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The statement containing the cast operator is shown below for convenient viewing.
myIntVar = (int)myDoubleVar; |
Syntax of a cast operator
The cast operator consists of the name of a type contained within a pair of matching parentheses.
A unary operator
The cast operator always appears to the left of an expression whose type is being converted to the type specified by the cast operator.
Assuming responsibility for potential problems
When dealing with primitive types, the cast operator is used to notify the compiler that the programmer is willing to assume the risk of a possible loss ofprecision in a narrowing conversion.
No loss of precision here
In this case, there was no loss in precision, but that was only because the value stored in the double variable was within the allowable value range for an int .
In fact, it was the largest positive value that can be stored in the type int . Had it been any larger, a loss of precision would have occurred.
More on this later ...
I will have quite a bit more to say about the cast operator in future modules. I will also have more to say about the use of the assignmentoperator in conjunction with the non-primitive types.
A. Compiler Error
Conversion from double to int is not automatic
This program attempts to assign a value of type double to a variable of type int .
Even though we know that the specific double value involved would fit in the int variable with no loss of precision, the conversion from double to int is not a widening conversion.
This is a narrowing conversion
In fact, it is a narrowing conversion because the allowable value range for an int is less than the allowable value range for a double .
The conversion is not allowed by the compiler. The following compiler error occurs under JDK 1.3:
Ap012.java:16: possible loss of precision
found : doublerequired: int
myIntVar = myDoubleVar myIntVar = myDoubleVar; |
C. 2.147483647E9
Declare a double
The method named doAsg first declares a local variable of type double named myDoubleVar without providing an initial value.
Declare and initialize an int
Then it declares an int variable named myIntVar and initializes its value to the integer value 2147483647 (you learned about Integer.MAX_VALUE in an earlier module) .
Assign the int to the double
Following this, the method assigns contents of the int variable to the double variable.
An assignment compatible conversion
This is an assignment compatible conversion. In particular, the integer value of 2147483647 is automatically converted to a double value and stored in the double variable.
The double representation of that value is what appears on the screen later when the value of myDoubleVar is displayed.
What is an assignment compatible conversion?
An assignment compatible conversion for the primitive types occurs when the required conversion is a widening conversion.
What is a widening conversion?
A widening conversion occurs when the allowable value range of the type of the left operand of the assignment operator is greater than the allowable valuerange of the right operand of the assignment operator.
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