String、StringBuffer、StringBuilder的底层源码
String、StringBuffer、StringBuilder的底层源码
String、StringBuilder和StringBuffer这三和类都是用来封装字符串并且提供了一系列的操作方法
1.String
源码：从源码上我们可以看出，String类实现了Serializable,Comparable,CharSequence三个接口，从
源码中可以看出String类是被关键字final修饰的，value是被final关键字修饰的字符，我们知道类被final
修饰不能被继承，变量被实例化后就不可以被修改了。这也是String和StringBuilder,StringBuffer的最显
著的区别。
public final class String
implements java.io.Serializable, Comparable<String>, CharSequence {
/** The value is used for character storage. */
private final char value[];
/** Cache the hash code for the string */
private int hash; // Default to 0
/** use serialVersionUID from JDK 1.0.2 for interoperability */
private static final long serialVersionUID = -6849794470754667710L;
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2.StringBuilder
源码：StringBuilder实现了Serializable和CharSequence,继承了AbstractStringBuilder，StringBuilder
类是被final关键字修饰的，StringBuilder不能被继承，StringBuilder中没有字符数组，因为StringBuilder继承了
AbstractStringBuilder,所以StringBuilder是一个可以被修改的动态字符数组，还可以看出StringBuilder
是JDK 1.5 引入的
* @since 1.5
*/
public final class StringBuilder
extends AbstractStringBuilder
implements java.io.Serializable, CharSequence
{
/** use serialVersionUID for interoperability */
static final long serialVersionUID = 4383685877147921099L;
AbstractStringBuilder .java 实现了Appendable和CharSequence,类是使用关键字abstract所修，有一
个可以被修改的动态数组
abstract class AbstractStringBuilder implements Appendable, CharSequence {
/**
* The value is used for character storage.
*/
char[] value;
/**
* The count is the number of characters used.
*/
int count;
3.StringBuffer
StringBuffer 和StringBuilder几乎一样，只是StringBuffer里面的共有方法都是被synchronized关键字
修饰的
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.
*
* 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.
*
* 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}.
*
* Let n 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 k in
* the new character sequence is equal to the character at index k
* in the old character sequence, if k is less than n;
* otherwise, it is equal to the character at index k-n in the
* argument {@code sb}.
*
* 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.
*
* The characters of the {@code CharSequence} argument are appended,
* in order, increasing the length of this sequence by the length of the
* argument.
*
*
The result of this method is exactly the same as if it were an
* invocation of this.append(s, 0, s.length());
*
*
This method synchronizes on {@code this}, the destination
* object, but does not synchronize on the source ({@code s}).
*
*
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;
}