String类中部分成员变量定义如下：
/** The value is used for character storage.这个值用于字符存储 */
private final char value[];String类中构造方法String(char value[], int offset, int count)源代码如下：
/**
* Allocates(分配) a new {@code String} that contains(包含) characters from a subarray
* of the character array argument. The {@code offset} argument is the
* index of the first character of the subarray and the {@code count}
* argument specifies the length of the subarray. The contents of the
* subarray are copied; subsequent modification of the character array does
* not affect the newly created string.
*
* @param(参数) value
* Array that is the source(来源) of characters
*
* @param offset
* The initial(最初的) offset
*
* @param count
* The length
*
* @throws IndexOutOfBoundsException
* If the {@code offset} and {@code count} arguments index
* characters outside the bounds of the {@code value} array
*/
//offset表示第一个被截取的字符在数组value[]中的下标，count表示从此字符开始向后截取的字符的数量
//将value[]数组按照传入的下标和指定的截取数组数据的数量进行截取，并且创建一个内容为此的string对象
public String(char value[], int offset, int count) {
if (offset < 0) {
throw new StringIndexOutOfBoundsException(offset);
}
if (count < 0) {
throw new StringIndexOutOfBoundsException(count);
}
// Note: offset or count might be near -1>>>1.
//可以举具体例子进行理解offset不能大于value.length - count
if (offset > value.length - count) {
throw new StringIndexOutOfBoundsException(offset + count);
}
//copyOfRange( , , )将指定数组的指定范围复制到一个新数组
this.value = Arrays.copyOfRange(value, offset, offset+count);
}