深入学习java源码之StringBuilder.append()与StringBuilder.appendCodePoint()
深入学习java源码之StringBuilder.append()与StringBuilder.appendCodePoint()
CharSequence接口
charSequence是一个接口，表示char值的一个可读序列。此接口对许多不同种类的char序列提供统一的自读访问。此接口不修改该equals和hashCode方法的常规协定，因此，通常未定义比较实现 CharSequence 的两个对象的结果。他有几个实现类：CharBuffer、String、StringBuffer、StringBuilder。
CharSequence与String都能用于定义字符串，但CharSequence的值是可读可写序列，而String的值是只读序列。
对于一个抽象类或者是接口类，不能使用new来进行赋值，但是可以通过以下的方式来进行实例的创建：
CharSequence b = "s";
CharSequence cs=”hello”;通常我们都把它等价于String类型，将参数使用toString()等方法转换为String再传入。实际上，CharSequence是一个接口，String、StringBuilder和StringBuffer都实现了CharSequence接口，所以以后再调用含有CharSequence类型参数的方法时，可以直接将StringBuilder或StringBuffer直接传入进去，减少一次转换。另外，有些工具类中跟字符串相关的类型，也会实现这个接口，便于减少方法调用直接传参。
Appendable接口
Appendable接口的实现类的对象能够被添加 char 序列和值。如果某个类的实例打算接收取自 java.util.Formatter 的格式化输出，那么该类必须实现 Appendable 接口。格式化主要用在文本输出方面,比如,数字、日期、金额等
要添加的字符应该是有效的 Unicode 字符。
Appendable 对于多线程访问而言没必要是安全的。线程安全由扩展和实现此接口的类负责。
所有已知实现类：
BufferedWriter, CharArrayWriter, CharBuffer, FileWriter, FilterWriter, LogStream, OutputStreamWriter, PipedWriter, PrintStream, PrintWriter, StringBuffer, StringBuilder, StringWriter, Writer
java的super与this关键字
super关键字
在JAVA类中使用super来引用父类的成分，用this来引用当前对象，如果一个类从另外一个类继承，我们new这个子类的实例对象的时候，这个子类对象里面会有一个父类对象。怎么去引用里面的父类对象呢？使用super来引用，this指的是当前对象的引用，super是当前对象里面的父对象的引用。
super关键字有两种用法：
1 在子类的构造方法中使用super关键字调用直接父类的构造方法，super语句必须是该子类构造方法中的第一句 。如果在子类中没有使用super关键字，系统会默认调用父类没有参数的构造方法，即隐含的使用super();语句，注意这时父类一定应该有一个无参数的构造方法。
2 使用super关键字调用被子类隐藏的成员变量和方法。如果子类定义了和父类具有相同名字的成员变量，子类就隐藏了从父类继承的成员变量。如果子类定义的某个方法与父类定义的某个方法完全相同，从父类继承的该方法就被完全隐藏（即override，覆盖）。
this关键字
this关键字可以出现在构造方法和实例方法中，不能出现在静态方法中，这是因为静态方法可以用类名来调用，这时可能还没有任何对象诞生。
this主要有两种用法：
1 用在构造方法中，调用本类的其他构造方法。 格式为 this([实参列表]); 通过this调用本类的其他构造方法时，该语句必须出现在构造方法中，并且必须作为第一条语句出现。
2 代指当前对象
StringBuilder的append方法
StringBuilder sb = new StirngBuilder();
sb.append("a").append("b").append("c").append("d");String str = "";
str += "a";
str += "b";
str += "c";
str += "d";上面的例子中，“a”、“b”、“c”、“d”都应该会首先在常量池中分配空间，是第一段代码效率更高
而str + "a" 会创建一个新的String对象，就慢了。你要知道String对象一旦创建就是不能被改变的，要达到字符串拼接的效果，就得不停创建新对象，而创建新的对象时，有可能会触发GC。
StringBuilder直到最后sb.toString()才会创建String对象，之前都没有创建新对象，但是如果你append的总长度超过一定范围——默认是16——就会创建一个新的数组，来装下更多的String。
也就是说 快得原因就是因为StringBuilder预先开辟了空间, append的时候只是向内存地址赋值; 而String总要不断的现开辟空间. 也因此String占的空间也会相对大。
StringBuilder和StringBuffer，字符串是存放在char[]中的，char[]是存放在堆中的。
相比String每次+都重新创建一个String对象，重新开辟一段内存不同，StringBuilder和StringBuffer的append都是直接把String对象中的char[]的字符直接拷贝到StringBuilder和StringBuffer的char[]上，效率比String的+高得多。当然，当StringBuilder和StringBuffer的char[]长度不够时，也会重新开辟一段内存。
另外，StringBuffer是线程安全的，StringBuilder不是线程安全的。
StringBuilder()的高性能
public final class StringBuilder
extends AbstractStringBuilder
implements java.io.Serializable, CharSequence
{
static final long serialVersionUID = 4383685877147921099L;
public StringBuilder() {
super(16);
}
public StringBuilder(int capacity) {
super(capacity);
}
}
父类AbstractStringBuilder 中的AbstractStringBuilder(int capacity)构造方法
abstract class AbstractStringBuilder implements Appendable, CharSequence {
char[] value;
int count;
AbstractStringBuilder() {
}
AbstractStringBuilder(int capacity) {
value = new char[capacity];
}
}StringBuilder的内部有一个char[]， 在调用StringBuilder的无参构造方法时其内部char[]的默认长度是16。当我们调用StringBuilder的append方法时，其实就是不断的往char[]里填东西的过程。
public StringBuilder append(String str) {
super.append(str);
return this;
}其中，super.append是调用AbstractStringBuilder 的append(String str)方法，如下：
public AbstractStringBuilder append(String str) {
if (str == null) str = "null";
int len = str.length();
ensureCapacityInternal(count + len);
str.getChars(0, len, value, count);
count += len;
return this;
}StringBuilder的扩容和ArrayList有些类似
private void ensureCapacityInternal(int minimumCapacity) {
// overflow-conscious code
if (minimumCapacity - value.length > 0)
expandCapacity(minimumCapacity);
}
void expandCapacity(int minimumCapacity) {
int newCapacity = value.length * 2 + 2;
if (newCapacity - minimumCapacity < 0)
newCapacity = minimumCapacity;
if (newCapacity < 0) {
if (minimumCapacity < 0) // overflow
throw new OutOfMemoryError();
newCapacity = Integer.MAX_VALUE;
}
value = Arrays.copyOf(value, newCapacity);
}StringBuilder默认长度是16，然后，如果要append第17个字符，怎么办？
答案是采用 Arrays.copyOf()成倍复制扩容！
扩容的性能代价是很严重的：一来有数组拷贝的成本，二来原来的char[]也白白浪费了要被GC掉。可以想见，一个129字符长度的字符串，经过了16，32，64, 128四次的复制和丢弃，合共申请了496字符的数组，在高性能场景下，这几乎不能忍。
由此可见，合理设置一个初始值多重要。使用之前先仔细评估一下要保存的字符串最大长度。
复用StringBuilder
StringBuilder.setLength()方法只重置它的count指针，而char[]则会继续重用
public void setLength(int newLength) {
if (newLength < 0)
throw new StringIndexOutOfBoundsException(newLength);
ensureCapacityInternal(newLength);
if (count < newLength) {
for (; count < newLength; count++)
value[count] = '\0';
} else {
count = newLength;
}
}toString()方法：
public String toString() {
// Create a copy, don't share the array
return new String(value, 0, count);
}
而toString()时会把当前的count指针也作为参数传给String的构造函数，所以不用担心把超过新内容大小的旧内容也传进去了。可见，StringBuilder是完全可以被重用的。
String[] history_steps = ruleDetailInfo.getHistory_steps().split(",");
for (String step : history_steps){
sb.setLength(0);
sb.append(ruleDetailInfo.getBusiness_id()).append("\t").append(step);
results.add(sb.toString());
}＋ 与 StringBuilder的区别
String s ＝ "hello" + user.getName();这一行代码经过javac编译后的效果，的确等价于使用StringBuilder，但没有设定长度。
String s ＝ new StringBuilder().append(“hello”).append(user.getName());但是，如果像下面这样：
String s ＝ “hello ”;
// 中间插入了其他一些代码
s = s + user.getName();每一条语句，都会生成一个新的StringBuilder，这里就有了两个StringBuilder，性能就完全不一样了。
如果是在循环体里s+=i; 就更加多得没谱，例如：
String str = "";
for(int i=0; i<10000;i++){
str += i;
}StringBuffer与StringBuilder都是继承于AbstractStringBuilder，唯一的区别就是StringBuffer的函数上都有synchronized关键字。
public final class StringBuffer
extends AbstractStringBuilder
implements java.io.Serializable, CharSequence {
/** use serialVersionUID from JDK 1.0.2 for interoperability */
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 setLength(int newLength) {
super.setLength(newLength);
}
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 final class StringBuilder
extends AbstractStringBuilder
implements java.io.Serializable, CharSequence {
/** use serialVersionUID for interoperability */
static final long serialVersionUID = 4383685877147921099L;
public StringBuilder() {
super(16);
}
public StringBuilder(int capacity) {
super(capacity);
}
public StringBuilder(String str) {
super(str.length() + 16);
append(str);
}
public StringBuilder(CharSequence seq) {
this(seq.length() + 16);
append(seq);
}
public StringBuilder append(Object obj) {
return append(String.valueOf(obj));
}
public StringBuilder append(String str) {
super.append(str);
return this;
}
// Appends the specified string builder to this sequence.
private StringBuilder append(StringBuilder sb) {
if (sb == null)
return append("null");
int len = sb.length();
int newcount = count + len;
if (newcount > value.length)
expandCapacity(newcount);
sb.getChars(0, len, value, count);
count = newcount;
return this;
}
public StringBuilder append(StringBuffer sb) {
super.append(sb);
return this;
}
......
}StringBuilder是非线程安全的，所以不能在多线程环境下共享使用。StringBuilder在使用的时候一定要指定其初始大小，另外，对性能要求比较高的场景下，可以考虑用一个ThreadLocal 缓存可重用的StringBuilder。
下划线转驼峰
public static String underlineToCamel(String param){
if (param==null||"".equals(param.trim())){
return "";
}
int len=param.length();
StringBuilder sb=new StringBuilder(len);
for (int i = 0; i < len; i++) {
char c = Character.toLowerCase(param.charAt(i));
if (c == UNDERLINE){
if (++i
| Modifier and Type | Method and Description |
|---|---|
StringBuilder |
append(boolean b) 将 |
StringBuilder |
append(char c) 将 |
StringBuilder |
append(char[] str) 将 |
StringBuilder |
append(char[] str, int offset, int len) 将 |
StringBuilder |
append(CharSequence s) 将指定的字符序列追加到此 Appendable 。 |
StringBuilder |
append(CharSequence s, int start, int end) 追加指定的序列 |
StringBuilder |
append(double d) 将 |
StringBuilder |
append(float f) 将 |
StringBuilder |
append(int i) 将 |
StringBuilder |
append(long lng) 将 |
StringBuilder |
append(Object obj) 追加 |
StringBuilder |
append(String str) 将指定的字符串附加到此字符序列。 |
StringBuilder |
append(StringBuffer sb) 将指定 |
StringBuilder |
appendCodePoint(int codePoint) 将 |
int |
capacity() 返回当前容量。 |
char |
charAt(int index) 返回 |
void |
ensureCapacity(int minimumCapacity) 确保容量至少等于规定的最小值。 |
void |
getChars(int srcBegin, int srcEnd, char[] dst, int dstBegin) 字符从该序列复制到目标字符数组 |
java源码
package java.lang;
public final class StringBuilder
extends AbstractStringBuilder
implements java.io.Serializable, CharSequence
{
static final long serialVersionUID = 4383685877147921099L;
public StringBuilder() {
super(16);
}
public StringBuilder(int capacity) {
super(capacity);
}
public StringBuilder(String str) {
super(str.length() + 16);
append(str);
}
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;
}
public StringBuilder append(StringBuffer sb) {
super.append(sb);
return this;
}
@Override
public StringBuilder append(CharSequence s) {
super.append(s);
return this;
}
@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;
}
@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;
}
@Override
public StringBuilder appendCodePoint(int codePoint) {
super.appendCodePoint(codePoint);
return this;
}
}package java.lang;
import sun.misc.FloatingDecimal;
import java.util.Arrays;
abstract class AbstractStringBuilder implements Appendable, CharSequence {
char[] value;
int count;
AbstractStringBuilder() {
}
AbstractStringBuilder(int capacity) {
value = new char[capacity];
}
@Override
public int length() {
return count;
}
public int capacity() {
return value.length;
}
private int newCapacity(int minCapacity) {
// overflow-conscious code
int newCapacity = (value.length << 1) + 2;
if (newCapacity - minCapacity < 0) {
newCapacity = minCapacity;
}
return (newCapacity <= 0 || MAX_ARRAY_SIZE - newCapacity < 0)
? hugeCapacity(minCapacity)
: newCapacity;
}
private void ensureCapacityInternal(int minimumCapacity) {
// overflow-conscious code
if (minimumCapacity - value.length > 0) {
value = Arrays.copyOf(value,
newCapacity(minimumCapacity));
}
}
public void getChars(int srcBegin, int srcEnd, char[] dst, int dstBegin)
{
if (srcBegin < 0)
throw new StringIndexOutOfBoundsException(srcBegin);
if ((srcEnd < 0) || (srcEnd > count))
throw new StringIndexOutOfBoundsException(srcEnd);
if (srcBegin > srcEnd)
throw new StringIndexOutOfBoundsException("srcBegin > srcEnd");
System.arraycopy(value, srcBegin, dst, dstBegin, srcEnd - srcBegin);
}
public AbstractStringBuilder append(Object obj) {
return append(String.valueOf(obj));
}
public AbstractStringBuilder append(String str) {
if (str == null)
return appendNull();
int len = str.length();
ensureCapacityInternal(count + len);
str.getChars(0, len, value, count);
count += len;
return this;
}
public AbstractStringBuilder append(StringBuffer sb) {
if (sb == null)
return appendNull();
int len = sb.length();
ensureCapacityInternal(count + len);
sb.getChars(0, len, value, count);
count += len;
return this;
}
AbstractStringBuilder append(AbstractStringBuilder asb) {
if (asb == null)
return appendNull();
int len = asb.length();
ensureCapacityInternal(count + len);
asb.getChars(0, len, value, count);
count += len;
return this;
}
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());
}
private AbstractStringBuilder appendNull() {
int c = count;
ensureCapacityInternal(c + 4);
final char[] value = this.value;
value[c++] = 'n';
value[c++] = 'u';
value[c++] = 'l';
value[c++] = 'l';
count = c;
return this;
}
@Override
public AbstractStringBuilder append(CharSequence s, int start, int end) {
if (s == null)
s = "null";
if ((start < 0) || (start > end) || (end > s.length()))
throw new IndexOutOfBoundsException(
"start " + start + ", end " + end + ", s.length() "
+ s.length());
int len = end - start;
ensureCapacityInternal(count + len);
for (int i = start, j = count; i < end; i++, j++)
value[j] = s.charAt(i);
count += len;
return this;
}
public AbstractStringBuilder append(char[] str) {
int len = str.length;
ensureCapacityInternal(count + len);
System.arraycopy(str, 0, value, count, len);
count += len;
return this;
}
public AbstractStringBuilder append(char str[], int offset, int len) {
if (len > 0) // let arraycopy report AIOOBE for len < 0
ensureCapacityInternal(count + len);
System.arraycopy(str, offset, value, count, len);
count += len;
return this;
}
public AbstractStringBuilder append(boolean b) {
if (b) {
ensureCapacityInternal(count + 4);
value[count++] = 't';
value[count++] = 'r';
value[count++] = 'u';
value[count++] = 'e';
} else {
ensureCapacityInternal(count + 5);
value[count++] = 'f';
value[count++] = 'a';
value[count++] = 'l';
value[count++] = 's';
value[count++] = 'e';
}
return this;
}
@Override
public AbstractStringBuilder append(char c) {
ensureCapacityInternal(count + 1);
value[count++] = c;
return this;
}
public AbstractStringBuilder append(int i) {
if (i == Integer.MIN_VALUE) {
append("-2147483648");
return this;
}
int appendedLength = (i < 0) ? Integer.stringSize(-i) + 1
: Integer.stringSize(i);
int spaceNeeded = count + appendedLength;
ensureCapacityInternal(spaceNeeded);
Integer.getChars(i, spaceNeeded, value);
count = spaceNeeded;
return this;
}
public AbstractStringBuilder append(long l) {
if (l == Long.MIN_VALUE) {
append("-9223372036854775808");
return this;
}
int appendedLength = (l < 0) ? Long.stringSize(-l) + 1
: Long.stringSize(l);
int spaceNeeded = count + appendedLength;
ensureCapacityInternal(spaceNeeded);
Long.getChars(l, spaceNeeded, value);
count = spaceNeeded;
return this;
}
public AbstractStringBuilder append(float f) {
FloatingDecimal.appendTo(f,this);
return this;
}
public AbstractStringBuilder append(double d) {
FloatingDecimal.appendTo(d,this);
return this;
}
public AbstractStringBuilder appendCodePoint(int codePoint) {
final int count = this.count;
if (Character.isBmpCodePoint(codePoint)) {
ensureCapacityInternal(count + 1);
value[count] = (char) codePoint;
this.count = count + 1;
} else if (Character.isValidCodePoint(codePoint)) {
ensureCapacityInternal(count + 2);
Character.toSurrogates(codePoint, value, count);
this.count = count + 2;
} else {
throw new IllegalArgumentException();
}
return this;
}
}
package java.lang;
import java.util.NoSuchElementException;
import java.util.PrimitiveIterator;
import java.util.Spliterator;
import java.util.Spliterators;
import java.util.function.IntConsumer;
import java.util.stream.IntStream;
import java.util.stream.StreamSupport;
public interface CharSequence {
int length();
char charAt(int index);
CharSequence subSequence(int start, int end);
public String toString();
public default IntStream chars() {
class CharIterator implements PrimitiveIterator.OfInt {
int cur = 0;
public boolean hasNext() {
return cur < length();
}
public int nextInt() {
if (hasNext()) {
return charAt(cur++);
} else {
throw new NoSuchElementException();
}
}
@Override
public void forEachRemaining(IntConsumer block) {
for (; cur < length(); cur++) {
block.accept(charAt(cur));
}
}
}
return StreamSupport.intStream(() ->
Spliterators.spliterator(
new CharIterator(),
length(),
Spliterator.ORDERED),
Spliterator.SUBSIZED | Spliterator.SIZED | Spliterator.ORDERED,
false);
}
public default IntStream codePoints() {
class CodePointIterator implements PrimitiveIterator.OfInt {
int cur = 0;
@Override
public void forEachRemaining(IntConsumer block) {
final int length = length();
int i = cur;
try {
while (i < length) {
char c1 = charAt(i++);
if (!Character.isHighSurrogate(c1) || i >= length) {
block.accept(c1);
} else {
char c2 = charAt(i);
if (Character.isLowSurrogate(c2)) {
i++;
block.accept(Character.toCodePoint(c1, c2));
} else {
block.accept(c1);
}
}
}
} finally {
cur = i;
}
}
public boolean hasNext() {
return cur < length();
}
public int nextInt() {
final int length = length();
if (cur >= length) {
throw new NoSuchElementException();
}
char c1 = charAt(cur++);
if (Character.isHighSurrogate(c1) && cur < length) {
char c2 = charAt(cur);
if (Character.isLowSurrogate(c2)) {
cur++;
return Character.toCodePoint(c1, c2);
}
}
return c1;
}
}
return StreamSupport.intStream(() ->
Spliterators.spliteratorUnknownSize(
new CodePointIterator(),
Spliterator.ORDERED),
Spliterator.ORDERED,
false);
}
}package java.lang;
import java.io.IOException;
public interface Appendable {
Appendable append(CharSequence csq) throws IOException;
Appendable append(CharSequence csq, int start, int end) throws IOException;
Appendable append(char c) throws IOException;
}