Check type of primitive field

问题:

I'm trying to determine the type of a field on an object. I don't know the type of the object when it is passed to me but I need to find fields which are longs. It is easy enough to distinguish the boxed Longs but the primitive long seems more difficult.

I can make sure that the objects passed to me only have Longs, not the primitives, but I'd rather not. So what I have is:

for (Field f : o.getClass().getDeclaredFields()) {
    Class<?> clazz = f.getType();
    if (clazz.equals(Long.class)) {
        // found one -- I don't get here for primitive longs
    }
}

A hacky way, which seems to work, is this:

for (Field f : o.getClass().getDeclaredFields()) {
    Class<?> clazz = f.getType();
    if (clazz.equals(Long.class) ||  clazz.getName().equals("long")) {
        // found one
    }
}

I'd really like a cleaner way to do this if there is one. If there is no better way then I think that requiring the objects I receive to only use Long (not long) would be a better API.

Any ideas?

回答1:

You're using the wrong constant to check for Long primitives - use Long.TYPE, each other primitive type can be found with a similarly named constant on the wrapper. eg: Byte.TYPE, Character.TYPE, etc.

回答2:

o.getClass().getField("fieldName").getType().isPrimitive();

回答3:

You can just use

boolean.class
byte.class
char.class
short.class
int.class
long.class
float.class
double.class
void.class

If you are using reflection, why do you care, why do this check at all. The get/set methods always use objects so you don't need to know if the field is a primitive type (unless you try to set a primitive type to the null value.)

In fact, for the method get() you don't need to know which type it is. You can do

// any number type is fine.
Number n = field.get(object);
long l = n.longValue();

If you are not sure if it is a Number type you can do

Object o = field.get(object); // will always be an Object or null.
if (o instanceof Number) {
     Number n = (Number) o;
     long l = n.longValue();

回答4:

• To detect fields with long type use long.class or Long.TYPE.

• To detect fields with Long type use Long.class.

Example:

for (Field f : o.getClass().getDeclaredFields()) {
    Class<?> clazz = f.getType();
    // to detect both Long and long types
    if (Long.class.equals(clazz) || long.class.equals(clazz)) {
        // found one
    }
}

Notice:

Long.TYPE is static Constant member and is equivalent to long.class.

snippet code form Long Class

/**
 * The {@link Class} object that represents the primitive type {@code long}.
 */
@SuppressWarnings("unchecked")
public static final Class<Long> TYPE
        = (Class<Long>) long[].class.getComponentType();

Also check for answer for Difference between Integer.class and Integer.TYPE question