Three ways in android studio for adding a external library.
if you want to add libarary project dependency in your project :
A. In file menu click new and choose import module choose your library project path and click ok, library project automatically add in your android studio project .
B. Now open your main module(like app) gradle file and add project dependency in dependency section dependencies {
compile project(':library project name')
if you want to add jar file : A. add jar file in libs folder. B. And Add dependency
compile fileTree(dir: 'libs', include: '*.jar') // add all jar file from libs folder, if you want to add particular jar from libs add below dependency.
compile files('libs/abc.jar')
Add Dependency from url (recommended). like
compile 'com.mcxiaoke.volley:library-aar:1.0.0'