* are valid.
   *
   * @param rowKey row key of mapping to be erased
   * @param columnKey column key of mapping to be erased
   * @return the value previously associated with the keys, or {@code null} if no mapping existed
   *     for the keys
   */
  @CanIgnoreReturnValue
  public @Nullable V erase(@Nullable Object rowKey, @Nullable Object columnKey) {
    Integer rowIndex = rowKeyToIndex.get(rowKey);

    public void test_get_nullProperty() {
        // Test getting non-existent property
        try {
            fessConfig.get("non.existent.property");
            fail("Should throw exception for non-existent property");
        } catch (ConfigPropertyNotFoundException e) {
            // Expected exception
            assertTrue(e.getMessage().contains("non.existent.property"));
        }
    }

        // Test with property that doesn't exist and default value
        cmdList.clear();
        execJob.testAddSystemProperty(cmdList, "non.existent.property", "default.value", null);
        assertEquals(1, cmdList.size());
        assertEquals("-Dnon.existent.property=default.value", cmdList.get(0));

        // Test with property that doesn't exist and no default value
        cmdList.clear();

        emptyDir.mkdirs();
        assertTrue(emptyDir.exists());

        // Simulate the behavior of postVisitDirectory - delete empty directories
        if (emptyDir.isDirectory() && emptyDir.list().length == 0) {
            emptyDir.delete();
        }

        assertFalse(emptyDir.exists());
    }

    // Test FilePurgeVisitor postVisitDirectoryWithException

        assertEquals(123, dataStoreParams.get("key2"));

        dataStoreParams.put("key3", null);
        assertNull(dataStoreParams.get("key3"));
    }

    // Test get for non-existent key
    public void test_getNonExistentKey() {
        assertNull(dataStoreParams.get("nonexistent"));
    }

    // Test getAsString with String value
    public void test_getAsStringWithStringValue() {

        assertTrue(permissionDenied.getMessage().contains("Permission denied"));

        // Scenario 4: Exit code error
        CommandExecutionException exitCodeError = new CommandExecutionException("Command exited with non-zero code: 127");
        assertTrue(exitCodeError.getMessage().contains("non-zero code"));
    }

        try {
            fessConfig.get("non.existent.property");
            fail("Should throw ConfigPropertyNotFoundException");
        } catch (ConfigPropertyNotFoundException e) {
            assertEquals("non.existent.property", e.getMessage());
        }
    }

    // Test isExtensionAllowed method
    /*
    // Commented out - methods don't exist in FessConfig
    public void test_isExtensionAllowed() {

        // but we can test that the class exists and basic functionality
        try {
            // Use reflection to verify ProcessDestroyer class exists
            final Class<?> destroyerClass = Class.forName("org.codelibs.fess.thumbnail.impl.CommandGenerator$ProcessDestroyer");
            assertNotNull("ProcessDestroyer class should exist", destroyerClass);

                                .get("response.plugins");
                boolean exists = installed.stream()
                        .map(this::getArtifactFromMap)
                        .anyMatch(a -> a.getName().equals(target.getName()) && a.getVersion().equals(target.getVersion()));
                if (!exists) {
                    ThreadUtil.sleep(500);
                    continue;
                }

        }

        if (outputFile.exists()) {
            if (logger.isDebugEnabled()) {
                logger.debug("The thumbnail file exists: {}", outputFile.getAbsolutePath());
            }
            return true;
        }

        final File parentFile = outputFile.getParentFile();
        if (!parentFile.exists()) {
            parentFile.mkdirs();
        }