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Using a StringBuffer for concatenation can in fact produce code that is significantly faster than using a String.

After the concatenation is performed on the StringBuffer object, it must be converted back into a String. This is done with the call to the toString method at location 26. This method creates a new String object from the temporary StringBuffer object. The creation of this temporary StringBuffer object and its subsequent conversion back into a String object are very expensive.

The HashMap
class is roughly equivalent to Hashtable, except that it is
unsynchronized and permits nulls

If multiple
threads access this map concurrently, and at least one of the threads
modifies the map structurally, it must be synchronized externally.

both the ArrayList and Vector hold onto their contents using an Array.

access specifier

There are a number of situations in software engineering when it is
important for disparate groups of programmers to agree to a "contract"
that spells out how their software interacts.

an interface is a reference type, similar to a class, that can contain only
constants, method signatures, and nested types.

Abstract Classes versus Interfaces

If an abstract class
contains only abstract method declarations, it should be declared as an interface instead.

The ability to examine and manipulate a Java class from within itself

The reflection classes, such as Method, are found in
java.lang.reflect. There are three steps that must be followed to use
these classes

Surprisingly, the answer is "no", because Box<Integer> and Box<Double> are not subtypes of Box<Number>