restore the state of the StringBuffer from
* a stream.
*/
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();
}
/**
* readObject is called to restore the state of the StringBuffer from
* a stream.
*/
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);
}
}
StringBuilder
StringBuilder的實現與StringBuffer代碼上差不多，但StringBuffer是使用了鎖是線程安全的。
public final class StringBuilder
extends AbstractStringBuilder
implements java.io.Serializable, CharSequence
{
/** use serialVersionUID for interoperability */
static final long serialVersionUID = 4383685877147921099L;
/**
* Constructs a string builder with no characters in it and an
* initial capacity of 16 characters.
*/
public StringBuilder() {
super(16);
}
/**
* Constructs a string builder with no characters in it and an
* initial capacity specified by the {@code capacity} argument.
*
* @param capacity the initial capacity.
* @throws NegativeArraySizeException if the {@code capacity}
* argument is less than {@code 0}.
*/
public StringBuilder(int capacity) {
super(capacity);
}
/**
* Constructs a string builder initialized to the contents of the
* specified string. The initial capacity of the string builder is
* {@code 16} plus the length of the string argument.
*
* @param str the initial contents of the buffer.
*/
public StringBuilder(String str) {
super(str.length() + 16);
append(str);
}
/**
* Constructs a string builder that contains the same characters
* as the specified {@code CharSequence}. The initial capacity of
* the string builder is {@code 16} plus the length of the
* {@code CharSequence} argument.
*
* @param seq the sequence to copy.
*/
public StringBuilder(CharSequence seq) {
this(seq.length() + 16);
append(seq);
}
@Override
public StringBuilder append(Object obj) {
return append(String.valueOf(obj));
}
@Override
public StringBuilder append(String str) {
super.append(str);
return this;
}
/**
* Appends the specified {@code StringBuffer} to this sequence.
* <p>
* The characters of the {@code StringBuffer} argument are appended,
* in order, to this sequence, increasing the
* length of this sequence by the length of the argument.
* If {@code sb} is {@code null}, then the four characters
* {@code "null"} are appended to this sequence.
* <p>
* Let <i>n</i> be the length of this character sequence just prior to
* execution of the {@code append} method. Then the character at index
* <i>k</i> in the new character sequence is equal to the character at
* index <i>k</i> in the old character sequence, if <i>k</i> is less than
* <i>n</i>; otherwise, it is equal to the character at index <i>k-n</i>
* in the argument {@code sb}.
*
* @param sb the {@code StringBuffer} to append.
* @return a reference to this object.
*/
public StringBuilder append(StringBuffer sb) {
super.append(sb);
return this;
}
@Override
public StringBuilder append(CharSequence s) {
super.append(s);
return this;
}
/**
* @throws IndexOutOfBoundsException {@inheritDoc}
*/
@Override
public StringBuilder append(CharSequence s, int start, int end) {
super.append(s, start, end);
return this;
}
@Override
public StringBuilder append(char[] str) {
super.append(str);
return this;
}
/**
* @throws IndexOutOfBoundsException {@inheritDoc}
*/
@Override
public StringBuilder append(char[] str, int offset, int len) {
super.append(str, offset, len);
return this;
}
@Override
public StringBuilder append(boolean b) {
super.append(b);
return this;
}
@Override
public StringBuilder append(char c) {
super.append(c);
return this;
}
@Override
public StringBuilder append(int i) {
super.append(i);
return this;
}
@Override
public StringBuilder append(long lng) {
super.append(lng);
return this;
}
@Override
public StringBuilder append(float f) {
super.append(f);
return this;
}
@Override
public StringBuilder append(double d) {
super.append(d);
return this;
}
/**
* @since 1.5
*/
@Override
public StringBuilder appendCodePoint(int codePoint) {
super.appendCodePoint(codePoint);
return this;
}
/**
* @throws StringIndexOutOfBoundsException {@inheritDoc}
*/
@Override
public StringBuilder delete(int start, int end) {
super.delete(start, end);
return this;
}
/**
* @throws StringIndexOutOfBoundsException {@inheritDoc}
*/
@Override
public StringBuilder deleteCharAt(int index) {
super.deleteCharAt(index);
return this;
}
/**
* @throws StringIndexOutOfBoundsException {@inheritDoc}
*/
@Override
public StringBuilder replace(int start, int end, String str) {
super.replace(start, end, str);
return this;
}
/**
* @throws StringIndexOutOfBoundsException {@inheritDoc}
*/
@Override
public StringBuilder insert(int index, char[] str, int offset,
int len)
{
super.insert(index, str, offset, len);
return this;
}
/**
* @throws StringIndexOutOfBoundsException {@inheritDoc}
*/
@Override
public StringBuilder insert(int offset, Object obj) {
super.insert(offset, obj);
return this;
}
/**
* @throws StringIndexOutOfBoundsException {@inheritDoc}
*/
@Override
public StringBuilder insert(int offset, String str) {
super.insert(offset, str);
return this;
}
/**
* @throws StringIndexOutOfBoundsException {@inheritDoc}
*/
@Override
public StringBuilder insert(int offset, char[] str) {
super.insert(offset, str);
return this;
}
/**
* @throws IndexOutOfBoundsException {@inheritDoc}
*/
@Override
public StringBuilder insert(int dstOffset, CharSequence s) {
super.insert(dstOffset, s);
return this;
}
/**
* @throws IndexOutOfBoundsException {@inheritDoc}
*/
@Override
public StringBuilder insert(int dstOffset, CharSequence s,
int start, int end)
{
super.insert(dstOffset, s, start, end);
return this;
}
/**
* @throws StringIndexOutOfBoundsException {@inheritDoc}
*/
@Override
public StringBuilder insert(int offset, boolean b) {
super.insert(offset, b);
return this;
}
/**
* @throws IndexOutOfBoundsException {@inheritDoc}
*/
@Override
public StringBuilder insert(int offset, char c) {
super.insert(offset, c);
return this;
}
/**
* @throws StringIndexOutOfBoundsException {@inheritDoc}
*/
@Override
public StringBuilder insert(int offset, int i) {
super.insert(offset, i);
return this;
}
/**
* @throws StringIndexOutOfBoundsException {@inheritDoc}
*/
@Override
public StringBuilder insert(int offset, long l) {
super.insert(offset, l);
return this;
}
/**
* @throws StringIndexOutOfBoundsException {@inheritDoc}
*/
@Override
public StringBuilder insert(int offset, float f) {
super.insert(offset, f);
return this;
}
/**
* @throws StringIndexOutOfBoundsException {@inheritDoc}
*/
@Override
public StringBuilder insert(int offset, double d) {
super.insert(offset, d);
return this;
}
@Override
public int indexOf(String str) {
return super.indexOf(str);
}
@Override
public int indexOf(String str, int fromIndex) {
return super.indexOf(str, fromIndex);
}
@Override
public int lastIndexOf(String str) {
return super.lastIndexOf(str);
}
@Override
public int lastIndexOf(String str, int fromIndex) {
return super.lastIndexOf(str, fromIndex);
}
@Override
public StringBuilder reverse() {
super.reverse();
return this;
}
@Override
public String toString() {
// Create a copy, don't share the array
return new String(value, 0, count);
}
/**
* Save the state of the {@code StringBuilder} instance to a stream
* (that is, serialize it).
*
* @serialData the number of characters currently stored in the string
* builder ({@code int}), followed by the characters in the
* string builder ({@code char[]}). The length of the
* {@code char} array may be greater than the number of
* characters currently stored in the string builder, in which
* case extra characters are ignored.
*/
private void writeObject(java.io.ObjectOutputStream s)
throws java.io.IOException {
s.defaultWriteObject();
s.writeInt(count);
s.writeObject(value);
}
/**
* readObject is called to restore the state of the StringBuffer from
* a stream.
*/
private void readObject(java.io.ObjectInputStream s)
throws java.io.IOException, ClassNotFoundException {
s.defaultReadObject();
count = s.readInt();
value = (char[]) s.readObject();
}
}
如何推導結論？
衆所周知，static 的變量是線程之間共享的，加synchronized字段的方法和代碼塊在多線程調度中也能起到資源集中佔用的優勢。
結論：線程安全的就是獨佔的，所以效率上StringBuffer比StringBuilder的效率要高。