翻阅String类源代码发现几个有趣的地方

1、看以下代码 Stirng类的hashcode 方法

/**
     * Returns a hash code for this string. The hash code for a
     * <code>String</code> object is computed as
     * <blockquote><pre>
     * s[0]*31^(n-1) + s[1]*31^(n-2) + ... + s[n-1]   * </pre></blockquote>
     * using <code>int</code> arithmetic, where <code>s[i]</code> is the
     * <i>i</i>th character of the string, <code>n</code> is the length of
     * the string, and <code>^</code> indicates exponentiation.
     * (The hash value of the empty string is zero.)
     *
     * @return  a hash code value for this object.
     */
    public int hashCode() {
	int h = hash;
	if (h == 0) {
	    int off = offset;
	    char val[] = value;
	    int len = count;

            for (int i = 0; i < len; i++) {
                h = 31*h + val[off++];
            }
            hash = h;
        }
        return h;
    }

s[0]*31^(n-1) + s[1]*31^(n-2) + ... + s[n-1]   *
这部分说明是这个string 的char数组的ascII拼接后31位进制的值。
不知道为什么是31,有什麽依据?。

2、1、看以下代码 Stirng类的toLowerCase方法片段

/* Now check if there are any characters that need to be changed. */
	scan: {
            int c;
	    for (firstUpper = 0 ;
                    firstUpper < count ; 
                    firstUpper += Character.charCount(c)) {
		c = codePointAt(firstUpper);
		if (c != Character.toLowerCase(c)) {
                    break scan;
                }
	    }
	    return this;
	}

像是个1次循环，不过这种写法估计没怎么有人用过
何意？
我想在某些时候是可以减少点代码量的,不过貌似没多大意义

情况1:
action:{
   for(...){
    if(..)break action
   }
    do something 
}

或
  boolena flag =false;
   for(...){
    if(..){
     flag =true;
     break;
    }
   }
   if(flag)
   do something 


情况2:
action:{
   do something 1 
   if(flag) break action
   do something 2
}

或
do Some thing 1 
 if(flag)
  do Some thing 2

3、String 的intern 方法

/**
     * Returns a canonical representation for the string object.
     * <p>
     * A pool of strings, initially empty, is maintained privately by the
     * class <code>String</code>.
     * <p>
     * When the intern method is invoked, if the pool already contains a
     * string equal to this <code>String</code> object as determined by
     * the {@link #equals(Object)} method, then the string from the pool is
     * returned. Otherwise, this <code>String</code> object is added to the
     * pool and a reference to this <code>String</code> object is returned.
     * <p>
     * It follows that for any two strings <code>s</code> and <code>t</code>,
     * <code>s.intern()&nbsp;==&nbsp;t.intern()</code> is <code>true</code>
     * if and only if <code>s.equals(t)</code> is <code>true</code>.
     * <p>
     * All literal strings and string-valued constant expressions are
     * interned. String literals are defined in &sect;3.10.5 of the
     * <a href="http://java.sun.com/docs/books/jls/html/">Java Language
     * Specification</a>
     *
     * @return  a string that has the same contents as this string, but is
     *          guaranteed to be from a pool of unique strings.
     */
    public native String intern();

其实该方法没多大用，但其说明String 在内部是缓存的。

才发现,其实臧圩人已经说得很清楚了 我就不多赘述了

"在JAVA虚拟机（JVM）中存在着一个字符串池，其中保存着很多String对象，并且可以被共享使用，因此它提高了效率。由于String类是final的，它的值一经创建就不可改变，因此我们不用担心String对象共享而带来程序的混乱。字符串池由String类维护，我们可以调用intern()方法来访问字符串池。" 

参见
http://zangweiren.iteye.com/blog/216005