src/main/java/org/apache/commons/text/StrBuilder.java [2249:2289]:
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        validateIndex(index);
        if (str == null) {
            str = nullText;
        }
        if (str != null) {
            final int strLen = str.length();
            if (strLen > 0) {
                final int newSize = size + strLen;
                ensureCapacity(newSize);
                System.arraycopy(buffer, index, buffer, index + strLen, size - index);
                size = newSize;
                str.getChars(0, strLen, buffer, index);
            }
        }
        return this;
    }

    /**
     * Tests if the string builder is empty (convenience Collections API style method).
     * <p>
     * This method is the same as checking {@link #length()} and is provided to match the
     * API of Collections.
     * </p>
     *
     * @return {@code true} if the size is {@code 0}.
     */
    public boolean isEmpty() {
        return size == 0;
    }

    /**
     * Tests if the string builder is not empty (convenience Collections API style method).
     * <p>
     * This method is the same as checking {@link #length()} and is provided to match the
     * API of Collections.
     * </p>
     *
     * @return {@code true} if the size is greater than {@code 0}.
     * @since 1.10.0
     */
    public boolean isNotEmpty() {
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src/main/java/org/apache/commons/text/TextStringBuilder.java [2264:2302]:
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        validateIndex(index);
        if (str == null) {
            str = nullText;
        }
        if (str != null) {
            final int strLen = str.length();
            if (strLen > 0) {
                final int newSize = size + strLen;
                ensureCapacity(newSize);
                System.arraycopy(buffer, index, buffer, index + strLen, size - index);
                size = newSize;
                str.getChars(0, strLen, buffer, index);
            }
        }
        return this;
    }

    /**
     * Checks is the string builder is empty (convenience Collections API style method).
     * <p>
     * This method is the same as checking {@link #length()} and is provided to match the API of Collections.
     * </p>
     *
     * @return {@code true} if the size is {@code 0}.
     */
    public boolean isEmpty() {
        return size == 0;
    }

    /**
     * Checks is the string builder is not empty.
     * <p>
     * This method is the same as checking {@link #length()}.
     * </p>
     *
     * @return {@code true} if the size is not {@code 0}.
     * @since 1.9
     */
    public boolean isNotEmpty() {
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