Obtener el valor predeterminado para los tipos primitivos de java

Es posible obtener el valor predeterminado de cualquier tipo creando una matriz de un elemento y recuperando su primer valor.

private static <T> T getDefaultValue(Class<T> clazz) {
    return (T) Array.get(Array.newInstance(clazz, 1), 0);
}

De esta manera no hay necesidad de tener en cuenta todos los tipos primitivos posibles, al costo generalmente insignificante de crear una matriz de un elemento.

Esto es lo que estoy pensando (sin embargo, falla la prueba de elegancia):

public class PrimitiveDefaults {
    // These gets initialized to their default values
    private static boolean DEFAULT_BOOLEAN;
    private static byte DEFAULT_BYTE;
    private static short DEFAULT_SHORT;
    private static int DEFAULT_INT;
    private static long DEFAULT_LONG;
    private static float DEFAULT_FLOAT;
    private static double DEFAULT_DOUBLE;

    public static Object getDefaultValue(Class clazz) {
        if (clazz.equals(boolean.class)) {
            return DEFAULT_BOOLEAN;
        } else if (clazz.equals(byte.class)) {
            return DEFAULT_BYTE;
        } else if (clazz.equals(short.class)) {
            return DEFAULT_SHORT;
        } else if (clazz.equals(int.class)) {
            return DEFAULT_INT;
        } else if (clazz.equals(long.class)) {
            return DEFAULT_LONG;
        } else if (clazz.equals(float.class)) {
            return DEFAULT_FLOAT;
        } else if (clazz.equals(double.class)) {
            return DEFAULT_DOUBLE;
        } else {
            throw new IllegalArgumentException(
                "Class type " + clazz + " not supported");
        }
    }
}

Una alternativa a la de Guayaba Defaults.java, que permite a la implementación calcular los valores predeterminados (mejorado usando la respuesta de Antag99):

import static java.util.stream.Collectors.toMap;

import java.lang.reflect.Array;
import java.util.Map;
import java.util.stream.Stream;

public class DefaultValue {
    /**
     * @param clazz
     *            the class for which a default value is needed
     * @return A reasonable default value for the given class (the boxed default
     *         value for primitives, <code>null</code> otherwise).
     */
    @SuppressWarnings("unchecked")
    public static <T> T forClass(Class<T> clazz) {
        return (T) DEFAULT_VALUES.get(clazz);
    }

    private static final Map<Class<?>, Object> DEFAULT_VALUES = Stream
            .of(boolean.class, byte.class, char.class, double.class, float.class, int.class, long.class, short.class)
            .collect(toMap(clazz -> (Class<?>) clazz, clazz -> Array.get(Array.newInstance(clazz, 1), 0)));

    public static void main(String... args) {
        System.out.println(DefaultValue.forClass(int.class)); // 0
        System.out.println(DefaultValue.forClass(Integer.class)); // null
    }
}

Puede hacer esto con reflexión, pero es más fácil y claro escribirlo, por ejemplo,

Object defaultValue(Class cls)
{
  Map defaults = new HashMap();
  defaults.put(Integer.TYPE, Integer.valueOf(0));  
  defaults.put(Double.TYPE, Double.valueOf(0));  
  defaults.put(Boolean.TYPE, Boolean.FALSE);  
  //... etc
  return defaults.get(cls);
}

Por supuesto, es probable que desee mover la inicialización del mapa a un constructor o similar para inicialización de una sola vez.

Razonablemente conciso - es elegante?

No hay una manera elegante de hacer esto. De hecho, ni siquiera es posible declarar la firma de un método que devolverá los valores primitivos per se.

Lo más cerca que puedes llegar es algo como esto:

public Object defaultValue(Class cls) {
    if (class == Boolean.TYPE) {
        return Boolean.FALSE;
    } else if (class == Byte.TYPE) {
        return Byte.valueOf(0);
    } else if (class == Short.TYPE) {
        ...
    } else {
        return null;
    }
}

Las variables de clase de las primitivas no necesitan inicializarse ni establecerse con un valor predeterminado. Sin embargo, las variables que se declaran en otro ámbito deben inicializarse o obtendrás errores de compilación.

public class PrimitiveStuff {
private int aInt;
private long aLong;
private boolean aBoolean;

public PrimitiveStuff() {
    System.out.println("aInt : "  + aInt); //prints 0
    System.out.println("aLong: "+ aLong);//prints 0
    System.out.println("aBoolean: " + aBoolean);//prints false
}


public void doStuff(){
    int outherInt;
    System.out.println(outherInt); //will not compile
}

public static void main(String[] args) {
    new PrimitiveStuff();
}

}

Basado en Respuesta de Jack Leow, Creé esta clase:

/**
   <P>{@code java InitializedObjectUtil}</P>
 **/
public class InitializedObjectUtil  {
      public static final void main(String[] igno_red)  {
         printDefault("boolean");
         printDefault("char");
         printDefault("byte");
         printDefault("short");
         printDefault("int");
         printDefault("long");
         printDefault("float");
         printDefault("double");
         printDefault("java.lang.AnythingAndEverythingElse");
      }
         private static final void printDefault(String s_type)  {
            Object oDflt = InitializedObjectUtil.getForClassName(s_type);
            System.out.println(s_type + " default is \"" + oDflt + "\"");
         }
      /**
         <P>The default value for a boolean is {@code false}.</P>

         <P>Viewed 1/21/2014
         <BR><CODE><A HREF="http://docs.oracle.com/javase/tutorial/java/nutsandbolts/datatypes.html">http://docs.oracle.com/javase/tutorial/java/nutsandbolts/datatypes.html</A></CODE>:</P>

         <P><B>Default Values:</B> </P>

         <P>It's not always necessary to assign a value when a field is declared. Fields that are declared but not initialized will be set to a reasonable default by the compiler. Generally speaking, this default will be zero or null, depending on the data type. Relying on such default values, however, is generally considered bad programming style. The following chart summarizes the default values for the above data types.</P>

   <PRE>{@literal
   Data Type   Default Value (for fields)
   --------------------------------------
   byte                       0
   short                      0
   int                        0
   long                       0L
   float                      0.0f
   double                     0.0d
   char                       '\u0000'
   String (or any object)     null
   boolean                    false}</PRE>

      @see  #getForClass(String) getForClass(s)
      @see  #getForClassName(String) getForClassName(s)
      @see  #DEFAULT_CHAR
      @see  #DEFAULT_BYTE
      @see  #DEFAULT_SHORT
      @see  #DEFAULT_INT
      @see  #DEFAULT_LONG
      @see  #DEFAULT_FLOAT
      @see  #DEFAULT_DOUBLE
    **/
   public static final Boolean DEFAULT_BOOLEAN = false;
   /**
      <P>The default value for a char {@code '\u0000'}.</P>

         @see  #DEFAULT_BOOLEAN
    **/
   public static final Character DEFAULT_CHAR = '\u0000';
   /**
      <P>The default value for a byte is {@code 0}.</P>

         @see  #DEFAULT_BOOLEAN
    **/
   public static final Byte DEFAULT_BYTE = 0;
   /**
      <P>The default value for a short is {@code 0}.</P>

         @see  #DEFAULT_BOOLEAN
    **/
   public static final Short DEFAULT_SHORT = 0;
   /**
      <P>The default value for a int is {@code 0}.</P>

         @see  #DEFAULT_BOOLEAN
    **/
   public static final Integer DEFAULT_INT = 0;
   /**
      <P>The default value for a long is {@code 0L}.</P>

         @see  #DEFAULT_BOOLEAN
    **/
   public static final Long DEFAULT_LONG = 0L;
   /**
      <P>The default value for a float {@code 0.0f}.</P>

         @see  #DEFAULT_BOOLEAN
    **/
   public static final Float DEFAULT_FLOAT = 0.0f;
   /**
      <P>The default value for a double {@code 0.0d}.</P>

         @see  #DEFAULT_BOOLEAN
    **/
   public static final Double DEFAULT_DOUBLE = 0.0d;
    /**
        <P>Get an object containing an initialized value for the static class-type.</P>

        @param  cls_static  May not be {@code null}.
        @return  <CODE>{@link getForClassName(String) getForClassName}(cls_static.getName())</CODE>
     **/
    public static final Object getForClass(Class cls_static)  {
       try  {
          return  getForClassName(cls_static.getName());
       }  catch(RuntimeException rtx)  {
          throw  new NullPointerException("getForClass: cls_static");
       }
    }
   /**
      <P>Get an object containing an initialized value for the type whose name is in a string.</P>

         <P>Idea from (viewed 1/2/2014)
      <BR> &nbsp; &nbsp; {@code <A HREF="https://stackoverflow.com/questions/2891970/getting-default-value-for-java-primitive-types/2892067#2892067">https://stackoverflow.com/questions/2891970/getting-default-value-for-java-primitive-types/2892067#2892067</A>}</P>

      @param  s_type  May not be {@code null}.
      @return  If {@code s_type} is equal to<UL>
         <LI>{@code "boolean"}: {@link #DEFAULT_BOOLEAN}</LI>
         <LI>{@code "char"}: {@link #DEFAULT_CHAR}</LI>
         <LI>{@code "byte"}: {@link #DEFAULT_BYTE}</LI>
         <LI>{@code "short"}: {@link #DEFAULT_SHORT}</LI>
         <LI>{@code "int"}: {@link #DEFAULT_INT}</LI>
         <LI>{@code "long"}: {@link #DEFAULT_LONG}</LI>
         <LI>{@code "float"}: {@link #DEFAULT_FLOAT}</LI>
         <LI>{@code "double"}: {@link #DEFAULT_DOUBLE}</LI>
         <LI><I>anything else</I>: {@code null}</LI>
      </UL>
      @see  #getForClass(Class) getForClass(cls)
    **/
   public static final Object getForClassName(String s_type)  {
      try  {
         if(s_type.equals("boolean"))  {
            return  DEFAULT_BOOLEAN;
         }
      }  catch(NullPointerException npx)  {
         throw  new NullPointerException("getForClassName: s_type");
      }
      if(s_type.equals("char"))  {
         return  DEFAULT_CHAR;
      }
      if(s_type.equals("byte"))  {
         return  DEFAULT_BYTE;
      }
      if(s_type.equals("short"))  {
         return  DEFAULT_SHORT;
      }
      if(s_type.equals("int"))  {
         return  DEFAULT_INT;
      }
      if(s_type.equals("long"))  {
         return  DEFAULT_LONG;
      }
      if(s_type.equals("float"))  {
         return  DEFAULT_FLOAT;
      }
      if(s_type.equals("double"))  {
         return  DEFAULT_DOUBLE;
      }

      //Non-primitive type
      return  null;
   }
}