Wrapper Class in Java Interview Questions and Answers for 7 years experience

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Java Wrapper Classes Interview Questions

  1. What are Wrapper classes in Java?

    • Answer: Wrapper classes in Java provide a way to convert primitive data types (like `int`, `float`, `boolean`, etc.) into objects. This is crucial for using primitives in collections (like `ArrayList`), which only accept objects. Each primitive type has a corresponding wrapper class (e.g., `Integer` for `int`, `Float` for `float`, `Boolean` for `boolean`).

  2. Explain autoboxing and unboxing.

    • Answer: Autoboxing is the automatic conversion of a primitive type to its corresponding wrapper class. Unboxing is the reverse – converting a wrapper class object back to its primitive type. This is handled implicitly by the Java compiler, simplifying code.

  3. What is the purpose of the `valueOf()` method in wrapper classes?

    • Answer: The `valueOf()` method in wrapper classes is used to convert a primitive type or a String representation of a primitive value into a wrapper object. For example, `Integer.valueOf("10")` converts the String "10" into an `Integer` object.

  4. How do you convert an `Integer` object to an `int` primitive?

    • Answer: You can use the `intValue()` method. For example: `int primitiveInt = integerObject.intValue();`

  5. What are the advantages of using wrapper classes?

    • Answer: Advantages include the ability to use primitives in collections, null values for primitives (unlike primitives which cannot be null), and methods provided by wrapper classes for tasks like parsing and converting to String representations.

  6. Explain the concept of immutability in wrapper classes.

    • Answer: Wrapper class objects are immutable. Once a wrapper object is created, its value cannot be changed. Any operation that appears to modify the object actually creates a new object with the modified value.

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

    • Answer: `int` is a primitive data type, while `Integer` is its corresponding wrapper class. `int` holds a value directly, while `Integer` is an object that encapsulates an `int` value. `Integer` can be null, `int` cannot.

  8. How does `Integer.parseInt()` work?

    • Answer: `Integer.parseInt()` takes a String as input and parses it to return the equivalent `int` value. It throws a `NumberFormatException` if the string cannot be parsed as an integer.

  9. What is the purpose of `Integer.MAX_VALUE` and `Integer.MIN_VALUE`?

    • Answer: These constants represent the maximum and minimum values that can be stored in an `int` primitive, respectively.

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