java.lang.InstantiationException: can't instantiate class?

Question

I'm trying to make an Android application with automatic form generation http://labs.makemachine.net/2010/04/android-form-generator/

I've been reading a lot of topic about the "InstantiationException" but none solve my problem.

Here is my Activity

    package com.example.makemachine;

import java.io.IOException;
import java.io.InputStream;
import java.util.ArrayList;
import java.util.Collections;
import java.util.Comparator;
import java.util.HashMap;
import java.util.Map;

import org.json.JSONArray;
import org.json.JSONException;
import org.json.JSONObject;

import android.app.Activity;
import android.content.Context;
import android.util.Log;
import android.view.View;
import android.widget.LinearLayout;
import android.widget.ScrollView;
import android.widget.LinearLayout.LayoutParams;

/**
 * FormActivity allows you to create dynamic form layouts based upon a json schema file. 
 * This class should be sub-classed. 
 * 
 * @author Jeremy Brown
 */
public abstract class FormActivity extends Activity
{
    public static String SCHEMA_KEY_TYPE        = "type";
    public static String SCHEMA_KEY_BOOL        = "boolean";
    public static String SCHEMA_KEY_INT         = "integer";
    public static String SCHEMA_KEY_STRING      = "string";
    public static String SCHEMA_KEY_PRIORITY    = "priority";
    public static String SCHEMA_KEY_TOGGLES     = "toggles";
    public static String SCHEMA_KEY_DEFAULT     = "default";
    public static String SCHEMA_KEY_MODIFIERS   = "modifiers";
    public static String SCHEMA_KEY_OPTIONS     = "options";
    public static String SCHEMA_KEY_META        = "meta";
    public static String SCHEMA_KEY_HINT        = "hint";

    public static final LayoutParams defaultLayoutParams = new LinearLayout.LayoutParams( LayoutParams.FILL_PARENT, LayoutParams.WRAP_CONTENT);

    // -- data
    protected Map<String, FormWidget> _map;
    protected ArrayList<FormWidget> _widgets;

    // -- widgets
    protected LinearLayout _container;
    protected LinearLayout _layout;
    protected ScrollView   _viewport;

    // -----------------------------------------------
    //
    // parse data and build view
    //
    // -----------------------------------------------

    /**
     * parses a supplied schema of raw json data and creates widgets
     * @param data - the raw json data as a String
     */
    public FormActivity(){
        super();
    }
    public void generateForm( String data )
    {
        _widgets = new ArrayList<FormWidget>();
        _map = new HashMap<String, FormWidget>();

        try
        {
            String name;
            FormWidget widget;
            JSONObject property;
            JSONObject schema = new JSONObject( data ); 
            JSONArray names = schema.names();

            for( int i= 0; i < names.length(); i++ ) 
            {
                name = names.getString( i );

                if( name.equals( SCHEMA_KEY_META )  ) continue;

                property = schema.getJSONObject( name );    

                boolean toggles  = hasToggles( property );
                String defaultValue   = getDefault( property );
                int priority = property.getInt( FormActivity.SCHEMA_KEY_PRIORITY );

                widget = getWidget( name, property );
                if( widget == null) continue;

                widget.setPriority( priority );
                widget.setValue( defaultValue );

                if( toggles ){
                    widget.setToggles( processToggles( property ) );
                    widget.setToggleHandler( new FormActivity.FormWidgetToggleHandler() );
                }

                if( property.has(FormActivity.SCHEMA_KEY_HINT)) widget.setHint( property.getString( FormActivity.SCHEMA_KEY_HINT ) );

                _widgets.add( widget );
                _map.put( name, widget );
            }
        } catch( JSONException e ) {
            Log.i( "MakeMachine", e.getMessage() );
        }

        // -- sort widgets on priority
        Collections.sort( _widgets, new PriorityComparison() ); 

        // -- create the layout
        _container = new LinearLayout( this );
        _container.setOrientation( LinearLayout.VERTICAL );
        _container.setLayoutParams( FormActivity.defaultLayoutParams );

        _viewport  = new ScrollView( this );
        _viewport.setLayoutParams( FormActivity.defaultLayoutParams );

        _layout = new LinearLayout( this );
        _layout.setOrientation( LinearLayout.VERTICAL );
        _layout.setLayoutParams( FormActivity.defaultLayoutParams );

        initToggles();

        for( int i = 0; i < _widgets.size(); i++ ) {
            _layout.addView( ( View ) _widgets.get(i).getView() );
        }

        _viewport.addView( _layout );
        _container.addView( _viewport );

        setContentView( _container );
    }

    // -----------------------------------------------
    //
    // populate and save
    //
    // -----------------------------------------------

    /**
     * this method fills the form with existing data
     * get the json string stored in the record we are editing
     * create a json object ( if this fails then we know there is now existing record )
     * create a list of property names from the json object
     * loop through the map returned by the Form class that maps widgets to property names
     * if the map contains the property name as a key that means there is a widget to populate w/ a value
     */
    protected void populate( String jsonString )
    {
        try
        {   
            String prop;
            FormWidget widget;
            JSONObject data = new JSONObject( jsonString );
            JSONArray properties = data.names();

            for( int i = 0; i < properties.length(); i ++ )
            {
                prop = properties.getString( i );
                if( _map.containsKey(prop) )  {
                    widget = _map.get( prop );
                    widget.setValue( data.getString(prop) );
                }
            }
        } catch ( JSONException e ) {

        }
    }

    /**
     * this method preps the data and saves it
     * if there is a problem w/ creating the json string, the method fails
     * loop through each widget and set a property on a json object to the value of the widget's getValue() method
     */
    protected JSONObject save()
    {
        FormWidget widget;
        JSONObject data = new JSONObject();

        boolean success = true;

        try{
            for( int i = 0; i < _widgets.size(); i++ ) 
            {
                widget = _widgets.get(i);
                data.put( widget.getPropertyName(), widget.getValue() );
            }
        } catch( JSONException e )
        {
            success = false;
            Log.i( "MakeMachine", "Save error - " + e.getMessage() );
            return null;
        }

        if( success ) {
            Log.i( "MakeMachine", "Save success " + data.toString() );
            return data;
        }
        return null;
    }

    // -----------------------------------------------
    //
    // toggles
    //
    // -----------------------------------------------

    /**
     * creates the map a map of values for visibility and references to the widgets the value affects
     */
    protected HashMap<String, ArrayList<String>> processToggles( JSONObject property )
    {
        try{
            ArrayList<String> toggled;
            HashMap<String, ArrayList<String>> toggleMap = new HashMap<String, ArrayList<String>>();

            JSONObject toggleList = property.getJSONObject( FormActivity.SCHEMA_KEY_TOGGLES );
            JSONArray toggleNames = toggleList.names();

            for( int j = 0; j < toggleNames.length(); j++ )
            {
                String toggleName = toggleNames.getString(j);
                JSONArray toggleValues = toggleList.getJSONArray( toggleName );
                toggled = new ArrayList<String>();
                toggleMap.put( toggleName, toggled );
                for( int k = 0; k < toggleValues.length(); k++ ) {
                    toggled.add( toggleValues.getString(k) );
                }
            }

            return toggleMap;

        } catch( JSONException e ){
            return null;
        }
    }

    /**
     * returns a boolean indicating that the supplied json object contains a property for toggles
     */
    protected boolean hasToggles( JSONObject obj ){
        try{
            obj.getJSONObject( FormActivity.SCHEMA_KEY_TOGGLES );
            return true;
        } catch ( JSONException e ){
            return false;
        }
    }

    /**
     * initializes the visibility of widgets that are togglable 
     */
    protected void initToggles()
    {
        int i;
        FormWidget widget;

        for( i = 0; i < _widgets.size(); i++ )  {
            widget = _widgets.get(i);
            updateToggles( widget );
        }
    }

    /**
     * updates any widgets that need to be toggled on or off
     * @param widget
     */
    protected void updateToggles( FormWidget widget ) 
    {
        int i;
        String name;
        ArrayList<String> toggles;
        ArrayList<FormWidget> ignore = new ArrayList<FormWidget>();

        toggles = widget.getToggledOn();
        for( i = 0; i < toggles.size(); i++ ) 
        {
            name = toggles.get(i);
            if( _map.get(name) != null ) 
            {
                FormWidget toggle = _map.get(name);
                ignore.add( toggle );
                toggle.setVisibility( View.VISIBLE );
            }
        }

        toggles = widget.getToggledOff();
        for( i = 0; i < toggles.size(); i++ ) 
        {
            name = toggles.get(i);
            if( _map.get(name) != null ) 
            {
                FormWidget toggle = _map.get(name);
                if( ignore.contains(toggle) ) continue;
                toggle.setVisibility( View.GONE );
            }
        }
    }

    /**
     * simple callbacks for widgets to use when their values have changed
     */
    class FormWidgetToggleHandler
    {
        public void toggle( FormWidget widget ) {
            updateToggles( widget );
        }
    }

    // -----------------------------------------------
    //
    // utils
    //
    // -----------------------------------------------

    protected String getDefault( JSONObject obj ){
        try{
            return obj.getString( FormActivity.SCHEMA_KEY_DEFAULT );
        } catch ( JSONException e ){
            return null;
        }
    }

    /**
     * helper class for sorting widgets based on priority
     */
    class PriorityComparison implements Comparator<FormWidget>
    {
        public int compare( FormWidget item1, FormWidget item2 ) {
            return item1.getPriority() > item2.getPriority() ? 1 : -1;
        }
    }

    /**
     * factory method for actually instantiating widgets
     */
    protected FormWidget getWidget( String name, JSONObject property ) 
    {
        try
        {
            String type = property.getString( FormActivity.SCHEMA_KEY_TYPE );

            if( type.equals( FormActivity.SCHEMA_KEY_STRING ) ){
                return new FormEditText( this, name  );
            }

            if( type.equals( FormActivity.SCHEMA_KEY_BOOL ) ){
                return new FormCheckBox( this, name );
            }

            if( type.equals(  FormActivity.SCHEMA_KEY_INT ) )
            {   
                if( property.has( FormActivity.SCHEMA_KEY_OPTIONS ) ) 
                {
                    JSONObject options = property.getJSONObject( FormActivity.SCHEMA_KEY_OPTIONS );
                    return new FormSpinner(  this, name, options );
                }else{
                    return new FormNumericEditText( this, name );
                }
            }
        } catch( JSONException e ) {
            return null;
        }
        return null;
    }

    public static String parseFileToString( Context context, String filename )
    {
        try
        {
            InputStream stream = context.getAssets().open( filename );
            int size = stream.available();

            byte[] bytes = new byte[size];
            stream.read(bytes);
            stream.close();

            return new String( bytes );

        } catch ( IOException e ) {
            Log.i("MakeMachine", "IOException: " + e.getMessage() );
        }
        return null;
    }
}

Manifest.xml

<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
    package="com.example.makemachine"
    android:versionCode="1"
    android:versionName="1.0" >

    <uses-sdk
        android:minSdkVersion="8"
        android:targetSdkVersion="19" />

    <application
        android:allowBackup="true"
        android:icon="@drawable/ic_launcher"
        android:label="@string/app_name"
        android:theme="@style/AppTheme" >
        <activity
            android:name="com.example.makemachine.MainActivity"
            android:label="@string/app_name" >      

        </activity>
        <activity
            android:name="com.example.makemachine.FormActivity"
            android:label="@string/app_name" >
            <intent-filter>
                <action android:name="android.intent.action.MAIN" />

                <category android:name="android.intent.category.LAUNCHER" />
            </intent-filter>
        </activity>
    </application>

</manifest>

Logcat Error

12-22 10:41:01.791: E/AndroidRuntime(2953): FATAL EXCEPTION: main
12-22 10:41:01.791: E/AndroidRuntime(2953): java.lang.RuntimeException: Unable to instantiate activity ComponentInfo{com.example.makemachine/com.example.makemachine.FormActivity}: java.lang.InstantiationException: can't instantiate class com.example.makemachine.FormActivity
12-22 10:41:01.791: E/AndroidRuntime(2953):     at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2106)
12-22 10:41:01.791: E/AndroidRuntime(2953):     at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2230)
12-22 10:41:01.791: E/AndroidRuntime(2953):     at android.app.ActivityThread.access$600(ActivityThread.java:141)
12-22 10:41:01.791: E/AndroidRuntime(2953):     at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1234)
12-22 10:41:01.791: E/AndroidRuntime(2953):     at android.os.Handler.dispatchMessage(Handler.java:99)
12-22 10:41:01.791: E/AndroidRuntime(2953):     at android.os.Looper.loop(Looper.java:137)
12-22 10:41:01.791: E/AndroidRuntime(2953):     at android.app.ActivityThread.main(ActivityThread.java:5041)
12-22 10:41:01.791: E/AndroidRuntime(2953):     at java.lang.reflect.Method.invokeNative(Native Method)
12-22 10:41:01.791: E/AndroidRuntime(2953):     at java.lang.reflect.Method.invoke(Method.java:511)
12-22 10:41:01.791: E/AndroidRuntime(2953):     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:793)
12-22 10:41:01.791: E/AndroidRuntime(2953):     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:560)
12-22 10:41:01.791: E/AndroidRuntime(2953):     at dalvik.system.NativeStart.main(Native Method)
12-22 10:41:01.791: E/AndroidRuntime(2953): Caused by: java.lang.InstantiationException: can't instantiate class com.example.makemachine.FormActivity
12-22 10:41:01.791: E/AndroidRuntime(2953):     at java.lang.Class.newInstanceImpl(Native Method)
12-22 10:41:01.791: E/AndroidRuntime(2953):     at java.lang.Class.newInstance(Class.java:1319)
12-22 10:41:01.791: E/AndroidRuntime(2953):     at android.app.Instrumentation.newActivity(Instrumentation.java:1054)
12-22 10:41:01.791: E/AndroidRuntime(2953):     at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2097)
12-22 10:41:01.791: E/AndroidRuntime(2953):     ... 11 more

What is wrong in my code?

Answer 1

Your FormActivity is abstract and abstract classes cannot be instantiated.

Reading the comments from the generated code, it says "This class should be sub-classed". So extend the base FormActivity and only declare the non-abstract, derived activity classes in your manifest.

Answer 2

quite simple easy .. you can Remove abstract (keyword) non access modifier from your Activity

eg: from :public abstract class FormActivity extends Activity

to: public class FormActivity extends Activity `