Add interview for java skills

README.md

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 2. [Services](https://github.com/farhad/android-interview/blob/master/services.md)
 3. [Gradle](https://github.com/farhad/android-interview/blob/master/gradle.md)
 4. [ConstraintLayout](https://github.com/farhad/android-interview/blob/master/constraintlayout.md)
+5. [Java Skills](https://github.com/ImanX/android-interview/blob/master/java-skills.md)
+

java-skills.md

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+#### Java Skills Interview Questions
+
+1. Describe OOP.
+2. What is `interfaces` ?
+3. What diff between Class and Object ?
+4. ٌWhat is `final` keyword in Java ?
+5. ٌWhat diff between func and method ?
+6. ٌWhat is `abstract` in Java ?
+7. When a class defined `abstract` What happen ?
+8. ٌWhat is `static block` in Java ?
+9. What is `IIB` in Java?
+10. What is `annotation` in Java ?
+11. What is `Generic`s in Java?
+12. How to return more of one value from a method ?
+13. What is diff between `abstract` and `interface` ?
+14. ْWhat is class variables ?
+15. ٌDescribe OverLoading and Overriding.
+16. What diff between OverLoading and Overriding ?
+17. What `Reflaction` in Java ?
+18. When a method defined as `static` What happen ?
+19. When a variable defined as `static` What happen ?
+20. Typically when We will use a `observer pattern` impelement it by `interface`, but interfaces require us that override all of the methods them
+    now, if We will override method by optional we must what of things use?
+21. What types of inheritance in Java ?
+22. What is `wildcard` Generics in Java ?
+23. Describe `Multi Thread` in Java ?
+24. Descrine `Constructor` & `Deconstructor`.
+25. What diff between `inner class` and `nested class` ?
+26. What is `Anonymous class` ?
+27. Do you use `Design Pattern`s ? Which ? Describe ?
+28. What diff between `List<?>` and `ArrayList<?>` ?
+29. Where we use `interface`s ?
+30. What is `Primitive types`? How to allocate in memory them ?
+31. When we define `Primitive types` what is default value ?
+
+
+