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Question and answer

What is the difference between an abstract class and an interface in Java?

Answer: An abstract class is a class that cannot be instantiated, and it can have abstract and non-abstract methods. An interface, on the other hand, is a collection of abstract methods and constants that can be implemented by any class. One key difference between the two is that a class can implement multiple interfaces, but it can only extend one abstract class.

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209 viewsedited 04:34

Android interview

Question and answer

What is the difference between the “==” operator and the “.equals()” method in Java?

Answer: The “==” operator checks for object reference equality, while the “.equals()” method checks for object value equality. For example, two different String objects with the same value will return true when compared using “.equals()”, but false when compared using “==”.

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249 viewsedited 04:39

Android interview

Java and Kotlin

1. What is the difference between an abstract class and an interface in Java?
Answer: An abstract class is a class that cannot be instantiated, and it can have abstract and non-abstract methods. An interface, on the other hand, is a collection of abstract methods and constants that can be implemented by any class. One key difference between the two is that a class can implement multiple interfaces, but it can only extend one abstract class.

2. What is the difference between the “==” operator and the “.equals()” method in Java?

Answer: The “==” operator checks for object reference equality, while the “.equals()” method checks for object value equality. For example, two different String objects with the same value will return true when compared using “.equals()”, but false when compared using “==”.

3. What is a lambda expression in Kotlin?

Answer: A lambda expression is a way to define a function in Kotlin without creating a separate named function. It allows you to define a function inline, using a more concise syntax. For example, the following code defines a lambda expression that takes two integers and returns their sum: “(x: Int, y: Int) -> x + y”.

4. What is the difference between a lateinit property and an initialized property in Kotlin?

Answer: A lateinit property is a property that is declared without an initial value, but is guaranteed to be initialized before it is used. This is useful for properties that cannot be initialized in the constructor, but need to be initialized before they are used. An initialized property, on the other hand, is a property that is declared with an initial value and can be used immediately.

5. What is the difference between a HashSet and a TreeSet in Java?

Answer: A HashSet is an unordered set of unique elements, implemented using a hash table. A TreeSet, on the other hand, is an ordered set of unique elements, implemented using a red-black tree. The elements in a TreeSet are stored in sorted order.

6. What is a companion object in Kotlin?

Answer: A companion object is an object that is associated with a class, rather than an instance of the class. It can be used to define static methods and properties for the class. For example, the following code defines a companion object for the MyClass class, with a static method “myStaticMethod”:

class MyClass {
companion object {
fun myStaticMethod() {
/* code */ }
}
}

7. What is the difference between a class and an object in Kotlin?

Answer: A class is a blueprint for creating objects, while an object is a single instance of a class. Objects are often used to implement singletons in Kotlin.

8. What is polymorphism in Java?

Answer: Polymorphism is the ability of an object to take on multiple forms. In Java, this can be achieved through inheritance and method overriding. For example, a subclass can override a method from its superclass to provide a different implementation.

9. What is a functional interface in Java?

Answer: A functional interface is an interface that has only one abstract method. It is often used with lambda expressions and method references in Java. Java provides the

@FunctionalInterface

annotation to indicate that an interface is a functional interface.

10. What is the difference between a private and a protected method in Java?

Answer: A private method is a method that can only be accessed within the same class. A protected method, on the other hand, can be accessed within the same class and any subclasses.

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