java中String StringBuilder StringBuffer比较和效率（性能）测试

string stringbuilder stringbuffer三者的区别

从JDK源码看，String、StringBuilder、StringBuffer都是存放在char[] 数组字符串。
简单看下三者的部分源码：
String定义属性和构造方法：

public final class String implements java.io.Serializable, Comparable<String>, CharSequence { private final char value[]; public String() { this.value = "".value; } public String(String original) { this.value = original.value; this.hash = original.hash; } public String(char value[]) { this.value = Arrays.copyOf(value, value.length); }

StringBuilder源码：

public final class StringBuilder extends AbstractStringBuilder implements java.io.Serializable, CharSequence { public StringBuilder() { super(16); } public StringBuilder(int capacity) { super(capacity); } public StringBuilder(String str) { super(str.length() + 16); append(str); }

StringBuffer源码：

public final class StringBuffer extends AbstractStringBuilder implements java.io.Serializable, CharSequence { private transient char[] toStringCache; public StringBuffer() { super(16); }

比较明显的是：
String 中定义的char[] 数组是用final 修饰，所以，String 是不可变字符序列，而StringBuilder和StringBuffer是可变字符序列；
如果Sting 需要改变则需要重新创建新对象；
StringBuffer 和 StringBuilder 都继承 AbstractStringBuilder类，他们在初始化时，都是调用父类的构造器。

接下来，我们在简单看下AbstractStringBuilder类源码：

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]; }

可以看到 AbstractStringBuilder 其实也定义了char[] 数组，不同的是，AbstractStringBuilder 中的char[] 数组可以可变的，在细看一点，可以看到AbstractStringBuilder 有扩容的方法：

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 int hugeCapacity(int minCapacity) { if (Integer.MAX_VALUE - minCapacity < 0) { // overflow throw new OutOfMemoryError(); } return (minCapacity > MAX_ARRAY_SIZE) ? minCapacity : MAX_ARRAY_SIZE; }