Tuesday, April 15, 2014

Programmatic Access to Sizes of Java Primitive Types

One of the first things many developers new to Java learn about is Java's basic primitive data types, their fixed (platform independent) sizes (measured in bits or bytes in terms of two's complement), and their ranges (all numeric types in Java are signed). There are many good online resources that list these characteristics and some of these resources are the Java Tutorial lesson on Primitive Data Types, The Eight Data Types of Java, Java's Primitive Data Types, and Java Basic Data Types.

Java allows one to programmatically access these characteristics of the basic Java primitive data types. Most of the primitive data types' maximum values and minimum values have been available for some time in Java via the corresponding reference types' MAX_VALUE and MIN_VALUE fields. J2SE 5 introduced a SIZE field for most of the types that provides each type's size in bits (two's complement). JDK 8 has now provided most of these classes with a new field called BYTES that presents the type's size in bytes (two's complement).

DataTypeSizes.java
package dustin.examples.jdk8;

import static java.lang.System.out;
import java.lang.reflect.Field;

/**
 * Demonstrate JDK 8's easy programmatic access to size of basic Java datatypes.
 * 
 * @author Dustin
 */
public class DataTypeSizes
{
   /**
    * Print values of certain fields (assumed to be constant) for provided class.
    * The fields that are printed are SIZE, BYTES, MIN_VALUE, and MAX_VALUE.
    * 
    * @param clazz Class which may have static fields SIZE, BYTES, MIN_VALUE,
    *    and/or MAX_VALUE whose values will be written to standard output.
    */
   private static void printDataTypeDetails(final Class clazz)
   {
      out.println("\nDatatype (Class): " + clazz.getCanonicalName() + ":");
      final Field[] fields = clazz.getDeclaredFields();
      for (final Field field : fields)
      {
         final String fieldName = field.getName();
         try  
         {
            switch (fieldName)
            {
               case "SIZE" :  // generally introduced with 1.5 (twos complement)
                  out.println("\tSize (in bits):  " + field.get(null));
                  break;
               case "BYTES" : // generally introduced with 1.8 (twos complement)
                  out.println("\tSize (in bytes): " + field.get(null));
                  break;
               case "MIN_VALUE" :
                  out.println("\tMinimum Value:   " + field.get(null));
                  break;
               case "MAX_VALUE" :
                  out.println("\tMaximum Value:   " + field.get(null));
                  break;
               default :
                  break;
            }
         }
         catch (IllegalAccessException illegalAccess)
         {
            out.println("ERROR: Unable to reflect on field " + fieldName);
         }
      }
   }

   /**
    * Demonstrate JDK 8's ability to easily programmatically access the size of
    * basic Java data types.
    * 
    * @param arguments Command-line arguments: none expected.
    */
   public static void main(final String[] arguments)
   {
      printDataTypeDetails(Byte.class);
      printDataTypeDetails(Short.class);
      printDataTypeDetails(Integer.class);
      printDataTypeDetails(Long.class);
      printDataTypeDetails(Float.class);
      printDataTypeDetails(Double.class);
      printDataTypeDetails(Character.class);
      printDataTypeDetails(Boolean.class);
   }
}

When executed, the code above writes the following results to standard output.

The Output

Datatype (Class): java.lang.Byte:
 Minimum Value:   -128
 Maximum Value:   127
 Size (in bits):  8
 Size (in bytes): 1

Datatype (Class): java.lang.Short:
 Minimum Value:   -32768
 Maximum Value:   32767
 Size (in bits):  16
 Size (in bytes): 2

Datatype (Class): java.lang.Integer:
 Minimum Value:   -2147483648
 Maximum Value:   2147483647
 Size (in bits):  32
 Size (in bytes): 4

Datatype (Class): java.lang.Long:
 Minimum Value:   -9223372036854775808
 Maximum Value:   9223372036854775807
 Size (in bits):  64
 Size (in bytes): 8

Datatype (Class): java.lang.Float:
 Maximum Value:   3.4028235E38
 Minimum Value:   1.4E-45
 Size (in bits):  32
 Size (in bytes): 4

Datatype (Class): java.lang.Double:
 Maximum Value:   1.7976931348623157E308
 Minimum Value:   4.9E-324
 Size (in bits):  64
 Size (in bytes): 8

Datatype (Class): java.lang.Character:
 Minimum Value:   

UPDATE: Note that, as Attila-Mihaly Balazs has pointed out in the comment below, the MIN_VALUE values showed for java.lang.Float and java.lang.Double above are not negative numbers even though these constant values are negative for Byte, Short, Int, and Long. For the floating-point types of Float and Double, the MIN_VALUE constant represents the minimum absolute value that can stored in those types.

Although the characteristics of the Java primitive data types are readily available online, it's nice to be able to programmatically access those details easily when so desired. I like to think about the types' sizes in terms of bytes and JDK 8 now provides the ability to see those sizes directly measured in bytes.

2 comments:

Attila-Mihaly Balazs said...

One thing to note here is that float/double has for MIN_VALUE the "closest positive value to zero" it can represent, not the smallest in absolute terms.

If you want to have a "smaller than every other value" you need to use -1*Float/Double.MAX_VALUE or better yet, -Infinity.

@DustinMarx said...

Attila-Mihaly,

Thanks for pointing out that subtlety of MIN_VALUE being the minimum absolute value in the case of Float and Double.

Dustin