Android 8.0 Behavior Changes

Along with new features and capabilities, Android 8.0 (API level 26) includes a variety of system and API behavior changes. This document highlights some of the key changes that you should understand and account for in your apps.

Most of these changes affect all apps, regardless of what version of Android they target. However, several changes only affect apps targeting Android 8.0. To maximize clarity, this page is divided into two sections: Changes for all apps and Changes for apps targeting Android 8.0.

Changes for all apps

These behavior changes apply to all apps when they run on the Android 8.0 (API level 26) platform, regardless of the API level that they target. All developers should review these changes and modify their apps to support them properly, where applicable to the app.

Background execution limits

As one of the changes that Android 8.0 (API level 26) introduces to improve battery life, when your app enters the cached state, with no active components, the system releases any wakelocks that the app holds.

In addition, to improve device performance, the system limits certain behaviors by apps that are not running in the foreground. Specifically:

  • Apps that are running in the background now have limits on how freely they can access background services.
  • Apps cannot use their manifests to register for most implicit broadcasts (that is, broadcasts that are not targeted specifically at the app).

By default, these restrictions only apply to apps that target O. However, users can enable these restrictions for any app from the Settings screen, even if the app has not targeted O.

Android 8.0 (API level 26) also includes the following changes to specific methods:

  • The startService() method now throws an IllegalStateException if an app targeting Android 8.0 tries to use that method in a situation when it isn't permitted to create background services.
  • The new Context.startForegroundService() method starts a foreground service. The system allows apps to call Context.startForegroundService() even while the app is in the background. However, the app must call that service's startForeground() method within five seconds after the service is created.

For more information, see Background Execution Limits.

Android background location limits

In order to preserve battery, user experience, and system health, background apps receive location updates less frequently when used on a device running Android 8.0. This behavior change affects all apps that receive location updates, including Google Play services.

These changes affect the following APIs:

  • Fused Location Provider (FLP)
  • Geofencing
  • GNSS Measurements
  • Location Manager
  • Wi-Fi Manager

To ensure that your app runs as expected, complete the following steps:

  • Review your app's logic and ensure that you're using the latest location APIs.
  • Test that your app exhibits the behavior that you expect for each use case.
  • Consider using the Fused Location Provider (FLP) or geofencing to handle the use cases that depend on the user's current location.

For more information about these changes, see Background Location Limits.

App shortcuts

Android 8.0 (API level 26) includes the following changes to app shortcuts:

  • The com.android.launcher.action.INSTALL_SHORTCUT broadcast no longer has any effect on your app, because it is now a private, implicit broadcast. Instead, you should create an app shortcut by using the requestPinShortcut() method from the ShortcutManager class.
  • The ACTION_CREATE_SHORTCUT intent can now create app shortcuts that you manage using the ShortcutManager class. This intent can also create legacy launcher shortcuts that don't interact with ShortcutManager. Previously, this intent could create only legacy launcher shortcuts.
  • Shortcuts created using requestPinShortcut() and shortcuts created in an activity that handles the ACTION_CREATE_SHORTCUT intent are now fully-fledged app shortcuts. As a result, apps can now update them using the methods in ShortcutManager.
  • Legacy shortcuts retain their functionality from previous versions of Android, but you must convert them to app shortcuts manually in your app.

To learn more about changes to app shortcuts, see the Pinning Shortcuts and Widgets feature guide.

Locales and internationalization

Android 7.0 (API level 24) introduced the concept of being able to specify a default Category Locale, but some APIs continued to use the generic Locale.getDefault() method, without arguments, when they should have instead used default DISPLAY category Locale. In Android 8.0 (API level 26), the following methods now use Locale.getDefault(Category.DISPLAY) instead of Locale.getDefault():

Locale.getDisplayScript(Locale) also falls back to Locale.getDefault() when the displayScript value specified for the Locale argument is not available.

Additional locale and internationalization-related changes are as follows:

  • Calling Currency.getDisplayName(null) throws a NullPointerException, matching the documented behavior.
  • Time zone name parsing has changed. Previously, Android devices used the system clock value sampled at boot time to cache the time zone names used for parsing date times. As a result, parsing could be negatively affected if the system clock was wrong at boot time or in other, rarer cases.

    Now, in common cases the parsing logic uses ICU and the current system clock value when parsing time zone names. This change provides more correct results, which may differ from earlier Android versions when your app uses classes like SimpleDateFormat.

  • Android 8.0 (API level 26) updates the version of ICU to version 58.

Alert windows

If an app uses the SYSTEM_ALERT_WINDOW permission and uses one of the following window types to attempt to display alert windows above other apps and system windows:

...then these windows always appear beneath the windows that use the TYPE_APPLICATION_OVERLAY window type. If an app targets Android 8.0 (API level 26), the app uses the TYPE_APPLICATION_OVERLAY window type to display alert windows.

For more information, see the Common window types for alert windows section within the behavior changes for Apps targeting Android 8.0.

Input and navigation

With the advent of Android apps on ChromeOS and other large form factors, such as tablets, we're seeing a resurgence of keyboard navigation use within Android apps. Within Android 8.0 (API level 26), we've re-addressed using the keyboard as a navigation input device, resulting in a more reliable, predictable model for arrow- and tab-based navigation.

In particular, we've made the following changes to element focus behavior:

  • If you haven't defined any focus state colors for a View object (either its foreground or background drawable), the framework now sets a default focus highlight color for the View. This focus highlight is a ripple drawable that's based on the activity's theme.

    If you don't want a View object to use this default highlight when it receives focus, set the android:defaultFocusHighlightEnabled attribute to false in the layout XML file containing the View, or pass in false to setDefaultFocusHighlightEnabled() in your app's UI logic.

  • To test how keyboard input affects UI element focus, you can enable the Drawing > Show layout bounds developer option. In Android 8.0, this option displays an "X" icon over the element that currently has focus.

Also, all toolbar elements in Android 8.0 are automatically keyboard navigation clusters, making it easier for users to navigate into and out of each toolbar as a whole.

To learn more about how to improve support for keyboard navigation within your app, read the Supporting Keyboard Navigation guide.

Web form autofill

Now that the Android Autofill Framework provides built-in support for autofill functionality, the following methods related to WebView objects have changed for apps installed on devices running Android 8.0 (API level 26):

WebSettings
WebViewDatabase
  • Calling clearFormData() no longer has any effect.
  • The hasFormData() method now returns false. Previously, this method returned true when the form contained data.

Accessibility

Android 8.0 (API level 26) includes the following changes to accessibility:

  • The accessibility framework now converts all double-tap gestures into ACTION_CLICK actions. This change allows TalkBack to behave more like other accessibility services.

    If your app's View objects use custom touch handling, you should verify that they still work with TalkBack. You might just need to register the click handler that your View objects use. If TalkBack still doesn't recognize gestures performed on these View objects, override performAccessibilityAction().

  • Accessibility services are now aware of all ClickableSpan instances within your app's TextView objects.

To learn more about how to make your app more accessible, see Accessibility.

Networking and HTTP(S) connectivity

Android 8.0 (API level 26) includes the following behavior changes to networking and HTTP(S) connectivity:

  • OPTIONS requests with no body have a Content-Length: 0 header. Previously they had no Content-Length header.
  • HttpURLConnection normalizes URLs containing empty paths by appending a slash after the host or authority name with a slash. For example, it converts http://example.com to http://example.com/.
  • A custom proxy selector set via ProxySelector.setDefault() only targets the address (scheme, host and port) of a requested URL. As a result, proxy selection may only be based on those values. A URL passed to a custom proxy selector does not include the requested URL’s path, query parameters, or fragments.
  • URIs cannot contain empty labels.

    Previously, the platform supported a workaround to accept empty labels in host names, which is an illegal use of URIs. This workaround was for compatibility with older libcore releases. Developers using the API incorrectly would see an ADB message: "URI example..com has empty labels in the hostname. This is malformed and will not be accepted in future Android releases." Android 8.0 removes this workaround; the system returns null for malformed URIs.

  • Android 8.0’s implementation of HttpsURLConnection does not perform insecure TLS/SSL protocol version fallback.
  • Handling of tunneling HTTP(S) connections has changed as follows:
    • When tunneling HTTPS connection over connection, the system correctly places the port number (:443) in the Host line when sending this information to an intermediate server. Previously, the port number only occurred in the CONNECT line.
    • The system no longer sends user-agent and proxy-authorization headers from a tunneled request to the proxy server.

      The system no longer sends a proxy-authorization header on a tunneled Http(s)URLConnection to the proxy when setting up the tunnel. Instead, the system generates a proxy-authorization header, and sends it to the proxy when that proxy sends HTTP 407 in response to the initial request.

      Similarly, the system no longer copies the user-agent header from the tunneled request to the proxy request that sets up the tunnel. Instead, the library generates a user-agent header for that request.

  • The send(java.net.DatagramPacket) method throws a SocketException if the previously executed connect() method failed.
    • DatagramSocket.connect() sets a pendingSocketException if there is an internal error. Prior to Android 8.0, a subsequent recv() call threw a SocketException even though a send() call would have succeeded. For consistency, both calls now throw a SocketException.
  • InetAddress.isReachable() attempts ICMP before falling back to TCP Echo protocol.
    • Some hosts that block port 7 (TCP Echo), such as google.com, may now become reachable if they accept ICMP Echo protocol.
    • For truly unreachable hosts, this change means that twice the amount of time is spent before the call returns.

Bluetooth

Android 8.0 (API level 26) makes the following changes to the length of the data that the ScanRecord.getBytes() method retrieves:

  • The getBytes() method makes no assumptions as to the number of bytes received. Therefore, apps should not rely on any minimum or maximum number of bytes returned. Instead, they should evaluate the length of the resulting array.
  • Bluetooth 5-compatible devices may return data length exceeding the previous maximum of ~60 bytes.
  • If a remote device does not provide a scan response, fewer than 60 bytes may be returned as well.

Seamless Connectivity

Android 8.0 (API level 26) makes a number of improvements to Wi-Fi Settings to make it easier to choose the Wi-Fi network that offers the best user experience. Specific changes include:

  • Stability and reliability improvements.
  • A more intuitively readable UI.
  • A single, consolidated Wi-Fi Preferences menu.
  • On compatible devices, automatic activation of Wi-Fi when a high quality saved network is nearby.

Security

Android 8.0 includes the following security-related changes:

  • The platform no longer supports SSLv3.
  • When establishing an HTTPS connection to a server that incorrectly implements TLS protocol-version negotiation, HttpsURLConnection no longer attempts the workaround of falling back to earlier TLS protocol versions and retrying.
  • Android 8.0 (API level 26) applies a Secure Computing (SECCOMP) filter to all apps. The list of allowed syscalls is restricted to those exposed through bionic. Although there are several other syscalls provided for backwards compatibility, we recommend against their use.
  • Your app's WebView objects now run in multiprocess mode. Web content is handled in a separate, isolated process from the containing app's process for enhanced security.
  • You can no longer assume that APKs reside in directories whose names end in -1 or -2. Apps should use sourceDir to get the directory, and not rely on the directory format directly.
  • For information about security enhancements related to use of native libraries, see Native Libraries.

In addition, Android 8.0 (API level 26) introduces the following changes related to installing unknown apps from unknown sources:

For additional details about installing unknown apps, see the Unknown App Install Permissions guide.

For additional guidelines on making your app more secure, see Security for Android Developers.

Privacy

Android 8.0 (API level 26) makes the following privacy-related changes to the platform.

  • The platform now handles identifiers differently.
    • For apps that were installed prior to an OTA to a version of Android 8.0 (API level 26) (API level 26), the value of ANDROID_ID remains the same unless uninstalled and then reinstalled after the OTA. To preserve values across uninstalls after OTA, developers can associate the old and new values by using Key/Value Backup.
    • For apps installed on a device running Android 8.0, the value of ANDROID_ID is now scoped per app signing key, as well as per user. The value of ANDROID_ID is unique for each combination of app-signing key, user, and device. As a result, apps with different signing keys running on the same device no longer see the same Android ID (even for the same user).
    • The value of ANDROID_ID does not change on package uninstall or reinstall, as long as the signing key is the same (and the app was not installed prior to an OTA to a version of Android 8.0).
    • The value of ANDROID_ID does not change even if a system update causes the package signing key to change.
    • On devices shipping with Google Play services and Advertising ID, you must use Advertising ID. A simple, standard system to monetize apps, Advertising ID is a unique, user-resettable ID for advertising. It is provided by Google Play services.

      Other device manufacturers should continue to provide ANDROID_ID.

  • Querying the net.hostname system property produces a null result.

Logging of uncaught exceptions

If an app installs a Thread.UncaughtExceptionHandler that does not call through to the default Thread.UncaughtExceptionHandler, the system does not kill the app when an uncaught exception occurs. Starting from Android 8.0 (API level 26), the system logs the exception stacktrace in this situation; in earlier versions of the platform, the system would not have logged the exception stacktrace.

We recommend that custom Thread.UncaughtExceptionHandler implementations always call through to the default handler; apps that follow this recommendation are unaffected by the change in Android 8.0.

findViewById() signature change

All instances of the findViewById() method now return <T extends View> T instead of View. This change has the following implications:

  • This may result in existing code now having ambiguous return type, for example if there is both someMethod(View) and someMethod(TextView) that takes the result of a call to findViewById().
  • When using Java 8 source language, this requires an explicit cast to View when the return type is unconstrained (for example, assertNotNull(findViewById(...)).someViewMethod()).
  • Overrides of non-final findViewById() methods (for example, Activity.findViewById()) will need their return type updated.

Contacts provider usage stats change

In previous versions of Android, the Contacts Provider component allows developers to get usage data for each contact. This usage data exposes information for each email address and each phone number associated with a contact, including the number of times the contact has been contacted and the last time the contact was contacted. Apps that request the READ_CONTACTS permission can read this data.

Apps can still read this data if they request READ_CONTACTS permission. In Android 8.0 (API level 26) and higher, queries for usage data return approximations rather than exact values. The Android system maintains the exact values internally, so this change does not affect the auto-complete API.

This behavior change affects the following query parameters:

Collection handling

AbstractCollection.removeAll() and AbstractCollection.retainAll() now always throw a NullPointerException; previously, the NullPointerException was not thrown when the collection was empty. This change makes the behavior consistent with the documentation.

Android enterprise

Android 8.0 (API level 26) changes the behavior of some APIs and features for enterprise apps, including device policy controllers (DPCs). The changes include:

  • New behaviors to help apps support work profiles on fully managed devices.
  • Changes to system update handling, app verification, and authentication to increase device and system integrity.
  • Improvements to the user experience for provisioning, notifications, the Recents screen, and always-on VPN.

To see the all the enterprise changes in Android 8.0 (API level 26) and learn how they might affect your app, read Android in the Enterprise.

Apps targeting Android 8.0

These behavior changes apply exclusively to apps that are targeting Android 8.0 (API level 26) or higher. Apps that compile against Android 8.0, or set targetSdkVersion to Android 8.0 or higher must modify their apps to support these behaviors properly, where applicable to the app.

Alert windows

Apps that use the SYSTEM_ALERT_WINDOW permission can no longer use the following window types to display alert windows above other apps and system windows:

Instead, apps must use a new window type called TYPE_APPLICATION_OVERLAY.

When using the TYPE_APPLICATION_OVERLAY window type to display alert windows for your app, keep the following characteristics of the new window type in mind:

  • An app's alert windows always appear under critical system windows, such as the status bar and IMEs.
  • The system can move or resize windows that use the TYPE_APPLICATION_OVERLAY window type to improve screen presentation.
  • By opening the notification shade, users can access settings to block an app from displaying alert windows shown using the TYPE_APPLICATION_OVERLAY window type.

Content change notifications

Android 8.0 (API level 26) changes how ContentResolver.notifyChange() and registerContentObserver(Uri, boolean, ContentObserver) behave for apps targeting Android 8.0.

These APIs now require that a valid ContentProvider is defined for the authority in all Uris. Defining a valid ContentProvider with relevant permissions will help defend your app against content changes from malicious apps, and prevent you from leaking potentially private data to malicious apps.

View focus

Clickable View objects are now also focusable by default. If you want a View object to be clickable but not focusable, set the android:focusable attribute to false in the layout XML file containing the View, or pass in false to setFocusable() in your app's UI logic.

User-agent matching in browser detection

Android 8.0 (API level 26) and higher include the build identifier string OPR. Some pattern matches may cause browser-detection logic to misidentify a non-Opera browser as Opera. An example of such a pattern match might be:

if(p.match(/OPR/)){k="Opera";c=p.match(/OPR\/(\d+.\d+)/);n=new Ext.Version(c[1])}

To avoid issues arising from such a misidentification, use a string other than OPR as a pattern-match for the Opera browser.

Security

The following changes affect security in Android 8.0 (API level 26):

  • If your app's network security configuration opts out of supporting cleartext traffic, your app's WebView objects cannot access websites over HTTP. Each WebView object must use HTTPS instead.
  • The Allow unknown sources system setting has been removed; in its place, the Install unknown apps permission manages unknown app installs from unknown sources. To learn more about this new permission, see the Unknown App Install Permissions guide.

For additional guidelines on making your app more secure, see Security for Android Developers.

Account access and discoverability

In Android 8.0 (API level 26), apps can no longer get access to user accounts unless the authenticator owns the accounts or the user grants that access. The GET_ACCOUNTS permission is no longer sufficient. To be granted access to an account, apps should either use AccountManager.newChooseAccountIntent() or an authenticator-specific method. After getting access to accounts, an app can call AccountManager.getAccounts() to access them.

Android 8.0 deprecates LOGIN_ACCOUNTS_CHANGED_ACTION. Apps should instead use addOnAccountsUpdatedListener() to get updates about accounts during runtime.

For information about new APIs and methods added for account access and discoverability, see Account Access and Discoverability in the New APIs section of this document.

Privacy

The following changes affect privacy in Android 8.0 (API level 26).

  • The system properties net.dns1, net.dns2, net.dns3, and net.dns4 are no longer available, a change that improves privacy on the platform.
  • To obtain networking information such as DNS servers, apps with the ACCESS_NETWORK_STATE permission can register a NetworkRequest or NetworkCallback object. These classes are available in Android 5.0 (API level 21) and higher.
  • Build.SERIAL is deprecated. Apps needing to know the hardware serial number should instead use the new Build.getSerial() method, which requires the READ_PHONE_STATE permission.
  • The LauncherApps API no longer allows work profile apps to get information about the primary profile. When a user is in a work profile, the LauncherApps API behaves as if no apps are installed in other profiles within the same profile group. As before, attempts to access unrelated profiles causes SecurityExceptions.

Permissions

Prior to Android 8.0 (API level 26), if an app requested a permission at runtime and the permission was granted, the system also incorrectly granted the app the rest of the permissions that belonged to the same permission group, and that were registered in the manifest.

For apps targeting Android 8.0, this behavior has been corrected. The app is granted only the permissions it has explicitly requested. However, once the user grants a permission to the app, all subsequent requests for permissions in that permission group are automatically granted.

For example, suppose an app lists both READ_EXTERNAL_STORAGE and WRITE_EXTERNAL_STORAGE in its manifest. The app requests READ_EXTERNAL_STORAGE and the user grants it. If the app targets API level 25 or lower, the system also grants WRITE_EXTERNAL_STORAGE at the same time, because it belongs to the same STORAGE permission group and is also registered in the manifest. If the app targets Android 8.0 (API level 26), the system grants only READ_EXTERNAL_STORAGE at that time; however, if the app later requests WRITE_EXTERNAL_STORAGE, the system immediately grants that privilege without prompting the user.

Media

  • The framework can perform automatic audio ducking by itself. In this case, when another application requests focus with AUDIOFOCUS_GAIN_TRANSIENT_MAY_DUCK, the application that has focus reduces its volume but usually does not receive an onAudioFocusChange() callback and will not lose audio focus. New APIs are available to override this behavior for applications that need to pause instead of ducking.
  • When the user takes a phone call, active media streams mute for the duration of the call.
  • All audio-related APIs should use AudioAttributes rather than audio stream types to describe the audio playback use case. Continue to use audio stream types for volume controls only. Other uses of stream types still work (for example, the streamType argument to the deprecated AudioTrack constructor), but the system logs this as an error.
  • When using an AudioTrack, if the application requests a large enough audio buffer, the framework will try to use the deep buffer output if it is available.
  • In Android 8.0 (API level 26) the handling of media button events is different:
    1. The handling of media buttons in a UI activity has not changed: foreground activities still get priority in handling media button events.
    2. If the foreground activity does not handle the media button event, the system routes the event to the app that most recently played audio locally. The active status, flags, and playback state of a media session are not considered when determining which app receives media button events.
    3. If the app's media session has been released, the system sends the media button event to the app's MediaButtonReceiver if it has one.
    4. For every other case, the system discards the media button event.

Native libraries

In apps targeting Android 8.0 (API level 26), native libraries no longer load if they contain any load segment that is both writable and executable. Some apps might stop working because of this change if they have native libraries with incorrect load segments. This is a security-hardening measure.

For more information, see Writable and Executable Segments.

Linker changes are tied to the API level that an app targets. If there is a linker change at the targeted API level, the app cannot load the library. If you are targeting an API level lower than the API level where the linker change occurs, logcat shows a warning.

Collection handling

In Android 8.0 (API level 26), Collections.sort() is implemented on top of List.sort(). The reverse was true in Android 7.x (API levels 24 and 25): The default implementation of List.sort() called Collections.sort().

This change allows Collections.sort() to take advantage of optimized List.sort() implementations, but has the following constraints:

  • Implementations of List.sort() must not call Collections.sort(), because doing so would result in stack overflow due to infinite recursion. Instead, if you want the default behavior in your List implementation, you should avoid overriding sort().

    If a parent class implements sort() inappropriately, it’s usually fine to override List.sort() with an implementation built on top of List.toArray(), Arrays.sort(), and ListIterator.set(). For example: 

    @Override
public void sort(Comparator<? super E> c) {
  Object[] elements = toArray();
  Arrays.sort(elements, c);
  ListIterator<E> iterator = (ListIterator<Object>) listIterator();
  for (Object element : elements) {
    iterator.next();
    iterator.set((E) element);
  }
}

    In most cases, you can also override List.sort() with an implementation that delegates to different default implementations depending on API level. For example: 

    @Override
public void sort(Comparator<? super E> comparator) {
  if (Build.VERSION.SDK_INT <= 25) {
    Collections.sort(this);
  } else {
    super.sort(comparator);
  }
}

    If you’re doing the latter only because you want to have a sort() method available on all API levels, consider giving it a unique name, such as sortCompat(), instead of overriding sort().

  • Collections.sort() now counts as a structural modification in List implementations that call sort(). For example, in versions of the platform prior to Android 8.0 (API level 26), iterating over an ArrayList and calling sort() on it partway through the iteration would have thrown a ConcurrentModificationException if the sorting was done by calling List.sort(). Collections.sort() did not throw an exception.

    This change makes the platform behavior more consistent: Either approach now results in a ConcurrentModificationException.

Class-loading behavior

Android 8.0 (API level 26) checks to make sure that class loaders do not break the assumptions of the runtime when loading new classes. These checks are performed whether the class is referenced from Java (from forName()), Dalvik bytecode, or JNI. The platform does not intercept direct calls from Java to the loadClass() method, nor does it check the results of such calls. This behavior should not affect the functioning of well-behaved class loaders.

The platform checks that the descriptor of the class that the class loader returns matches the expected descriptor. If the returned descriptor does not match, the platform throws a NoClassDefFoundError error, and stores in the exception a detailed message noting the discrepancy.

The platform also checks that the descriptors of the requested classes are valid. This check catches JNI calls that indirectly load classes such as GetFieldID(), passing invalid descriptors to those classes. For example, a field with signature java/lang/String is not found because that signature is invalid; it should be Ljava/lang/String;.

This is different from a JNI call to FindClass() where java/lang/String is a valid fully-qualified name.

Android 8.0 (API level 26) does not support having multiple class loaders try to define classes using the same DexFile object. An attempt to do so causes the Android runtime to throw an InternalError error with the message "Attempt to register dex file <filename> with multiple class loaders".

DexFile API is now deprecated, and you are strongly encouraged to use one of the platform classloaders, including PathClassLoader or BaseDexClassLoader, instead.

Note: You can create multiple class loaders that reference the same APK or JAR file container from the file system. Doing so normally does not result in much memory overhead: If DEX files in the container are stored instead of compressed, the platform can perform an mmap operation on them rather than directly extracting them. However, if the platform must extract the DEX file from the container, referencing a DEX file in this fashion may consume a lot of memory.

In Android, all class loaders are considered parallel-capable. When multiple threads race to load the same class with the same class loader, the first thread to complete the operation wins, and the result is used for the other threads. This behavior occurs regardless of whether the class loader has returned the same class, returned a different class, or thrown an exception. The platform silently ignores such exceptions.

Caution: In versions of the platform lower than Android 8.0 (API level 26), breaking these assumptions can lead to defining the same class multiple times, heap corruption due to class confusion, and other undesirable effects.