more trouble with Generics - inserting in to a base class

[Stuart Rogers]

Hello again,

following up on my previous post, I've created an interface to handle calls to a function getAsArray which exists in each of several object classes
ie. Asset_HP, Asset_GPS, etc.

Using generics to write out the various lists works great (thanks guys!)

Now I'd like to use generics to have one function to populate any list rather than one function for each list due to having to hardcode the call to the
object class' constructor.

package gps.lisa;

import java.io.*;
import java.util.*;

import gps.lisa.Asset_HP;
import gps.lisa.Asset_GPS;

interface Asset {
    public String[] getAsArray();
}

public class Lisa {
    LinkedList<Asset> asset_HP_list   = new LinkedList<Asset>();
    LinkedList<Asset> asset_GPS_list = new LinkedList<Asset>();

    public void WriteOutLinkedList( LinkedList<Asset> lisst, String outFileName ) {
        
        ListIterator<Asset> li = lisst.listIterator();
       
        File           outFile;
        FileWriter     outFW;
        BufferedWriter outFBW;
        PrintWriter    outPW;
        try { 
            outFile = new File( outFileName);
            outFW   = new FileWriter( outFile);
            outFBW  = new BufferedWriter( outFW);
            outPW   = new PrintWriter( outFBW);
            while( li.hasNext()) {
                Asset roww = li.next();
                outPW.println( Arrays.toString( roww.getAsArray() ) );
            }
            outPW.flush();
            outPW.close();
        } catch (IOException e) {
            System.out.println("Cannot write file" + outFileName);
            return;
        }
    }

// what I'd like
    public void populate_Asset_list( LinkedList<Asset> list_to_populate, String innFileName)
    {
        String[] innline_arry;
        String         innline, tmp_str, out_str;
 
        File           innFile;
        FileReader     innFR;
        BufferedReader innFBR;
        try {
            innFile = new File( innFileName);
            innFR = new FileReader( innFile);
            innFBR = new BufferedReader( innFR);
            try {
                while( (innline = innFBR.readLine() ) != null) {
                	innline_arry = innline.split("\t");
//                  System.out.println( Arrays.toString( innline_arry) );
                    list_to_populate.add( new Asset( innline_arry)); //   <-- compiler complains "cannot instantiate the type Asset
                }
                innFBR.close();
            } catch (IOException e) {
                System.out.println("Cannot read file" + innFileName);
                return;
            }
        } catch (FileNotFoundException e) {
            System.out.println("Cannot open file" + innFileName + " for reading");
            return;
        }
    
    }

// what works for one specific list
    public void populate_Asset_HP_list( LinkedList<Asset> list_to_populate, String innFileName)
    {
        String[] innline_arry;
        String         innline, tmp_str, out_str;

        File           innFile;
        FileReader     innFR;
        BufferedReader innFBR;
        try {
            innFile = new File( innFileName);
            innFR = new FileReader( innFile);
            innFBR = new BufferedReader( innFR);
            try {
                while( (innline = innFBR.readLine() ) != null) {
                	innline_arry = innline.split("\t");
//                  System.out.println( Arrays.toString( innline_arry) );
//                  how can I avoid having to explicitly naming the correct constructor to use?
                    list_to_populate.add( new Asset_HP( innline_arry));
                }
                innFBR.close();
            } catch (IOException e) {
                System.out.println("Cannot read file" + innFileName);
                return;
            }
        } catch (FileNotFoundException e) {
            System.out.println("Cannot open file" + innFileName + " for reading");
            return;
        }
    
    }

    public static void main( String[] args) {

        Lisa theApp;

        theApp = new Lisa();
        theApp.asset_HP_list  = new LinkedList<Asset>();
        theApp.asset_GPS_list = new LinkedList<Asset>();

        //this works
        theApp.populate_Asset_HP_list( theApp.asset_HP_list, "C:/lisa_II/HP_stuff.csv");
        //but this is what I'd like
        theApp.populate_Asset_list(    theApp.asset_HP_list, "C:/lisa_II/HP_stuff.csv");
        theApp.WriteOutLinkedList( theApp.asset_HP_list, "C:/lisa_II/Asset_HP_out.txt");

        System.out.println("done");

    }

}

Any ideas/suggestions/constructive criticisms are all appreciated.


TIA,

Still-learning Stuart

[Campbell Ritchie]

Are you really using different methods to handle assets from different manufacturers? That looks very non-object-oriented to me.

[Stuart Rogers]

Thank you for your reply.

Having learned that the type information of an object is thrown away when you place it into a Collection object

LinkedList mylist = new LinkedList();
Cat blackCat = new Cat("black");
mylist.add( blackCat);

favoritePet = mylist.get(0); // errors because favoritePet doesn't know it came from a Cat

But what about when I don't use a collection object "in the raw"?

LinkedList<Cat> mylist = new LinkedList<Cat>();
Cat blackCat = new Cat("black");
mylist.add( blackCat);

favoritePet = mylist.get(0); // I'd hoped to make this work somehow

Or is it possible to extract the type of the stored objects ( Cat ) from a Collection with a specified type?

Still-learning Stuart

[Campbell Ritchie]

Collections don't store the actual parameter; so the <Cat> bit is lost. This is because Java implements generics by erasure.

What I meant about object-oriented is that you should have a field in your resource which stores the manufacturer's name. You can put those names into an enum.

[Stuart Rogers]

Back again armed with more code.

Essentially, I have several kinds of similar assets and I'd like to place each kind of asset in its own list.
Creating a class for each specific type of asset and having them all share an interface to expose a common method seems logical.

Been busy reading and it sounds like Generics, with a little help from reflection, will do what I need.
So I created a generic AssetLinkedList class that contains a .populate() and writeOut() method.
The idea is to instantiate several AssetLinkedList objects and populate them from some csv files.
The key is for each AssetLinkedList object to intrinsically know which kinds of assets it will be filled with.

The following code compiles without errors but fails at runtime on line 164 (I've flagged in in the code with a comment ) with the message:

Exception in thread "main" java.lang.IllegalArgumentException: wrong number of arguments
at sun.reflect.NativeConstructorAccessorImpl.newInstance0(Native Method)
at sun.reflect.NativeConstructorAccessorImpl.newInstance(NativeConstructorAccessorImpl.java:39)
at sun.reflect.DelegatingConstructorAccessorImpl.newInstance(DelegatingConstructorAccessorImpl.java:27)
at java.lang.reflect.Constructor.newInstance(Constructor.java:513)
at gps.lisa.AssetLinkedList.populate(Lisa.java:164)
at gps.lisa.Lisa.main(Lisa.java:323)

So it seems to be pretty close. Any help/suggestions/hints/constructive criticism is always welcome. Like, is there a
better way for a instance AssetLinkedList to know what type of object it should contain without having to pass in an example
of the object?

Still-learning Stuart

package gps.lisa;

import java.lang.reflect.*;
import java.io.*;
import java.util.*;

import gps.lisa.Cdw_Utils;

class Asset_HP implements Asset {

    // fields
    private String  receiving_no;
    private String  qty;
    private String  part_no;

    //constructors
    public Asset_HP()
    {
        setAsset_HP_Defaults();
    }
    
    public Asset_HP( String aa, String bb, String cc)
    {
        setAsset_HP_Defaults();
        if( !aa.equals("\\N")) receiving_no = aa;
        if( !bb.equals("\\N")) qty = bb;
        if( !cc.equals("\\N")) part_no = cc;
    }
  
    public Asset_HP( String[] str_arry)
    {
        setAsset_HP_Defaults();
        receiving_no = str_arry[0];
        qty = str_arry[1];
        part_no = str_arry[2];
    }

    private void setAsset_HP_Defaults()
    {
        receiving_no = null;
        qty = null;
        part_no = null;
    }
    
    //----------------------------------------------------------------------------
    // Asset_HP   G E T T E R S   A N D   S E T T E R S
    //----------------------------------------------------------------------------
    
    public String[] getAsArray()    // IMPORTANT - each class that implements Asset will contain this method
    {
        String[] str_arry = new String[7];
        str_arry[0] = receiving_no;
        str_arry[1] = qty;
        str_arry[2] = part_no;
        return str_arry;
    }
    
    public String getAsset_HP_Receiving_No()
    {
        return receiving_no;
    }
    
    public void setAsset_HP_Receving_No( String aa)
    {
        this.receiving_no = aa;
    }
    
    public String getAsset_HP_Qty()
    {
        return qty;
    }
    
    public void setAsset_HP_Qty( String aa)
    {
        this.qty = aa;
    }
    
    public String getAsset_HP_Part_No()
    {
        return part_no;
    }
    
    public void setAsset_HP_Part_No( String aa)
    {
        this.part_no = aa;
    }
 
}

interface Asset {
	
	public String[] getAsArray();
}

class AssetLinkedList<T extends Asset> extends LinkedList<T> {

    T obj;
    Class<?> objType;
    LinkedList<T> asset_list;
    
    //constructors
    AssetLinkedList() {
        obj = null;
        objType = null;
        asset_list = new LinkedList<T>();
    }
    
    AssetLinkedList( T aa) {     // list constructor called from within main()
        obj = aa;
        objType = aa.getClass();
        asset_list = new LinkedList<T>();
        System.out.println( aa.getClass().getName() ); // prints out lisa.gps.Asset_HP  great!
    }

    public void list_out_constructors() {
        Constructor<?>[] aawwr = this.objType.getConstructors();
        for( int i = 0; i < aawwr.length; i++) {
            System.out.println(" " + aawwr[i]);
        }
    }
    
    public void populate( String innFileName)
    {
        String[]       innline_arry;
        String         innline, tmp_str, out_str;
        int            col_cnt_desired = 6;
        int            row_cnt = 0;
       
        Constructor<?> objTypeConstructorIwant = null;

        File           innFile;
        FileReader     innFileReader;
        BufferedReader innFileBufferedReader;

        try {
            innFile = new File( innFileName);
            innFileReader = new FileReader( innFile);
            innFileBufferedReader = new BufferedReader( innFileReader);
            try {
                objTypeConstructorIwant = this.objType.getConstructor( new Class<?>[] { String[].class });
                try {
                    while( (innline = innFileBufferedReader.readLine() ) != null) {
                        row_cnt++;
    //                  System.out.println( "-----" + row_cnt);
    //                  System.out.println( innline);
                        tmp_str = Cdw_Utils.AddTabsAsNecessary( innline, col_cnt_desired);
    //                  System.out.println( tmp_str);
                        innline_arry = tmp_str.split("\t");
    //                  System.out.println( Arrays.toString( innline_arry) );
                        for( int ii = 0 ; ii < innline_arry.length ; ii++ ) {
                            if( innline_arry[ii] == null) {
                                innline_arry[ii] = "Null";
                            } else {
                                if( innline_arry[ii].equals("") ) {
                                    innline_arry[ii] = "BLanK";
                                } else {
                                    tmp_str = Cdw_Utils.TrimDoubleQuotes( innline_arry[ii]);
                                    innline_arry[ii] = tmp_str;
                                }
                            }
                        } //end-for
    //                  System.out.println( Arrays.toString( innline_arry) );
                        try {
                        	Object newobj = objTypeConstructorIwant.newInstance( (Object[]) innline_arry);
// FAILS EXECUTING PREVIOUS LINE
                        	this.add( (T) newobj);
                        } catch (InstantiationException e) {
                             e.printStackTrace();
                        } catch (IllegalAccessException e) {
                            e.printStackTrace();
                        } catch (InvocationTargetException e) {
                            e.printStackTrace();
                        }
                    } //end-while
                    innFileReader.close();
                } catch (IOException e) {
                    System.out.println("Cannot read file" + innFileName);
                    return;
                }
            } catch ( NoSuchMethodException e) {
                e.printStackTrace();
            }
        } catch (FileNotFoundException e) {
            System.out.println("Cannot open file" + innFileName + " for reading");
            return;
        }
    
    }

    public void writeOut( String outFileName) {
        
        ListIterator<T> li = this.listIterator();
       
        T              roww;      
        File           outFile;
        FileWriter     outFW;
        BufferedWriter outBW;
        PrintWriter    outPW;
        System.out.println("length of list=" + this.size() );
        try { 
            outFile = new File( outFileName);
            outFW = new FileWriter( outFile);
            outBW = new BufferedWriter( outFW);
            outPW = new PrintWriter( outBW);
            while( li.hasNext()) {
                roww = li.next();
                outPW.println( Arrays.toString( roww.getAsArray() ) );
            }
            outPW.flush();
            outPW.close();
        } catch (IOException e) {
            System.out.println("Cannot write file" + outFileName);
            return;
        }
    }

}

class Asset_HP_LinkedList extends LinkedList<Asset_HP> {

    LinkedList<Asset_HP> asset_list;
    
    //constructor
    Asset_HP_LinkedList() {
        asset_list = new LinkedList<Asset_HP>();
    }

    public void populate( String innFileName)
    {
        String[]       innline_arry;
        String         innline, tmp_str, out_str;
        int            col_cnt_desired = 6;
        int            row_cnt = 0;

        File           innFile;
        FileReader     innFileReader;
        BufferedReader innFileBufferedReader;
        try {
            innFile = new File( innFileName);
            innFileReader = new FileReader( innFile);
            innFileBufferedReader = new BufferedReader( innFileReader);
            try {
                while( (innline = innFileBufferedReader.readLine() ) != null) {
                    row_cnt++;
//                  System.out.println( "-----" + row_cnt);
//                  System.out.println( innline);
                    tmp_str = Cdw_Utils.AddTabsAsNecessary( innline, col_cnt_desired);
//                  System.out.println( tmp_str);
                    innline_arry = tmp_str.split("\t");
//                  System.out.println( Arrays.toString( innline_arry) );
                    for( int ii = 0 ; ii < innline_arry.length ; ii++ ) {
                        if( innline_arry[ii] == null) {
                            innline_arry[ii] = "Null";
                        } else {
                            if( innline_arry[ii].equals("") ) {
                                innline_arry[ii] = "BLanK";
                            } else {
                                tmp_str = Cdw_Utils.TrimDoubleQuotes( innline_arry[ii]);
                                innline_arry[ii] = tmp_str;
                            }
                        }
                    }
//                  System.out.println( Arrays.toString( innline_arry) );
                    this.add( new Asset_HP( innline_arry));
                }
                innFileReader.close();
            } catch (IOException e) {
                System.out.println("Cannot read file" + innFileName);
                return;
            }
        } catch (FileNotFoundException e) {
            System.out.println("Cannot open file" + innFileName + " for reading");
            return;
        }
    
    }

    public void writeOut( String outFileName) {
        
        ListIterator<Asset_HP> li = this.listIterator();
       
        Asset_HP       roww;
        File           outFile;
        FileWriter     outFW;
        BufferedWriter outBW;
        PrintWriter    outPW;
        System.out.println("length of list=" + this.size() );
        try { 
            outFile = new File( outFileName);
            outFW = new FileWriter( outFile);
            outBW = new BufferedWriter( outFW);
            outPW = new PrintWriter( outBW);
            while( li.hasNext()) {
                roww = li.next();
                outPW.println( Arrays.toString( roww.getAsArray() ) );
            }
            outPW.flush();
            outPW.close();
        } catch (IOException e) {
            System.out.println("Cannot write file" + outFileName);
            return;
        }
    }

}

public class Lisa {
    Asset_HP anobj_HP;
    Asset_HP_LinkedList  asset_HP_list;
    AssetLinkedList<Asset_HP>  asset_PH_list;

    public static void main( String[] args) {

        Lisa theApp;

        //hard-coded way works but is not generic
        theApp = new Lisa();
        theApp.asset_HP_list = new Asset_HP_LinkedList();
        theApp.asset_HP_list.populate( "/home/chris/lisa/HP_EqptInventory_V.csv");
        theApp.asset_HP_list.removeFirst();  // removes header row
        theApp.asset_HP_list.writeOut( "/home/chris/lisa_test/Asset_HP_out.txt");
        
        // generic way should work but does not
        theApp.anobj_HP = new Asset_HP();
        theApp.asset_PH_list = new AssetLinkedList<Asset_HP>( theApp.anobj_HP);
        theApp.asset_PH_list.populate( "/home/chris/lisa/HP_EqptInventory_V.csv");
        theApp.asset_PH_list.removeFirst();  // removes header row
        theApp.asset_PH_list.writeOut( "/home/chris/lisa_test/Asset_PH_out.txt");
        
        System.out.println("done");

    }

}