Conversions of numeric types to narrower types can result in lost or misinterpreted data if the value of the wider type is outside the range of values of the narrower type. Consequently, all narrowing conversions must be guaranteed safe by range-checking the value before conversion.

Java provides 22 possible narrowing primitive conversions. According to The Java Languagee Specification (JLS), §5.1.3, "Narrowing Primitive Conversions" [JLS 2015]:

• short to byte or char
• char to byte or short
• int to byte, short, or char
• long to byte, short, char, or int
• float to byte, short, char, int, or long
• double to byte, short, char, int, long, or float

Narrowing primitive conversions are allowed in cases where the value of the wider type is within the range of the narrower type.

Integer Narrowing

Integer type ranges are defined by the JLS, §4.2.1, "Integral Types and Values" [JLS 2015], and are also described in NUM00-J. Detect or prevent integer overflow.

The following table presents the rules for narrowing primitive conversions of integer types. In the table, for an integer type T, n represents the number of bits used to represent the resulting type T (precision).

When integers are cast to narrower data types, the magnitude of the numeric value and the corresponding sign can be affected. Consequently, data can be lost or misinterpreted.

Floating-Point to Integer Conversion

Floating-point conversion to an integral type T is a two-step procedure:

1. When converting a floating-point value to an int or long and the value is a NaN, a zero value is produced. Otherwise, if the value is not infinity, it is rounded toward zero to an integer value V:

• If T is long and V can be represented as a long, the long value V is produced.
• If V can be represented as an int, then the int value V is produced.

Otherwise,

• The value is negative infinity or a value too negative to be represented, and Integer.MIN_VALUE or Long.MIN_VALUE is produced.
• The value is positive infinity or a value too positive to be represented, and Integer.MAX_VALUE or Long.MAX_VALUE is produced.

2. If T is byte, char, or short, the result of the conversion is the result of a narrowing conversion to type T of the result of the first step

See the JLS, §5.1.3, "Narrowing Primitive Conversions" [JLS 2005], for more information.

Other Conversions

Narrower primitive types can be cast to wider types without affecting the magnitude of numeric values (see the JLS, §5.1.2, Widening Primitive Conversion" [JLS 2005]), for more information). Conversion from int or long to float or from long to double can lead to loss of precision (loss of least significant bits). No runtime exception occurs despite this loss.

Note that conversions from float to double or from double to float can also lose information about the overall magnitude of the converted value (see NUM53-J. Use the strictfp modifier for floating-point calculation consistency across platforms for additional information).

Noncompliant Code Example (Integer Narrowing)

In this noncompliant code example, a value of type int is converted to a value of type byte without range checking:

class CastAway {
  public static void main(String[] args) {
    int i = 128;
    workWith(i);
  }

  public static void workWith(int i) {
    byte b = (byte) i;  // b has value -128
    // Work with b
  }
}

The resulting value may be unexpected because the initial value (128) is outside of the range of the resulting type.

Compliant Solution (Integer Narrowing)

This compliant solution validates that the value stored in the wider integer type is within the range of the narrower type before converting to the narrower type. It throws an ArithmeticException if the value is out of range.

class CastAway {
  public static void workWith(int i) {
    // Check whether i is within byte range
    if ((i < Byte.MIN_VALUE) || (i > Byte.MAX_VALUE)) { 
      throw new ArithmeticException("Value is out of range");
    }

    byte b = (byte) i;
    // Work with b
  } 
}

 Alternatively, the workWith() method can explicitly narrow when the programmer's intent is to truncate the value:

class CastAway {
   public static void workWith(int i) { 
     byte b = (byte)(i % 0x100); // 2^8;
     // Work with b
   }
}

Range-checking is unnecessary because the truncation that is normally implicit in a narrowing conversion is made explicit. The compiler will optimize the operation away, and for that reason, no performance penalty is incurred. Similar operations may be used for converting to other integral types.

Noncompliant Code Example (Floating-Point to Integer Conversion)

The narrowing primitive conversions in this noncompliant code example suffer from loss in the magnitude of the numeric value as well as a loss of precision:

float i = Float.MIN_VALUE;
float j = Float.MAX_VALUE;
short b = (short) i;
short c = (short) j;

The minimum and maximum float values are converted to minimum and maximum int values (0x80000000 and 0x7fffffff respectively). The resulting short values are the lower 16 bits of these values (0x0000 and 0xffff). The resulting final values (0 and −1) might be unexpected.

Compliant Solution (Floating-Point to Integer Conversion)

This compliant solution range-checks both the i and j variables before converting to the resulting integer type. Because both values are out of the valid range for a short, this code will always throw an ArithmeticException.

float i = Float.MIN_VALUE;
float j = Float.MAX_VALUE;
if ((i < Short.MIN_VALUE) || (i > Short.MAX_VALUE) ||
    (j < Short.MIN_VALUE) || (j > Short.MAX_VALUE)) {
  throw new ArithmeticException ("Value is out of range");
}

short b = (short) i;
short c = (short) j;
// Other operations

Noncompliant Code Example (double to float Conversion)

The narrowing primitive conversions in this noncompliant code example suffer from a loss in the magnitude of the numeric value as well as a loss of precision. Because Double.MAX_VALUE is larger than Float.MAX_VALUE, c receives the value infinity, and because Double.MIN_VALUE is smaller than Float.MIN_VALUE, b receives the value 0.

double i = Double.MIN_VALUE;
double j = Double.MAX_VALUE;
float b = (float) i;
float c = (float) j;

Compliant Solution (double to float Conversion)

This compliant solution performs range checks on both i and j before proceeding with the conversions. Because both values are out of the valid range for a float, this code will always throw an ArithmeticException.

double i = Double.MIN_VALUE;
double j = Double.MAX_VALUE;
if ((i < Float.MIN_VALUE) || (i > Float.MAX_VALUE) ||
    (j < Float.MIN_VALUE) || (j > Float.MAX_VALUE)) {
  throw new ArithmeticException ("Value is out of range");
}

float b = (float) i;
float c = (float) j;
// Other operations

Risk Assessment

Casting a numeric value to a narrower type can result in information loss related to the sign and magnitude of the numeric value. As a result, data can be misrepresented or interpreted incorrectly.

Automated Detection

Automated detection of narrowing conversions on integral types is straightforward. Determining whether such conversions correctly reflect the intent of the programmer is infeasible in the general case. Heuristic warnings could be useful.

Related Guidelines

Bibliography

[Harold 1999]
[JLS 2005]	§4.2.1, "Integral Types and Values" §5.1.3, "Narrowing Primitive Conversions"
[Seacord 2015]	NUM12-J. Ensure conversions of numeric types to narrower types do not result in lost or misinterpreted data LiveLesson