Deferred Execution With Java's Supplier

In the third chapter of his 2014 book "Java SE 8 for the Really Impatient: Programming with Lambdas," Cay Horstmann writes, "The point of all lambdas is deferred execution." He adds, "After all, if you wanted to execute some code right now, you'd do that, without wrapping it inside a lambda." Horstmann then lists several examples of the "many reasons for executing code later" and his last listed reason is "Running the code only when necessary." In this post, I look at some examples of this from the JDK that use Supplier to do exactly that: to execute a "code block" represented as a lambda expression "only when necessary."

When JDK 8 introduced lambda expressions, it also introduced several standard functional interfaces in its java.util.function package. That package's Javadoc documentation states, "Functional interfaces provide target types for lambda expressions and method references." The package contains standard functional interfaces such as Predicate (accepts a single argument and represents a boolean), Function (accepts a single argument and provides a single result), Consumer (accepts a single argument and does not provide a result), and Supplier (accepts no arguments and represents/supplies a single result). [As a side note, the blog post "Java 8 Friday: The Dark Side of Java 8" provides a highly useful "decision tree to [determine] ... the [standard functional interface] you're looking for" when trying to determine which standard functional interface to use (or to roll your own).]

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