src/main/java/org/apache/commons/text/StrBuilder.java [380:395]:
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        if (chars == null) {
            return appendNull();
        }
        if (startIndex < 0 || startIndex > chars.length) {
            throw new StringIndexOutOfBoundsException("Invalid startIndex: " + length);
        }
        if (length < 0 || startIndex + length > chars.length) {
            throw new StringIndexOutOfBoundsException("Invalid length: " + length);
        }
        if (length > 0) {
            final int len = length();
            ensureCapacity(len + length);
            System.arraycopy(chars, startIndex, buffer, len, length);
            size += length;
        }
        return this;
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src/main/java/org/apache/commons/text/TextStringBuilder.java [450:465]:
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        if (chars == null) {
            return appendNull();
        }
        if (startIndex < 0 || startIndex > chars.length) {
            throw new StringIndexOutOfBoundsException("Invalid startIndex: " + length);
        }
        if (length < 0 || startIndex + length > chars.length) {
            throw new StringIndexOutOfBoundsException("Invalid length: " + length);
        }
        if (length > 0) {
            final int len = length();
            ensureCapacity(len + length);
            System.arraycopy(chars, startIndex, buffer, len, length);
            size += length;
        }
        return this;
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