Access a class's fields with Java reflection

Question

I'm trying to access a document class that has id, name, text and list of words. I try to compare id which I have with the ids and when found get the list of words attached to this id to find exact word. I was trying with java reflection but I'm unable to get it working?
Any help is highly appreciated.

public class Doc {
    private static int documentID;
    private static Doc docInstance = null;
    private String documentText;
    private ArrayList<String> listOfTokens;
    static int docCount = 0;

    public Doc() {
        documentID = 0;
        listOfTokens = new ArrayList<String>();
        tokFreq = 0;
        docCount++;
    }

    public static Doc getDocInstance() { 
        if (docInstance == null) {
            docInstance = new Doc();
        }
        return docInstance;
    }

    public ArrayList<String> getListOfTokens() { 
        return listOfTokens;
    }
}

and I am trying this

public static void createDocumentVector(TreeMap<Integer,Integer> 
documentVector, TreeMap<String, ArrayList<Integer>>qm, int N) throws 
NoSuchFieldException, SecurityException, IllegalArgumentException, 
IllegalAccessException, InstantiationException, NoSuchMethodException, 
InvocationTargetException 
{
    int eachDoc = 0;

    Collection<String> allKeys = qm.keySet();
    ArrayList<Integer> l1 = new ArrayList<Integer>();
    boolean addedTerm = false;

    /**
    Obtain an Iterator for Collection
    */
    Iterator<String> itr = allKeys.iterator();
    String key;
    int termFrequency = 0;
    int documentFrequency = 0;

    /**
     Iterate through TreeMap values iterator
     */
    while(itr.hasNext()) 
    {
        key = (String)itr.next();
        Integer LL = 0;
        l1 = qm.get(key); // Returns value of that key
        for (int k = 0; k < l1.size(); k++) 
        {
            LL = l2.get(k);

            Doc obj = new Doc();
            Class<? extends Doc> docOb = obj.getClass();
            Field field1 = docOb.getDeclaredField("documentID");
            field1.setAccessible(true);
            Field field2 = docOb.getDeclaredField("listOfTokens");
            field1.setAccessible(true);

            if (field1.isAccessible()) {
                Method setID = docOb.getDeclaredMethod("setDocumentID", new Class[]{int.class});
                setID.setAccessible(true);
                setID.invoke(docOb, LL);
            }

            Method listTock = docOb.getMethod("getListOfTokens");
            ArrayList<String> per = (ArrayList<String>) listTock.invoke(docOb, null);
            for (String tock : per) {
                if(tock.equals(key)) {
                    termFrequency++;
                }
            }

            documentFrequency = l1.size();
            eachDoc.add(getTFIDF(termFrequency, documentFrequency, N));
            documentVector.put(eachDoc, LL);
            addedTerm = true;
        }


    }
}

And I get this error

Exception in thread "main" java.lang.IllegalArgumentException: object is not an instance of declaring class
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(Unknown Source)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source)
at java.lang.reflect.Method.invoke(Unknown Source)

Answer 1

Did you post the complete definition of Doc? It's missing the setDocumentID method.

Anyway, I was able to reproduce your error with the following code:

public class DocReflection {

    private static int documentId; // static field

    private void setDocumentId(int docId) { // but setter is not static.
        documentId = docId;
    }


    public static void main(String [] args) throws NoSuchFieldException, NoSuchMethodException, InvocationTargetException, IllegalAccessException {
        DocReflection docReflection = new DocReflection();
        Class<? extends DocReflection> cls = docReflection.getClass();
        Field docId = cls.getDeclaredField("documentId");
        docId.setAccessible(true);

        if (docId.isAccessible()) {
            Method setId = cls.getDeclaredMethod("setDocumentId", new Class[]{int.class});
            setId.setAccessible(true);
            setId.invoke(cls, 1); // <-- Invoking non-static method with class object.
        }
    }
}

Output:

Exception in thread "main" java.lang.IllegalArgumentException: object is not an instance of declaring class
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
    at java.lang.reflect.Method.invoke(Method.java:498)
    at com.company.DocReflection.main(DocReflection.java:28)
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
    at java.lang.reflect.Method.invoke(Method.java:498)
    at com.intellij.rt.execution.application.AppMain.main(AppMain.java:144)

Solution is to make setDocumentId static.

Answer 2

TreeMap<List<Integer>,Integer> causes an error, due to this explanation in the javadoc.

The map is sorted according to the {@linkplain Comparable natural ordering} of its keys

But List does not implement the Comparable interface. So you can't use List<Integer> as a key in a TreeMap.

My English is poor, hope you can understand!