2.5 Java1610: polymorphism, type conversion, casting, etc.  (Page 2/6)

In other cases, multiple methods have the same name, same return type, and same formal argument list (overridden methods) .

Three distinct forms of polymorphism

From a practical programming viewpoint, polymorphism manifests itself in three distinct forms in Java:

• Method overloading
• Method overriding through inheritance
• Method overriding through the Java interface

I covered method overloading as one form of polymorphism in a previous module.

We need to backtrack

In this module, I will backtrack a bit and discuss the conversion of references from one type to another.

I will begin the discussion of polymorphism through method overriding and inheritance in the next module. I will cover interfaces in a future module.

Assignment compatibility and type conversion

As a background for polymorphism, you need to understand something about assignment compatibility and type conversion .

A value of a given type is assignment compatible with another type if

• a value of the first type
• can be successfully assigned to a variable of the second type.

Type conversion and the cast operator

In some cases, type conversion happens automatically. In other cases, type conversion must be forced through the use of a cast operator .

A cast operator is a unary operator, which has a single right operand. The physical representation of the cast operator is the name of a type inside a pair of matching parentheses, as in:

Applying a cast operator

Applying a cast operator to the name of a variable doesn't actually change the type of the variable. However, it does cause the contents of the variable to be treated as a different type for the evaluation of the expression in which the cast operator is contained. Thus, the application of a cast operator is a short-term operation.

Primitive values and type conversion

Assignment compatibility issues come into play for both primitive types and reference types.

Values of type boolean can only be assigned to variables of type boolean (you cannot change the type of a boolean ) .

Otherwise, a primitive value can be assigned to any variable of a type

• whose range is as wide or wider
• than the range of the type of the value.

In that case, the type of the value is automatically converted to the type of the variable.

(For example, types byte and short can be assigned to a variable of type int without the requirement for a cast because type int has a wider range than either type byte or type short .)

Conversion to narrower range

On the other hand, a primitive value of a given type cannot be assigned to a variable of a type with a narrower range than the type of the value, unless the cast operator is used to force a type conversion.

Oftentimes, such a conversion will result in the loss of data, and that loss is the responsibility of the programmer who performs the cast.

Assignment compatibility for references

Assignment compatibility, with respect to references, doesn't involve range issues, as is the case with primitives. Instead, the reference to an object instantiated from a given class can be assigned to: