Java how can I with reflection check if a field is initialized or is default value?

Question

So, should be straight forward question.

Lets say I have a class with a lot of fields like:

String thizz;
long that;
boolean bar;

How can I, with reflection, see if the fields thizz, that and bar have been initialized or left to their default values of null, 0 and false?

Because I dont know how many and what fields the other object has, it changes a lot... — rapadura, Aug 28 '11 at 19:57
Please note that the default value of `Boolean` is `null`. Perhaps you had `boolean` in mind? — BalusC, Aug 28 '11 at 20:01
Wait, so are you asking how to dynamically look up an object's fields and values? That's kind of a different question. — Paul Bellora, Aug 28 '11 at 20:03
This is the default values for java types: http://download.oracle.com/javase/tutorial/java/nutsandbolts/datatypes.html — Lynch, Aug 28 '11 at 20:10

Peter Lawrey · Accepted Answer · 2018-09-13T11:20:01.320

22

You have only 7 primitive types and one reference type to check. If you group all Number types together, you only have four values to check for.

Object o =
for (Field field : o.getClass().getDeclaredFields()) {
 Class t = field.getType();
 Object v = field.get(o);
 if(t == boolean.class && Boolean.FALSE.equals(v)) 
   // found default value
 else if(t == char.class && ((Character) v).charValue() == 0)
   // found default value
 else if(t.isPrimitive() && ((Number) v).doubleValue() == 0)
   // found default value
 else if(v == null)
   // found default value
}

edited Sep 13 '18 at 11:20

answered Aug 28 '11 at 22:15

Peter Lawrey

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@Rahul that's what the checks in the middle of the code are for, yes. – Peter Lawrey Sep 21 '16 at 13:00
What about `char`? (`Character` is not a `Number`) – DodgyCodeException Sep 12 '18 at 16:26
@DodgyCodeException a `char` is an unsigned 16-bit value. e.g. you can write `char ch = 48;` or `Character ch = 48;` or `char ch = '0'; ch /= 0.9;` It is `0` or `'\0'` before initialisation. – Peter Lawrey Sep 13 '18 at 06:16
1

@PeterLawrey yes but even so, you'll still get a ClassCastException in `else if(t.isPrimitive() && ((Number) v).doubleValue() == 0)` because v is of class Character, which is not castable to Number. – DodgyCodeException Sep 13 '18 at 10:16
@DodgyCodeException Thank you for the correction. I have added this case. – Peter Lawrey Sep 13 '18 at 11:20

score 8 · Answer 2 · edited May 23 '17 at 12:02

Peter Lawrey's answer works fine for me, except for fields of primitive data type char, which raises the following exception in my code :

java.lang.ClassCastException: java.lang.Character cannot be cast to java.lang.Number

So I added the char case :

Object o =
for (Field field : o.getClass().getDeclaredFields()) {
 Class t = field.getType();
 Object v = field.get(o);
 if (boolean.class.equals(t) && Boolean.FALSE.equals(v)) 
   // found default value
 else if (char.class.equals(t) && ((Character) v) != Character.MIN_VALUE)
   // found default value
 else if (t.isPrimitive() && ((Number) v).doubleValue() == 0)
   // found default value
 else if(!t.isPrimitive() && v == null)
   // found default value
}

Character.MIN_VALUE is '\u0000', which is the default value of char according to the Java Documentation on primitive data types.

JB Cha · Answer 3 · 2017-03-18T14:43:42.370

6

You can create a new instance via reflection (automatically initialized with default value) and compare yourObject with this default instance.

//default Object filled with default values:
Object defaultObject = yourObject.getClass().newInstance();
for (final Field field : yourObject.getClass().getDeclaredFields()) {
  final Object value = field.get(yourObject);
  if (value == null || value.equals(field.get(defaultObject))) {
    // found default value
  }
}

PS: This solution also solved the issue with char mentioned by Christophe Weis.

edited Mar 18 '17 at 14:43

answered Nov 21 '16 at 16:01

JB Cha

597
5
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1

This solution should have more upvotes. If you create the default object without a static type you do not even have to depend on `Foo`. Do it this way: `Object defaultObject = yourObject.getClass().newInstance();` (this can be even more refined if you get all the constructors and create one that has the least parameters). – Hash Mar 17 '17 at 09:22
This will throw an java.lang.NoSuchMethodException for class that does not have a default constructor, like Integer for example. – abarazal Sep 21 '20 at 18:33

score 6 · Answer 4 · edited May 09 '14 at 15:15

6

You don't need reflection...

if (thizz == null) {
    //it's not initialized
}
if (that == 0) {
   //it's not initialized
}
if(bar == false) {
    //it's not initialized
}

However, they could have been initialized then reset to their default values. If you truly want to know if they're been initialized you could do something like this:

private boolean isFooInitialized = false;
private Foo foo;
public void setFoo(Foo foo) {
    this.foo = foo;
    isFooInitialized = foo != null;
}

/edit To get all the fields from a class, check out Class.getDeclaredFields(). This will give every field, not just the public ones.

From here you can check the type of the field and get its value:

Foo foo = ...
Field[] fooFields = foo.getClass().getDeclaredFields();
for (Field fooField : fooFields) {
    Class<?> fooFieldClass = fooField.getClass();
    if (fooFieldClass.equals(int.class)) {
        if (fooField.getInt(foo) == 0) {
            // not initialized
        }
    } else if (fooFieldClass.equals(double.class)) {
        if (fooField.getDouble(foo) == 0) {
            // not initialized
        }
    } else if (fooFieldClass.equals(boolean.class)) {
        if (fooField.getBoolean(foo) == false) {
            // not initialized
        }
    } else if (fooFieldClass.equals(float.class)) {
        if (fooField.getFloat(foo) == 0) {
            // not initialized
        }
    } else if (fooFieldClass.equals(char.class)) {
        if (fooField.getChar(foo) == 0) {
            // not initialized
        }
    } else if (fooFieldClass.equals(byte.class)) {
        if (fooField.getByte(foo) == 0) {
            // not initialized
        }
    } else if (fooFieldClass.equals(long.class)) {
        if (fooField.getLong(foo) == 0) {
            // not initialized
        }
    } else if (fooField.get(foo) == null) {
        // not initialized
    }
}

edited May 09 '14 at 15:15

thetaiko

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answered Aug 28 '11 at 19:55

Jeffrey

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Ah, let me restate my question, I actually dont care if the value has been initialized and later set to null or 0, I just need to check the default for all Javas possible types, primitives as well as java.lang objects – rapadura Aug 28 '11 at 19:59
@AntonioP In that case, it's easy -- there are only ~7 (I could be wrong by one or two) different "default values". One for each primitive and one for non-primitives. A simple overloaded method could easily perform the check: `bool IsDefault(bool b)`, `bool IsDefault(long i)`, etc (some types can be doubled up, like `int/long` and `float/double`), modify for reflection as/if required. – Aug 28 '11 at 20:05
@Jeffrey, Yeah that was kind of what I did, except I used to check instance of Number, Boolean and String, the rest I assume are false anyway... – rapadura Aug 28 '11 at 20:12
@AntonioP If you already did this, where's the problem with it? – Jeffrey Aug 28 '11 at 20:13
I did it like this after I asked my question since I figured I could try it and it worked, but still there might be a smarter way of doing it :) I thought there would be a way to ask an object are you null false or 0 without doing instanceof on its value for different types – rapadura Aug 28 '11 at 20:32
@AntionoP Ahh, I don't think there's a better way of doing this. Cheers. – Jeffrey Aug 28 '11 at 20:36
@Jeffrey, Most of the values can be compared with 0, you don't need so many checks. See my answer. – Peter Lawrey Aug 28 '11 at 22:16
@Peter Lawrey I guess I was just being thorough. That would be a much better way to go about this. – Jeffrey Aug 28 '11 at 23:32

score 1 · Answer 5 · answered Aug 28 '11 at 20:08

This should get you going in the right direction.

    Class clazz = Class.forName("your.class");
    Field[] fields = clazz.getDeclaredFields();
    for (Field field : fields) {
      String dataType = field.getType().getName();
      if (dataType.equals("java.lang.String")) {
          System.out.println("found a string");
      }
    }

david van brink · Answer 6 · 2011-08-28T21:04:37.667

1

The gist of it is:

Field[] fields = yourObject.getClass().getFields();
for(Field f : fields)
{
  Class<?> k = f.getType();
  // depending on k, methods like f.getInt(yourObject),
  // f.getFloat(yourObject), 
  // f.getObject(hourObject) to get each member.
}

Now, this only lets you read the public fields.

Alternatively, IF your object follows getX/setX naming conventions, you can use getMethods(), and look for methods named "getXxx" and "setXxx", to infer the existence of settable fields -- and invoke those getters to look for the expected default values.

edited Aug 28 '11 at 21:04

answered Aug 28 '11 at 20:08

david van brink

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2

Note that `getFields()` only returns public fields. – Paul Bellora Aug 28 '11 at 20:11
`getDeclaredFields()` gives you all fields. – Matthieu Oct 17 '19 at 03:53

Java how can I with reflection check if a field is initialized or is default value?

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