What are Integer Boolean and Double wrapper classes?

Answer 1

In Java an integer and a double are "primitive" data types.

When dealing with functions, often you want to do more than send an "int" or a "double"'s VALE, you wish to send a REFERENCE of a number type to a function so that you can operate on that number type. What you can do is create a new Integer, Boolean, or Double, and send it to the function. This creates a "CLASS" that acts as an Integer (or whatever you chose).

Eg. You want a function that takes in a number variable, and adds two to it, and returns NOTHING. You cannot send a normal "int", because that integer cannot be "referenced" to after the function call. You would create a new Integer Object from the Integer Class, and insert that new object to the function.

Integer four = new Integer(4); // Makes a new Integer named "four" with the value 4.

There is a wrapper class for every primitive in Java. For instance, the wrapper class for int is Integer, the class for float is Float, and so on. Remember that the primitive name is simply the lowercase name of the wrapper except for char, which maps to Character, and int, which maps to Integer.

The Wrapper classes for each of the primitive types is as follows:

1. boolean - Boolean

2. byte - Byte

3. char - Character

4. double - Double

5. float - Float

6. int - Integer

7. long - Long

8. short - Short

A point to note here is that, the wrapper classes for numeric primitives have the capability to convert any valid number in string format into its corresponding primitive object. For ex: "10" can be converted into an Integer by using the below line

Integer intVal = new Integer("10");

I can do the same for all numeric types like long, short etc. But, if I pass an invalid value I will get a "NumberFormatException" Ex:

Integer intVal = new Integer("Haha");

The Wrapper Constructors

All of the wrapper classes except Character provide two constructors: one that takes a primitive of the type being constructed, and one that takes a String representation of the type being constructed-for example

Integer i1 = new Integer(42);

Integer i2 = new Integer("42");

or

Float f1 = new Float(3.14f);

Float f2 = new Float("3.14f");

The Character class provides only one constructor, which takes a char as an argument-for example

Character c1 = new Character('c');

The constructors for the Boolean wrapper take either a boolean value true or false, or a String. If the String's case-insensitive value is "true" the Boolean will be true-any other value will equate to any other value will equate to false. Until Java 5, a Boolean object couldn't be used as an expression in a boolean test-for instance

Boolean b = new Boolean("false");

if (b) // won't compile, using Java 1.4 or earlier

As of Java 5, a Boolean object can be used in a boolean test, because the compiler will automatically "unbox" the Boolean to a boolean.