If an enum implements an interface, and methods from the interface are called on an instance of the enum, the enum instance needs to be referenced as the interface, or the interface is somehow referenced in the context where the methods are called; Otherwise, there is possible compilation failure in a Maven build complaining about "cannot find symbol" on the interface methods being called.

WeakHashMap uses WeakReference for its keys. In order for an entry to be kept in such a map, the key object of the entry needs to be referenced somewhere outside of the map; otherwise, the entry is removed in the next garbage collection, according to the definition of WeakReference.

If an anonymous class instance is not going to reference the implicit this to its enclosing instance, which is actually the majority of the uses where we are just too lazy to define another class, the anonymous class instance needs to be created from a static context, such as a static factory method. Otherwise, there is highly likely a memory leak.

To lazily initialize a variable of a reference type in a multithreaded environment, we either use the Initialization-On-Demand Holder or use the Double-Checked Locking with the variable being qualified by the volatile keyword. The variable does not need to be qualified by volatile if it is a 32-bit primitive such as int and float.

The Java Language Specification 3.0 §14.4.2 defines a restriction on variable shadowing to help detect bugs.

Suppose that we need to make some changes to our source code during a Maven build, e.g., we want to add a special method to a class for a testing Maven profile, but not for other Maven profiles. This post shows a method to achieve this by calling the Java Compiler API from a Maven plugin. The method does not require the knowledge of annotation processing or bytecode manipulation.

Queue latency is the main performance metric that Disruptor sets itself apart from traditional queue implementations, e.g., ArrayBlockingQueue. To my understanding, Disruptor is based on volatile variable polling, rather than the blocking wait() and notify() mechanism. In this post, we benchmark the Disruptor polling style and the ArryaBlockingQueue blocking style using a simple queue with just one producer and one consumer. We define two latency metrics: one is the latency between a producer starts producing an item and ends producing the item, i.e., the producing latency; the other is the latency between a producer starts producing an item and a consumer starts consuming the same item, i.e., the queue latency. By definition, the queue latency includes the producing latency.

Immutable objects have many advantages, e.g., they can be shared, and the sharing is thread-safe and can help avoid creating unnecessary objects. The only real disadvantage of making objects immutable is that a separate object is needed for each distinct value. This seems to be a big overhead for large immutable objects, but it is not always true. In some cases, the immutability can actually lead to a cheaper implementation than you would otherwise. The idea is that a distinct value is often not a completely different value. It is possible that some internals within immutable objects which themselves are immutable can be shared.

Threads are a fundamental part of the Java platform. Java SE 5 and 6 are a huge step forward for the development of concurrent applications, with improvements to the Java Virtual Machine to support high-performance, highly scalable concurrent classes and a rich set of new concurrency building blocks. In Java Concurrency in Practice, the creators of these new facilities explain not only how they work and how to use them, but also the motivation and design patterns behind them.

Joshua J. Bloch (born August 28, 1961) is a software engineer, currently employed at Google, and a technology author. He led the design and implementation of numerous Java platform features, including the Java Collections Framework, the java.math package, and the assert mechanism. He is the author of the programming guide Effective Java, which won the 2001 Jolt Award, and is a co-author of two other Java books, Java Puzzlers (2005) and Java Concurrency In Practice (2006).