Wrapper Class in Java Interview Questions and Answers for 5 years experience

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100 Java Wrapper Class Interview Questions

  1. What are Wrapper classes in Java?

    • Answer: Wrapper classes in Java are classes that provide a way to use primitive data types (like `int`, `float`, `boolean`, etc.) as objects. Each primitive type has a corresponding wrapper class (e.g., `Integer` for `int`, `Float` for `float`, `Boolean` for `boolean`). This allows primitives to be used in situations where objects are required, such as adding them to collections (like `ArrayList` or `HashSet`).

  2. Explain autoboxing and unboxing in Java.

    • Answer: Autoboxing is the automatic conversion of a primitive type to its corresponding wrapper class. For example, `Integer i = 10;` automatically converts the `int` literal `10` into an `Integer` object. Unboxing is the reverse process – converting a wrapper class object back to its primitive type. For example, `int x = i;` automatically extracts the `int` value from the `Integer` object `i`.

  3. What are the benefits of using wrapper classes?

    • Answer: Wrapper classes offer several advantages: They allow primitives to be used in collections; they provide methods for manipulating primitive values (like `parseInt()`, `valueOf()`); they offer null values, which primitives cannot have; and they are useful in situations where object references are needed.

  4. What is the difference between `Integer` and `int`?

    • Answer: `int` is a primitive data type, while `Integer` is its corresponding wrapper class. `int` is stored directly on the stack, whereas `Integer` objects are stored on the heap. `Integer` can be null, while `int` cannot. `Integer` provides methods like `parseInt()`, `toString()`, etc., which `int` lacks.

  5. Explain the `valueOf()` method in wrapper classes.

    • Answer: The `valueOf()` method is a static method in each wrapper class that converts a primitive type or a String into a wrapper object. For example, `Integer.valueOf("10")` converts the String "10" into an `Integer` object. It's often used for autoboxing, but it also offers more control than implicit autoboxing.

  • How can you convert a String to an Integer in Java? Discuss multiple ways.

    • Answer: There are several ways: 1. Using `Integer.parseInt()`: This method parses a String and returns an `int`. You would then autobox this `int` to an `Integer`. 2. Using `Integer.valueOf()`: This method parses a String and directly returns an `Integer` object. 3. Using a `NumberFormat` object for more complex parsing scenarios.

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