Notes about Default methods in an interface in java 8

1) If you have 2 interfaces which contains same non-default methods then, inside an implementation class either 
   you provide InterfaceName.super.methodName(); to directly refer a method from super interface or else it will 
   call default implementation inside the implementation class itself.
  
2) If you have multiple interfaces with DEFAULT method/s, and if any one or all of them are OVERRIDDEN in
   client class, then while calling those methods in implementation class it always refers to the child 
   implementation method of client class. For examples refer to the defaults package inside this repository.
   
3) A method in an interface should have only one of these following modifiers at a time
   public, abstract, static, default, strictfp

Difference between Consumer Functional Interface and Supplier Functional Interface

  Consumer interface accepts single input argument and returns no result 
  while Supplier interface does not take any input but returns output

How to iterate a Map using lambda in forEach loop

filename and method name: StreamGoupingByExample.java , customizedGpaGroupingBy()

Code:

  public static Map<String, List<Student>> customizedGpaGroupingBy() {
	 		 Map<String, List<Student>> studentGapGrouping = 
                               StudentDataBase
                               .getAllStudents()
                               .stream()
                               .collect(
                               Collectors.groupingBy(stud -> stud.getGpa()>=5? "OUTSTANDING":"AVERAGE"));
	 return studentGapGrouping;
 }

code in main( )

  customizedGpaGroupingBy().forEach((x, y) -> {
                           System.out.println(x + "  ");
                           y.forEach(a -> 
                           System.out.println("name= " + a.getName() + " gpa=" + a.getGpa()));
});

OutPut: to remove square brackets and for clean output

	  AVERAGE                                   OUTSTANDING
	name= Adam gpa=3.6                     name= Dave gpa=6.0
	name= Jenny gpa=1.8                    name= Viccky gpa=7.0
	name= Emily gpa=4.0                    name= Gopal gpa=8.9
	name= Sophia gpa=3.5                   name= Vijay gpa=5.5
	name= James gpa=3.9

Java-8-Code Samples using Lambda, Streams, Functional Interfaces etc.

To avoid duplicate keys in Collectors.toMap() using List to Map with Key Mapper, Value Mapper and Merge Function and assign it to LinkedHashMap

    <T,K,U> Collector<T,?,Map<K,U>> toMap(Function<? super T,? extends K> keyMapper,
                                          Function<? super T,? extends U> valueMapper,
                                          BinaryOperator<U> mergeFunction)

Code :

  public static void main(String[] args) {
      List<Person> list = new ArrayList<>();
      list.add(new Person(100, "Mohan"));
      list.add(new Person(100, "Sohan"));
      list.add(new Person(300, "Mahesh"));
      LinkedHashMap<Integer, String> map =list.stream()
                                          .collect(Collectors.toMap(Person::getId, Person::getName, 
                                          (x, y) -> x+", "+ y, LinkedHashMap::new));
                                          map.forEach((x, y) -> System.out.println("Key: " + x +", value: "+ y));
    }

---

To avoid duplicate keys in Collectors.toMap() using BinaryOperator and Supplier
OR List to Map with Key Mapper, Value Mapper, Merge Function and Map Supplier

 <T,K,U,M extends Map<K,U>> Collector<T,?,M> toMap(Function<? super T,? extends K> keyMapper,
                                                   Function<? super T,? extends U> valueMapper,
                                                   BinaryOperator<U> mergeFunction,
                                                   Supplier<M> mapSupplier)

Code :

 public static void main(String[] args) { 
    List<Person> list = new ArrayList<>();
    list.add(new Person(100, "Mohan"));
    list.add(new Person(100, "Sohan"));
    list.add(new Person(300, "Mahesh"));
    list.add(new Person(100, "Gopal"));
      
    Map<Integer, String> map = list.stream()
                              .collect(Collectors.toMap(Person::getId, Person::getName, (x, y) -> x + ", " + y));
                               map.forEach((x, y) -> System.out.println("Key: " + x + ", value: " + y));
  }

Parameters	Description
keyMapper	A mapping function to produce the map keys for each input stream element.
valueMapper	A mapping function to produce the map values for each input stream element.
mergeFunction	A binary operator which is to resolve collisions between values associated with the same key. The inputs to this function are the values which belong to the same key.
mapSupplier	A function which provies a new instance of the desired implementation of the Map.