Get values of a Generic List<?> object in Java

Question

I am trying to created a commonMethod which acccepts a Generic List allData type and prints out the values of the alldata object received. Additionally, I am sending the className as a parameter so that we can retrieve the Class and the Fields through Reflection.

I am almost there except the part where I want to declare the className in a forloop. I know the rest of the code works, because i tried hardcoding a ClassName and it works.

public void commonMethod(List<?> alldata, String className) {
      Class c = Class.forName(className);
      Field[] fields = c.getDeclaredFields();

     //I would like to define incoming className here..but failing   
      for (className c : alldata) {
          for (Field field : fields) {
                field.setAccessible(true);
                Object value = null;
                value = field.get(c);
                System.out.println(value);
                //use the value in some other code
           }  
      }

 }

How do I change this part for (className c : alldata) to get it working..

Thanks

You could type the method: public <T> void commonMethod(List<T> alldata, Class<T> c) { /* removed: Class c = Class.forName(className); */ then use T — user180100
– user180100, Commented Jun 25, 2017 at 7:37

Chai T. Rex · Accepted Answer · 2017-06-25 07:53:13Z

2

You can't do it the way that you want, because declaring variables to be a certain type like that (typeName variableName) is reserved for type names that you know at compile-time.

The proper way is to use:

for (Object c : alldata) {

field.get(c) will work just fine with this, since the get method takes an Object, so c will have a good enough type.

answered Jun 25, 2017 at 7:53

Chai T. Rex

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Comments

kerner1000 · Accepted Answer · 2017-06-25 07:36:50Z

1

Change the generic type of the parameter List to be concrete instead of ?, e.g. List<T> alldata.

answered Jun 25, 2017 at 7:36

kerner1000

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Comments

mt eee · Accepted Answer · 2017-06-25 07:42:32Z

1

How about this:

public <T> void commonMethod(List<T> alldata) {
    // do something
}

answered Jun 25, 2017 at 7:42

mt eee

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1 Comment

Andy Turner Over a year ago

Sure; but this gives no advantage over ? and Object.

Blip · Accepted Answer · 2017-06-25 07:43:30Z

1

I suggest you modify your code as below to accommodate for the generics

public <T>void commonMethod(List<T> alldata) {
      for (T c : alldata) {
          .... 
          .....
      }

 }

Now Whatever class you supply, The compiler would easily understand what class it require to cast variable c into.

Still better is what is suggested by RC in the comments :

public <T> void commonMethod(List<T> alldata, Class<T> c) {
      Field[] fields = c.getDeclaredFields();

      for (T c : alldata) {
          for (Field field : fields) {
                field.setAccessible(true);
                Object value = null;
                value = field.get(c);
                System.out.println(value);
                //use the value in some other code
           }  
      }

 }

edited Jun 25, 2017 at 7:43

answered Jun 25, 2017 at 7:38

Blip

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1 Comment

Andy Turner Over a year ago

The first part of the answer is unnecessary: just change the loop variable type to Object; you get nothing from the type variable other than a shorter type. On the other hand, the second part is good. The only thing I'd change would be to make it Class<? super T> (or List<? extends T>), to make it a tiny bit more flexible.

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