As of Java 7 now, default size of ArrayList is 10 and default capacity of HashMap is 16, it must be power of 2. Here is code snippet from ArrayList and HashMap class :
// from ArrayList.java JDK 1.7
private static final int DEFAULT_CAPACITY = 10;
//from HashMap.java JDK 7
static final int DEFAULT_INITIAL_CAPACITY = 1 << 4; // aka 16