Java 8 Cheat Sheet Code Snippets

DefaultMethods.java

//Default methods in interfaces
@FunctionalInterface
interface Utilities {
  default Consumer<Runnable> m() {
    return (r) -> r.run();
  }
  // default methods, still functional
  Object function(Object o); 
}
  
class A implements Utilities { // implement 
  public Object function(Object o) {
    return new Object();
  }
  {
    // call a  default method
    Consumer<Runnable> n = new A().m();          
  }
}

Lambdas.java

// takes an Long, returns a String
Function<Long, String> f = (l) -> l.toString(); 
// takes nothing gives you Threads
Supplier<Thread> s = Thread::currentThread; 
// takes a string as the parameter 
Consumer<String> c = System.out::println;        
   
// use them in with streams
new ArrayList<String>().stream(). 
// peek: debug streams without changes
peek(e -> System.out.println(e)). 
// map: convert every element into something  
map(e -> e.hashCode()). 
// filter: pass some elements through
filter(e -> ((e.hashCode() % 2) == 0)).   
// collect the stream into a collection 
collect(Collectors.toCollection(TreeSet::new))

Optionals.java

// Create an optional
Optional<String> optional = Optional.ofNullable(a);
// process the optional 
optional.map(s -> "RebelLabs:" + s);
// map a function that returns Optional
optional.flatMap(s -> Optional.ofNullable(s));

// run if the value is there
optional.ifPresent(System.out::println); 
// get the value or throw an exception
optional.get(); 

// return the value or the default value
optional.orElse("Hello world!");  
// return empty Optional if not satisfied
optional.filter(s -> s.startsWith("RebelLabs"));