一、概念
StringBuffer
A thread-safe, mutable sequence of characters. A string buffer is like a {@link String}, but can be modified.
源码开头的一句话：线程安全的，可变字符顺序。一个类似String的字符串缓冲区，但是可以修改的！
1 public final class StringBuffer 2 extends AbstractStringBuilder 3 implements java.io.Serializable, CharSequence 4 { 5 6 /** 7 * A cache of the last value returned by toString. Cleared 8 * whenever the StringBuffer is modified. 9 */ 10 private transient char[] toStringCache; 11 12 /** use serialVersionUID from JDK 1.0.2 for interoperability */ 13 static final long serialVersionUID = 3388685877147921107L; 14 15 .......... 16 }
二、String定义成final
StringBuffer类跟String类一样定义成final形式，主要是为了“效率”和“安全性”的考虑，若StringBuffer 被继承，由于它的高使用率，可能会降低它的性能。
它继承了AbstractStringBuilder类，
1 abstract class AbstractStringBuilder implements Appendable, CharSequence { 2 /** 3 * The value is used for character storage.与String类一样，定义了一个char类型的数组存储值 4 */ 5 char value[]; 6 7 /** 8 * The count is the number of characters used. 9 */ 10 int count; 11 12 /** 13 * This no-arg constructor is necessary for serialization of subclasses. 14 */ 15 AbstractStringBuilder() { 16 }
而我们经常用到的是append方法，最常用的是 AbstractStringBuilder append(String str)
1 public AbstractStringBuilder append(String str) { 2 if (str == null) 3 return appendNull();//如果str==null，就会将null添加到尾部 4 int len = str.length(); 5 ensureCapacityInternal(count + len);//比较重要的的一步 6 str.getChars(0, len, value, count); 7 count += len; 8 return this; 9 }
ensureCapacityInternal(int minimumCapacity)，
确保容量至少等于指定的最小值。如果当前容量小于指定值，则创建新数组，新数组的容量为指定值的两倍加2；如果当前容量不小于指定值，则直接不做处理。
1 /** 2 * This method has the same contract as ensureCapacity, but is 3 * never synchronized. 4 */ 5 private void ensureCapacityInternal(int minimumCapacity) { 6 // overflow-conscious code 7 if (minimumCapacity - value.length > 0) 8 expandCapacity(minimumCapacity); 9 }
expandCapacity(int minimumCapacity)，
1 void expandCapacity(int minimumCapacity) { 2 int newCapacity = value.length * 2 + 2; //
-
定义一个是原容量的2倍+2大小的值
3 if (newCapacity - minimumCapacity < 0)
4 newCapacity = minimumCapacity;
5 if (newCapacity < 0) {
6 if (minimumCapacity < 0) // overflow
7 throw new OutOfMemoryError(); 8 newCapacity = Integer.MAX_VALUE; 9 } 10 value = Arrays.copyOf(value, newCapacity); //扩容 11 }
1 public static void main(String[] args) { 2 StringBuffer sb = new StringBuffer(); 3 4 System.out.println("容量"+sb.capacity()); 5 sb.ensureCapacity(10); 6 System.out.println("容量"+sb.capacity()); 7 8 sb.ensureCapacity(30); 9 System.out.println("容量"+sb.capacity());//（16+1）*2=34>30 10 sb.ensureCapacity(80); 11 System.out.println("容量"+sb.capacity());//80>(34+1)*2=68 12 sb.ensureCapacity(90); 13 System.out.println("容量"+sb.capacity());//(80+1)*2>90 14 15 //字符串返转 16 sb.append("abcde"); 17 System.out.println(sb); 18 sb.reverse(); 19 20 System.out.println(sb); 21 22 }
结果：扩容的大小与（之前的长度+1）*2相比较，那个大用哪个
1 容量16 2 容量16 3 容量34 4 容量80 5 容量162 6 abcde 7 edcba
----------------------------------------------------若有不正之处，欢迎大家指正，不胜感激！！！！！