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Java / Interface

Difference between Optional flatMap() and map() in Java8.

Use map if the function returns the object you need or flatMap if the function returns an Optional.

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What is a Marker(Tag) Interface? Advantages of Inner Interface. What are the access modifiers applicable for the nested interface declared inside a class? Can an interface inherit/extend multiple interface? Example of a nested interface from Java API. Inner Interface in Java. Can an Interface implement another interface? Are interfaces also inherited from Object class? Can a class be defined inside an Interface? Are Nested interfaces declared static? can we have non-static inner interface? Syntax for inner interface. Should the nested interface must be public? Can we define a class inside an interface? Advantage of having a class inside an interface. What is the difference between an Abstract class and Interface? Can a method inside a Interface be declared as final or static in Java? Can an Interface extend another Interface? Why interface is allowed to extend multiple interface whereas class can inherit only one class? Can an Interface be final? Give few examples of a marker interface? Can I define private and protected variables in an interface? What is Externalizable? Difference between Serializable and Externalizable. Does Externalizable extends Serializable interface? Can you specify the accessibility modifier for methods inside an interface? What is abstract Interfaces in Java? Can a interface define static initializer in Java ? What is functional interface in Java 8. Why do we implement interfaces in Java? What are the rules for writing lambda expressions in Java 8? What is SAM interface in Java 8? How will you call a default method of an interface in a class? Explain type inference in Java 8. Explain the difference between lambda expression and anonymous class in terms of “this” reference. What is Optional in Java8? Can we initialize Optional value as null in Java8? Difference between Optional flatMap() and map() in Java8. Does interface uses all Object class methods? Can interface have instance fields? Can interface have instance method? How JVM invoke special behavior in case of marker interface? How to write your own marker interface? Can we have private interface method? Can an interface have main method? Can you modify the value of an interface field? Can we have private method/static method in Interface ?
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