Java Basics - Interview Questions and Answers

  1. What are wrapper classes?

    Java provides specialized classes corresponding to each of the primitive data types. These are called wrapper classes.

    They are example: Integer, Character, Double etc.

  2. Why do we need wrapper classes?

    It is sometimes easier to deal with primitives as objects. Moreover most of the collection classes store objects and not primitive data types. And also the wrapper classes provide many utility methods also.

    Because of these resons we need wrapper classes. And since we create instances of these classes we can store them in any of the collection classes and pass them around as a collection. Also we can pass them around as method parameters where a method expects an object.

  3. What are checked exceptions?

    Checked exception are those which the Java compiler forces you to catch.

    Example: IOException are checked exceptions.

  4. What are runtime exceptions?
    Runtime exceptions are those exceptions that are thrown at runtime because of either wrong input data or because of wrong business logic etc. These are not checked by the compiler at compile time.
  5. What is the difference between error and an exception?

    An error is an irrecoverable condition occurring at runtime. Such as OutOfMemory error.

    These JVM errors and you can not repair them at runtime. While exceptions are conditions that occur because of bad input etc. Example: FileNotFoundException will be thrown if the specified file does not exist. Or a NullPointerException will take place if you try using a null reference.

    In most of the cases it is possible to recover from an exception (probably by giving user a feedback for entering proper values etc.).

  6. How to create custom exceptions?
    Your class should extend class Exception, or some more specific type thereof.

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