I've researched through the rest of the answers, but they all seem to suggest updating dependencies etc. However I'm developing for SDK 28, everything is up to date, and has been checked via Android Studio, so there would appear to be another cause.
Basically I have added a Floating Action Button via XML:
<android.support.design.widget.FloatingActionButton
android:id="@+id/dict_fab"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_gravity="end|bottom"
android:src="@drawable/ic_add_circle_black_24dp"
android:layout_margin="16dp" />
And get the below crash when I try to load the activity containing the FAB.
--------- beginning of crash
E/AndroidRuntime: FATAL EXCEPTION: main
Process: uk.ac.aber.dcs.haa14.assignment, PID: 2876
java.lang.RuntimeException: Unable to start activity ComponentInfo{uk.ac.aber.dcs.haa14.assignment/uk.ac.aber.dcs.haa14.assignment.ui.DictionaryActivity}: android.view.InflateException: Binary XML file line #10: Binary XML file line #10: Error inflating class android.support.design.widget.FloatingActionButton
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2778)
at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2856)
at android.app.ActivityThread.-wrap11(Unknown Source:0)
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1589)
at android.os.Handler.dispatchMessage(Handler.java:106)
at android.os.Looper.loop(Looper.java:164)
at android.app.ActivityThread.main(ActivityThread.java:6494)
at java.lang.reflect.Method.invoke(Native Method)
at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:438)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:807)
Caused by: android.view.InflateException: Binary XML file line #10: Binary XML file line #10: Error inflating class android.support.design.widget.FloatingActionButton
Caused by: android.view.InflateException: Binary XML file line #10: Error inflating class android.support.design.widget.FloatingActionButton
Caused by: java.lang.ClassNotFoundException: Didn't find class "android.support.design.widget.FloatingActionButton" on path: DexPathList[[zip file "/data/app/uk.ac.aber.dcs.haa14.assignment-5Y5qp-mx6ITFeGrYdObqMQ==/base.apk", zip file "/data/app/uk.ac.aber.dcs.haa14.assignment-5Y5qp-mx6ITFeGrYdObqMQ==/split_lib_dependencies_apk.apk", zip file "/data/app/uk.ac.aber.dcs.haa14.assignment-5Y5qp-mx6ITFeGrYdObqMQ==/split_lib_resources_apk.apk", zip file "/data/app/uk.ac.aber.dcs.haa14.assignment-5Y5qp-mx6ITFeGrYdObqMQ==/split_lib_slice_0_apk.apk", zip file "/data/app/uk.ac.aber.dcs.haa14.assignment-5Y5qp-mx6ITFeGrYdObqMQ==/split_lib_slice_1_apk.apk", zip file "/data/app/uk.ac.aber.dcs.haa14.assignment-5Y5qp-mx6ITFeGrYdObqMQ==/split_lib_slice_2_apk.apk", zip file "/data/app/uk.ac.aber.dcs.haa14.assignment-5Y5qp-mx6ITFeGrYdObqMQ==/split_lib_slice_3_apk.apk", zip file "/data/app/uk.ac.aber.dcs.haa14.assignment-5Y5qp-mx6ITFeGrYdObqMQ==/split_lib_slice_4_apk.apk", zip file "/data/app/uk.ac.aber.dcs.haa14.assignment-5Y5qp-mx6ITFeGrYdObqMQ==/split_lib_slice_5_apk.apk", zip file "/data/app/uk.ac.aber.dcs.haa14.assignment-5Y5qp-mx6ITFeGrYdObqMQ==/split_lib_slice_6_apk.apk", zip file "/data/app/uk.ac.aber.dcs.haa14.assignment-5Y5qp-mx6ITFeGrYdObqMQ==/split_lib_slice_7_apk.apk", zip file "/data/app/uk.ac.aber.dcs.haa14.assignment-5Y5qp-mx6ITFeGrYdObqMQ==/split_lib_slice_8_apk.apk", zip file "/data/app/uk.ac.aber.dcs.haa14.assignment-5Y5qp-mx6ITFeGrYdObqMQ==/split_lib_slice_9_apk.apk"],nativeLibraryDirectories=[/data/app/uk.ac.aber.dcs.haa14.assignment-5Y5qp-mx6ITFeGrYdObqMQ==/lib/x86, /system/lib, /vendor/lib]]
at dalvik.system.BaseDexClassLoader.findClass(BaseDexClassLoader.java:125)
at java.lang.ClassLoader.loadClass(ClassLoader.java:379)
at java.lang.ClassLoader.loadClass(ClassLoader.java:312)
at android.view.LayoutInflater.createView(LayoutInflater.java:606)
at android.view.LayoutInflater.createViewFromTag(LayoutInflater.java:790)
at android.view.LayoutInflater.createViewFromTag(LayoutInflater.java:730)
at android.view.LayoutInflater.rInflate(LayoutInflater.java:863)
at android.view.LayoutInflater.rInflateChildren(LayoutInflater.java:824)
at android.view.LayoutInflater.inflate(LayoutInflater.java:515)
at android.view.LayoutInflater.inflate(LayoutInflater.java:423)
at android.view.LayoutInflater.inflate(LayoutInflater.java:374)
at android.support.v7.app.AppCompatDelegateImpl.setContentView(AppCompatDelegateImpl.java:469)
at android.support.v7.app.AppCompatActivity.setContentView(AppCompatActivity.java:140)
at uk.ac.aber.dcs.haa14.assignment.ui.DictionaryActivity.onCreate(DictionaryActivity.java:32)
E/AndroidRuntime: at android.app.Activity.performCreate(Activity.java:7009)
at android.app.Activity.performCreate(Activity.java:7000)
at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1214)
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2731)
at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2856)
at android.app.ActivityThread.-wrap11(Unknown Source:0)
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1589)
at android.os.Handler.dispatchMessage(Handler.java:106)
at android.os.Looper.loop(Looper.java:164)
at android.app.ActivityThread.main(ActivityThread.java:6494)
at java.lang.reflect.Method.invoke(Native Method)
at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:438)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:807)
Suppressed: java.io.IOException: No original dex files found for dex location /data/app/uk.ac.aber.dcs.haa14.assignment-5Y5qp-mx6ITFeGrYdObqMQ==/split_lib_resources_apk.apk
at dalvik.system.DexFile.openDexFileNative(Native Method)
at dalvik.system.DexFile.openDexFile(DexFile.java:353)
at dalvik.system.DexFile.<init>(DexFile.java:100)
at dalvik.system.DexFile.<init>(DexFile.java:74)
at dalvik.system.DexPathList.loadDexFile(DexPathList.java:374)
at dalvik.system.DexPathList.makeDexElements(DexPathList.java:337)
at dalvik.system.DexPathList.<init>(DexPathList.java:157)
at dalvik.system.BaseDexClassLoader.<init>(BaseDexClassLoader.java:65)
at dalvik.system.PathClassLoader.<init>(PathClassLoader.java:64)
at com.android.internal.os.ClassLoaderFactory.createClassLoader(ClassLoaderFactory.java:73)
at com.android.internal.os.ClassLoaderFactory.createClassLoader(ClassLoaderFactory.java:88)
at android.app.ApplicationLoaders.getClassLoader(ApplicationLoaders.java:69)
at android.app.ApplicationLoaders.getClassLoader(ApplicationLoaders.java:35)
at android.app.LoadedApk.createOrUpdateClassLoaderLocked(LoadedApk.java:693)
at android.app.LoadedApk.getClassLoader(LoadedApk.java:727)
at android.app.LoadedApk.getResources(LoadedApk.java:954)
at android.app.ContextImpl.createAppContext(ContextImpl.java:2270)
at android.app.ActivityThread.handleBindApplication(ActivityThread.java:5639)
at android.app.ActivityThread.-wrap1(Unknown Source:0)
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1656)
... 6 more
Application terminated.
add implementation 'com.android.support:design:27.1.0' into app/build.gradle where all other dependencies are.