Some JDK APIs are obsolete and have preferred alternatives.

LinkedList

LinkedList almost never out-performs ArrayList or ArrayDeque[^1]. If you are using LinkedList as a list, prefer ArrayList. If you are using LinkedList as a stack or queue/deque, prefer ArrayDeque.

Migration gotcha: LinkedList permits null elements; ArrayDeque rejects them. The documentation for Deque strongly discourages users from inserting null, even into implementations that permit it. So, if you are using a LinkedList for this purpose, you should likely stop, and you will need to stop in order to migrate to ArrayDeque.

Vector

Vector performs synchronization that is usually unnecessary; prefer ArrayList.

If a synchronized collection is necessary, use Collections.synchronizedList or a data structure from java.util.concurrent.

Hashtable and Dictionary

This is a nonstandard class that predates the Java Collections Framework; prefer LinkedHashMap or HashMap.

If synchronization is necessary, java.util.concurrent.ConcurrentHashMap is usually a good choice.

java.util.Stack

Stack is a nonstandard class that predates the Java Collections Framework; prefer ArrayDeque.

If a synchronized collection is necessary, use a data structure from java.util.concurrent.

When migrating from Stack to Deque, note that the Stack methods push/pop/peek/add/iterator correspond to the Deque methods addFirst/removeFirst/peekFirst/addFirst/descendingIterator.

StringBuffer

StringBuffer performs synchronization that is rarely necessary and has significant performance overhead. Prefer StringBuilder, which does not do synchronization.

If synchronization is necessary, consider creating an explicit lock object and using synchronized blocks.

Enumeration

An ancient precursor to Iterator.

SortedSet and SortedMap

Replaced by NavigableSet and NavigableMap in Java 6.

[^1]: People generally choose LinkedList because they want fast insertion and removal. However, LinkedList has slow traversal, and typically you need to traverse the list to find the place to insert/remove. It turns out that the cost of traversing to a location in a LinkedList is approximately 4x the cost of copying an element in an ArrayList. Thus, LinkedList's traversal cost dominates and results in poorer performance. More info: https://stuartmarks.wordpress.com/2015/12/18/some-java-list-benchmarks/

Date

The Date API is full of major design flaws and pitfalls and should be avoided at all costs. Prefer the java.time APIs, specifically, java.time.Instant (for physical time) and java.time.LocalDate[Time] (for civil time).