Default value from a class

Question

How do I write a function which returns a default value for the given class? If a class is one of the primitives it should return a wrapper class with default value, else return null.

public <B> Object defaultValue(Class<B> clazz) {
    return clazz.isPrimitive() ? ... : null;
}

So for int.class it should return Integer(0).

You can't do it in a single line (if that's what you wanted to do). — Luiggi Mendoza, Apr 29 '13 at 15:55
Check this http://stackoverflow.com/questions/1704634/simple-way-to-get-wrapper-class-type-in-java — Pragnani, Apr 29 '13 at 15:56

Pshemo · Accepted Answer · 2013-04-29T16:51:24.540

How do I write a function which returns a default value for the given class? If a class is one of the primitives it should return a wrapper class with default value, else return null.

So I assume that for primitives like int.class you want to return new Integer(0), but for lets say Objects you want to return null.

If that is true you can use fact that arrays are filled with default values at start. Try maybe this way

import java.lang.reflect.Array;
...

public static <B> Object defaultValue(Class<B> clazz) {
    return Array.get(Array.newInstance(clazz, 1),0);
}

Array.newInstance(clazz, 1) will create array of type clazz with one default element,
Array.get(<someArray>, 0) will return its first element.
also since your method defaultValue returns Object it will be autoboxed to corresponding type int->Integer.

Example

System.out.println(defaultValue(int.class));
System.out.println(defaultValue(int.class).getClass());

prints

0
class java.lang.Integer

score 1 · Answer 2 · answered Apr 29 '13 at 16:07

One way of achieve it is using a map to store the values:

private static Map<Class, Object> defaultValues = new HashMap<Class, Object>();

static {
    defaultValues.put(byte.class, 0);
    defaultValues.put(short.class, 0);
    defaultValues.put(int.class, 0);
    defaultValues.put(long.class, 0L);
    defaultValues.put(float.class, 0.0f);
    defaultValues.put(double.class, 0.0d);
    defaultValues.put(char.class, '\u0000');
    defaultValues.put(boolean.class, false);
}

public <B> Object defaultValue(Class<B> clazz) {
    return defaultValues.containsKey(clazz) ? defaultValues.get(clazz) : null;
}

phoenix7360 · Answer 3 · 2013-04-29T16:30:22.543

You can use the ClassUtils class of the Apache lang3 library:

import org.apache.commons.lang3.ClassUtils;

public Object defaultValue(Class<?> clazz) throws InstantiationException, IllegalAccessException,
        IllegalArgumentException, InvocationTargetException {
    final Class<?> c = ClassUtils.primitiveToWrapper(clazz);
    if (c != null && !c.equals(clazz)) {
        final Constructor[] ctors = c.getConstructors();
        Constructor ctor = null;
        for (int i = 0; i < ctors.length; i++) {
            ctor = ctors[i];
            if (ctor.getGenericParameterTypes().length == 0)
            break;
        }
        return ctor.newInstance(0);
    } else {
        return null;
    }
}

It returns a null if the given class is not a primitive. You might want to handle the exception in a better way.

score 0 · Answer 4 · answered Apr 29 '13 at 16:00

Why do you need this? Defaulting is done by the compiler, not by calling existing methods.

Here are the default values for primitives, you could certainly write a function that turns this table into values...

Data Type   Default Value (for fields)
byte        0
short       0
int         0
long        0L
float       0.0f
double      0.0d
char        '\u0000'
Object      null
boolean     false

Read this tutorial link

Default value from a class

4 Answers4