Object-Oriented Programming (Narva College) 2018/19 spring

Session 4

Wrapper class

In Java, there are eight primitive data types: byte, short, integer, long, float, double, char and boolean. Unfortunately, not all methods want to operate on primitive data types, but want to get objects as an argument. Hence, we need to convert a primitive data type into an object.

Each of Java's eight primitive data types has a class dedicated to it. These are known as wrapper classes because they wrap the primitive data type into an object of that class.

Wrapper classes of numbers have two constructors: one requires a String type object as an argument and another constructor requires a value of the corresponding data type as an argument. Each of these classes has one of the following methods:

• doubleValue (returns the value of this Double object as a double)
• floatValue (returns the value of this Float object as a float)
• intValue (returns the value of this Integer object as an int)
• longValue (returns the value of this Long object as a long)
• shortValue (returns the value of this Short object as a short)
• byteValue (returns the value of this Byte object as a byte)

The wrapper classes which operate on numbers also have constants MAX_VALUE and MIN_VALUE which hold the values of the minimum and maximum of the corresponding type. See examples:

 System.out.println(Double.MIN_VALUE);
 System.out.println(Byte.MAX_VALUE);
 System.out.println(Integer.MAX_VALUE);

Also these wrapper classes have a static method valueOf which returns a new object of the specified String object:

 Integer intObject = Integer.valueOf("19");

It is also possible to create a value of primitive data type from a String object using the corresponding parsing methods, e.g.:

 double numDouble4 = Double.parseDouble("21");

Session 4