Java EE Specification to Java namespace

JSR-250: javax.annotation
JPA 2.0: javax.persistence
JSF 2.0: javax.faces
Servlet 2.5/ 3.0: javax.servlet / javax.servlet.jsp
JSR-330: javax.inject
JSR-303: javax.validation

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ConcurrentHashMap computeIfAbsent method in Java 8

The very nifty method computeIfAbsent has been added in the ConcurrentMap interface in Java 8 as part of the atomic operations of the ConcurrentMap interface. It’s more precisely a default method that provides an alternative to what we use to code ourselves:

if (map.get(key) == null) {
   V newValue = mappingFunction.apply(key);
   if (newValue != null)
      return map.putIfAbsent(key, newValue);
   }
}

but this time providing a function as a second argument.

Most often this method will be used in the context of ConcurrentHashMap in which case the method is implemented in a thread-safe synchronised way.

In terms of usage the method is handy for situations where we want to maintain a thread-safe cache of expensive one-off computed resources.

Here’s another example of holding a key-value pair where value is a thread-safe counter represented by an AtomicInteger:

private final Map counters = new ConcurrentHashMap();

private void accumulate(String name) {
    counters.computeIfAbsent(name, k -> new AtomicInteger()).incrementAndGet();
}