java 中StringBuffer类有getChars方法吗 为什么我的书上没有介绍呀?

有的
以下来自JDK的Docs:

public void getChars(int srcBegin,int srcEnd,char[] dst,int dstBegin)
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Characters are copied from this string buffer into the destination character array dst. The first character to be copied is at index srcBegin; the last character to be copied is at index srcEnd-1. The total number of characters to be copied is srcEnd-srcBegin. The characters are copied into the subarray of dst starting at index dstBegin and ending at index:

dstbegin + (srcEnd-srcBegin) - 1
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[Parameters]:
srcBegin - start copying at this offset in the string buffer.
srcEnd - stop copying at this offset in the string buffer.
dst - the array to copy the data into.
dstBegin - offset into dst.
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[Throws]:
NullPointerException - if dst is null.
IndexOutOfBoundsException - if any of the following is true:
srcBegin is negativ