How much of the core Java APIs are present in Android API

Question

How much of the core Java APIs are present in Android API? How proficient will I become in writing core Java applications by writing applications for Android phones?

Answer 1

Android includes the essential packages from the Java SE API (java.lang, java.util, java.net, etc.) but it is nowhere near the full Java SE API. It excludes AWT, Swing, Java 2D, Java Sound, RMI, CORBA, the scripting API, JMX, JAXB, the Java Print Service API, parts of JDBC and probably several other things.

You can easily see for yourself by comparing the package lists for Android and Java SE.