Chapter: Java The Complete Reference - The Java Language - Data Types, Variables, and Arrays

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Type Conversion and Casting

If you have previous programming experience, then you already know that it is fairly common to assign a value of one type to a variable of another type.

Type Conversion and Casting

 

If you have previous programming experience, then you already know that it is fairly common to assign a value of one type to a variable of another type. If the two types are compatible, then Java will perform the conversion automatically. For example, it is always possible to assign an int value to a long variable. However, not all types are compatible, and thus, not

all type conversions are implicitly allowed. For instance, there is no automatic conversion defined from double to byte. Fortunately, it is still possible to obtain a conversion between incompatible types. To do so, you must use a cast, which performs an explicit conversion between incompatible types. Let’s look at both automatic type conversions and casting.

 

Java’s Automatic Conversions

 

When one type of data is assigned to another type of variable, an automatic type conversion will take place if the following two conditions are met:

 

     The two types are compatible.

 

     The destination type is larger than the source type.

 

When these two conditions are met, a widening conversion takes place. For example, the int type is always large enough to hold all valid byte values, so no explicit cast statement is required.

 

For widening conversions, the numeric types, including integer and floating-point types, are compatible with each other. However, there are no automatic conversions from the numeric types to char or boolean. Also, char and boolean are not compatible with each other.

As mentioned earlier, Java also performs an automatic type conversion when storing a literal integer constant into variables of type byte, short, long, or char.

 

Casting Incompatible Types

 

Although the automatic type conversions are helpful, they will not fulfill all needs. For example, what if you want to assign an int value to a byte variable? This conversion will not be performed automatically, because a byte is smaller than an int. This kind of conversion is sometimes called a narrowing conversion, since you are explicitly making the value narrower so that it will fit into the target type.

To create a conversion between two incompatible types, you must use a cast. A cast is simply an explicit type conversion. It has this general form:

 

(target-type) value

Here, target-type specifies the desired type to convert the specified value to. For example, the following fragment casts an int to a byte. If the integer’s value is larger than the range of a byte, it will be reduced modulo (the remainder of an integer division by the) byte’s range.

 

int a; byte b;

// …

 

b = (byte) a;

A different type of conversion will occur when a floating-point value is assigned to an integer type: truncation. As you know, integers do not have fractional components. Thus, when a floating-point value is assigned to an integer type, the fractional component is lost. For example, if the value 1.23 is assigned to an integer, the resulting value will simply be 1. The 0.23 will have been truncated. Of course, if the size of the whole number component is too large to fit into the target integer type, then that value will be reduced modulo the target type’s range.

 

The following program demonstrates some type conversions that require casts:

 

// Demonstrate casts. class Conversion {

 

public static void main(String args[]) { byte b;

 

int i = 257; double d = 323.142;

 

System.out.println("\nConversion of   int  to byte.");

b = (byte) i;          

System.out.println("i and b " + i +   " "  + b);    

System.out.println("\nConversion of   double to int.");

i = (int) d;           

System.out.println("d and i " + d +   " "  + i);    

System.out.println("\nConversion of   double to byte.");

b = (byte) d;          

System.out.println("d and b " + d +   " "  + b);    

 

}

 

}

This program generates the following output:

 

Conversion of int to byte. i and b 257 1

 

Conversion of double to int. d and i 323.142 323

 

Conversion of double to byte. d and b 323.142 67

Let’s look at each conversion. When the value 257 is cast into a byte variable, the result is the remainder of the division of 257 by 256 (the range of a byte), which is 1 in this case. When

the d is converted to an int, its fractional component is lost. When d is converted to a byte, its fractional component is lost, and the value is reduced modulo 256, which in this case is 67.

 

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