标签：
面试的经历中，相信大家都经常被问到这三者的区别，说到String我相信绝大多数的人都会说:"String是不可变的，final修饰的"，你会看到面试官微微猥琐一笑，接着问到：“final修饰的类就是不可变的吗,那StringBuilder和StringBuffer不是final修饰的？”
看下JDK1.7 String成员变量的源码
/**
* @author Lee Boynton
* @author Arthur van Hoff
* @author Martin Buchholz
* @author Ulf Zibis
* @see java.lang.Object#toString()
* @see java.lang.StringBuffer
* @see java.lang.StringBuilder
* @see java.nio.charset.Charset
* @since JDK1.0
*/
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;
可以看到String定义的成员变量value和hash,其中value是个字节数组，而且是final修饰，这个才是String不可变的关键点；
JDK1.7 String的部分注解
The Java language provides special support for the string * concatenation operator ( + ), and for conversion of * other objects to strings. String concatenation is implemented * through the <code>StringBuilder</code>(or <code>StringBuffer</code>) * class and its <code>append</code> method. * String conversions are implemented through the method * <code>toString</code>, defined by <code>Object</code> and * inherited by all classes in Java. For additional information on * string concatenation and conversion, see Gosling, Joy, and Steele, * <i>The Java Language Specification</i>.上面解释了java为String提供了特殊的支持，例如：String a="a"; String b="b" ;当执行String c=a+b操作时，实际上是创建一个StringBuilder对象，再通过apend()进行拼接，最后调用toStirng()生成一个新的对象给c。
String提供修改内容的方法最终都是调用new String()。看下 String的部分注解
/**
* The <code>String</code> class represents character strings. All
* string literals in Java programs, such as <code>"abc"</code>, are
* implemented as instances of this class.
* <p>
* Strings are constant; their values cannot be changed after they
* are created. String buffers support mutable strings.
* Because String objects are immutable they can be shared. For example:
* <p><blockquote><pre>
* String str = "abc";
* </pre></blockquote><p>
* is equivalent to:
* <p><blockquote><pre>
* char data[] = {'a', 'b', 'c'};
* String str = new String(data);
* </pre></blockquote><p>
* Here are some more examples of how strings can be used:这里定义了一个String str = "abc";相当于char data[] = {‘a‘, ‘b‘, ‘c‘};String str = new String(data);
再来看一个substring()方法的源码
/**
* Returns a new string that is a substring of this string. The
* substring begins at the specified <code>beginIndex</code> and
* extends to the character at index <code>endIndex - 1</code>.
* Thus the length of the substring is <code>endIndex-beginIndex</code>.
* <p>
* Examples:
* <blockquote><pre>
* "hamburger".substring(4, 8) returns "urge"
* "smiles".substring(1, 5) returns "mile"
* </pre></blockquote>
*
* @param beginIndex the beginning index, inclusive.
* @param endIndex the ending index, exclusive.
* @return the specified substring.
* @exception IndexOutOfBoundsException if the
* <code>beginIndex</code> is negative, or
* <code>endIndex</code> is larger than the length of
* this <code>String</code> object, or
* <code>beginIndex</code> is larger than
* <code>endIndex</code>.
*/
public String substring(int beginIndex, int endIndex) {
if (beginIndex < 0) {
throw new StringIndexOutOfBoundsException(beginIndex);
}
if (endIndex > value.length) {
throw new StringIndexOutOfBoundsException(endIndex);
}
int subLen = endIndex - beginIndex;
if (subLen < 0) {
throw new StringIndexOutOfBoundsException(subLen);
}
return ((beginIndex == 0) && (endIndex == value.length)) ? this
: new String(value, beginIndex, subLen);
}
看完这些，你可以清楚的知道定义一个变量str="hello world",则是在内存中分配一个对象new String("hello world")，当你修改str="hello nimei"，变量重新指向内存中新分配的new String("hello nimei");原来内存中的new String("hello world")还在那里，没有改变，等待垃圾回收。难道真的没有办法修改new String("hello world")对象中的值而不重新在内存中重新new一次吗？让我们来看看一个例子。
原来通过反射可以修改String对象中的内容，反射太强大了。
StringBuffer的部分源码
* @author Arthur van Hoff
* @see java.lang.StringBuilder
* @see java.lang.String
* @since JDK1.0
*/
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;
/**
* 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</code>
* argument is less than <code>0</code>.
*/
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</code> plus the length of the string argument.
*
* @param str the initial contents of the buffer.
* @exception NullPointerException if <code>str</code> is <code>null</code>
*/
public StringBuffer(String str) {
super(str.length() + 16);
append(str);
}
StringBuilder的部分源码
* @author Michael McCloskey
* @see java.lang.StringBuffer
* @see java.lang.String
* @since 1.5
*/
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</code> argument.
*
* @param capacity the initial capacity.
* @throws NegativeArraySizeException if the <code>capacity</code>
* argument is less than <code>0</code>.
*/
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</code> plus the length of the string argument.
*
* @param str the initial contents of the buffer.
* @throws NullPointerException if <code>str</code> is <code>null</code>
*/
public StringBuilder(String str) {
super(str.length() + 16);
append(str);
}
StringBuffer和StringBuilder的这部分源码基本一样，构造函数初始化大小都为16，都是继承了AbstractStringBuilder。
看看AbstractStringBuilder定义成员变量的源码
/**
* A mutable sequence of characters.
* <p>
* Implements a modifiable string. At any point in time it contains some
* particular sequence of characters, but the length and content of the
* sequence can be changed through certain method calls.
*
* @author Michael McCloskey
* @author Martin Buchholz
* @author Ulf Zibis
* @since 1.5
*/
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;
/**
* This no-arg constructor is necessary for serialization of subclasses.
*/
AbstractStringBuilder() {
}
/**
* Creates an AbstractStringBuilder of the specified capacity.
*/
AbstractStringBuilder(int capacity) {
value = new char[capacity];
}
AbstractStringBuilder中定义的变量value,是个字节数组，和String的成员变量value相比，String的value是final修饰的，所以StringBuffer和StringBuilde的内容可以变。
在对比下StingBuffer和StringBuilder的实现其他细节，以append()方法为例。
public synchronized StringBuffer append(String str) {
super.append(str);
return this;
}
public StringBuilder append(String str) {
super.append(str);
return this;
}两者最大的区别是：StingBuffer所有的实现的方法都是sychronized修饰的，StringBuilder则不是。
java面试题String,StringBuilder,StringBuffer
标签：
原文地址：http://blog.csdn.net/dfdsggdgg/article/details/51347748
