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));
 }