Java - looping and through variables of instance of a class

Question

I have a class

Class Test{

private String something ;
private String somethingElse;
private String somethingMore;

}

I am creating an instance of this.

myInst = new Test();

and adding values to first and second variables.

Now I need to check if any of the variable is null.

I know I can do this like if(myInst.something == null)

but for each item I add to the class it's difficult to do.

Is there anyway that i can check the instance by looping through all elements and seeing anything is null.

just like -

 for(i=0; i< myInstanceVariables ; i++)
{

if(myInstanceVariable == null ){

//do something 
donotDisplay(myInstanceVariable)

}

TIA

By reflection::Get the variable names and loop through them and check the values in that loop?Whats the problem here? — NeverGiveUp161
– NeverGiveUp161, Commented Jun 6, 2014 at 10:31

lpratlong · Accepted Answer · 2014-06-06 10:36:36Z

2

You can use Reflection using Fields from your instance. In your class, add this code. It will take all the fields and get their value.

Field[] fields = getClass().getDeclaredFields(); // get all the fields from your class.
for (Field f : fields) {                         // iterate over each field...
    try {
        if (f.get(this) == null) {               // evaluate field value.
            // Field is null
        } 
    } catch (IllegalArgumentException ex) {
        ex.printStackTrace();
    } catch (IllegalAccessException ex) {
        ex.printStackTrace();
    }
}

Here is a sample code: https://ideone.com/58jSia

edited Jun 6, 2014 at 10:36

answered Jun 6, 2014 at 10:29

lpratlong

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1 Comment

Syam S Over a year ago

You have to use getDeclaredFields() as opposed to getFields() to get instance variables

Yohji · Accepted Answer · 2014-06-06 10:27:08Z

0

You have to use reflection over Field of the Class.

myInst = new Test();
for (Field field : myInst.getClass().getDeclaredFields())
  if (field.get(myInst) == null)
     // do something

answered Jun 6, 2014 at 10:27

Yohji

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Comments

Arash Saidi · Accepted Answer · 2014-06-06 10:38:14Z

0

You can use reflection, however, in your case you only have String values, and thus it would also make sense to use a HashMap (for instance):

HashMap hm = new HashMap();
hm.put("something", "itsValue");
hm.put("somethingElse", null);

Now you can put as many values as you would like, and iterate through them like this:

Set set = hm.entrySet();
Iterator i = set.iterator();

while(i.hasNext()){
  Map.Entry me = (Map.Entry)i.next();
  System.out.println(me.getKey() + " : " + me.getValue() );
}

answered Jun 6, 2014 at 10:38

Arash Saidi

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1 Comment

lpratlong Over a year ago

It makes sense, but String keys don't throw error at compilation, so personally I prefer not to use such a solution when it's possible to do it without String keys. (That's also why I prefer Annotation than XML configuration in frameworks).

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