An example of JDK8 functional interfaces.

FunctionalInterfaces.java

// Existing interfaces that are now considered functional interfaces:

interface java.lang.Runnable {
	void run();
}

interface java.util.concurrent.Executor {
	void execute(java.lang.Runnable command);
}

interface java.lang.Iterable<T> {
	java.util.Iterator<T> iterator();
}

interface java.awt.event.ActionListener {
  void actionPerformed(java.awt.event.ActionEvent e);
}

// New functional interfaces:

interface java.util.function.Consumer<T> {
  void accept(T t);
}

interface java.util.function.Function<T, R> {
  R applly(T t);
}

interface java.util.function.Predicate<T> {
  boolean test(T t);
}