Error:Execution failed for task ':app:transformClassesWithDexForDebug'

Just correct Google play services dependencies:

You are including all play services in your project. Only add those you want.

For example , if you are using only maps and g+ signin, than change

 compile 'com.google.android.gms:play-services:8.1.0'

to

compile 'com.google.android.gms:play-services-maps:8.1.0'compile 'com.google.android.gms:play-services-plus:8.1.0'

From the doc :

In versions of Google Play services prior to 6.5, you had to compile the entire package of APIs into your app. In some cases, doing so made it more difficult to keep the number of methods in your app (including framework APIs, library methods, and your own code) under the 65,536 limit.

From version 6.5, you can instead selectively compile Google Play service APIs into your app. For example, to include only the Google Fit and Android Wear APIs, replace the following line in your build.gradle file:
compile 'com.google.android.gms:play-services:8.3.0'
with these lines:

compile 'com.google.android.gms:play-services-fitness:8.3.0'
compile 'com.google.android.gms:play-services-wearable:8.3.0'

Whole list can be found here.

Try

dexOptions {    javaMaxHeapSize "4g"    preDexLibraries = false}

I don't know the reason.Something about preDexLibraries :https://sites.google.com/a/android.com/tools/tech-docs/new-build-system/tips

According to @lgdroid57 :

The following resource should help explain what this code does: link(http://google.github.io/android-gradle-dsl/current/com.android.build.gradle.internal.dsl.DexOptions.html) Property | Description javaMaxHeapSize | Sets the -JXmx* value when calling dx. Format should follow the 1024M pattern. preDexLibraries | Whether to pre-dex libraries. This can improve incremental builds, but clean builds may be slower.

Try adding multiDexEnabled true to your app build.gradle file.

  defaultConfig {    multiDexEnabled true}