Hjæp til Generic

<? extends E> betyder <et eller andet som er E eller en subklasse af E>

eksempel:

import java.util.ArrayList;

public class GWC {
    public static void mstd(ArrayList<P> lst) {
        P o = lst.get(0);
        lst.add(new C2());
    }
    public static void mspec(ArrayList<? extends P> lst) {
        P o = lst.get(0);
        // syntax error - for good reasons: we can not add a C2 to this list
        //lst.add(new C2());
    }
    public static void main(String[] args) {
        // basic
        ArrayList<P> lst1 = new ArrayList<P>();
        lst1.add(new C1());
        mstd(lst1);
        // problem
        ArrayList<C1> lst2 = new ArrayList<C1>();
        lst2.add(new C1());
        // syntax error - for good reasons: we can not add a C2 to this list
        //mstd(lst2);
        // solution
        mspec(lst2);
    }
}

class P {
}

class C1 extends P {
}

class C2 extends P {
}

Mange tak for hjælpen arne_v. Skal nemlig have det som emne i min uddannelse senere i semestret.

Men nu har jeg et nyt problem. Måske du kan hjælpe mig?

Jeg har fået en opgave vedr. Hashset. som lyder på :
In this exercise you're going to use the java.util.Hashset Class for storage and retrival of data in files on a disk.

Problemet her når jeg tilføjer 3 instancer af Person til HashSet og vil skrive til file kommer denne her fejl :
Output:
The_Use_Of_Hashset.Person@1eed786
3.1415
The_Use_Of_Hashset.Person@19f953d
true
4910
The_Use_Of_Hashset.Person@1fee6fc
java.io.NotSerializableException:

Her er mit program
package The_Use_Of_Hashset;

import java.io.FileNotFoundException;
import java.io.FileOutputStream;
import java.io.IOException;
import java.io.ObjectOutputStream;
import java.util.HashSet;
import java.util.Iterator;
import java.util.Set;

public class Saver {

    private double PI;
    private boolean b = true;
    private int value;

    public Saver() {
        Person p1 = new Person("Peter", "010364-1777",
"188 cm");
        Person p2 = new Person("Poul", "231064-5641", "180 cm");
        Person p3 = new Person("Mary", "170765-4242", "161 cm");
        Set<Object> obj = new HashSet<Object>();
        obj.add(value = 4910);
        obj.add(PI = 3.1415);
        obj.add(b);
        obj.add(value = 491 * 10);
        obj.add(p1);
        obj.add(p2);
        obj.add(p3);

        // printing the obj
        Iterator<Object> itr = obj.iterator();
        while (itr.hasNext()) {
            System.out.println(itr.next().toString());
        }
        try {
            FileOutputStream fos = new FileOutputStream("object", false);

            ObjectOutputStream oos = new ObjectOutputStream(fos);
            oos.writeObject(obj);
            oos.close();
        } catch (FileNotFoundException e) {
            System.out.println("Uable to write to Objects");
            e.printStackTrace();
        } catch (IOException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }
    }
}



package The_Use_Of_Hashset;

import java.io.EOFException;
import java.io.FileInputStream;
import java.io.FileNotFoundException;
import java.io.IOException;
import java.io.ObjectInputStream;
import java.util.HashSet;
import java.util.Iterator;
import java.util.Set;

public class Loader {

    /**
    * @param args
    */
    public static void main(String[] args) {
        // TODO Auto-generated method stub

        Saver save = new Saver();

        ObjectInputStream ois;
        try {
            ois = new ObjectInputStream(new FileInputStream("object"));
            try {
                @SuppressWarnings("unchecked")
                Set<Object> load = (HashSet<Object>) ois.readObject();

                System.out.println("Number of Elements " + load.size());

                Iterator<Object> itr = load.iterator();
                while (itr.hasNext()) {
                    System.out.println(itr.next());
                }

            } catch (ClassNotFoundException e) {
                // TODO Auto-generated catch block
                e.printStackTrace();
            }
        } catch (FileNotFoundException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        } catch (IOException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }

    }

}

package The_Use_Of_Hashset;

public class Person {

    private String Name;
    private String Cpr;
    private String height;

    public Person(String name, String cpr, String height) {

        Name = name;
        Cpr = cpr;
        this.height = height;
    }
}

java.io.NotSerializableException

siger at

public class Person {

skal vaere

public class Person implements Serializable {

Tak for oplysningen det hjalp ihvertfald:) Men nu har jeg et nyt problem når jeg printer ud?

Her er mit program med ændring:
package The_Use_Of_Hashset;

import java.io.Serializable;

public class Person implements Serializable {

    private static final long serialVersionUID = 1L;
   
    private String Name;
    private String Cpr;
    private String height;

    public Person(String name, String cpr, String height) {

        Name = name;
        Cpr = cpr;
        this.height = height;
    }
}

package The_Use_Of_Hashset;

import java.io.FileNotFoundException;
import java.io.FileOutputStream;
import java.io.IOException;
import java.io.ObjectOutputStream;
import java.util.HashSet;
import java.util.Iterator;
import java.util.Set;

public class Saver {

    private Person peter = new Person("Peter", "010364-1777", "188 cm");
    private Person poul = new Person("Poul", "231064-5641", "180 cm");
    private Person mary = new Person("Mary", "170765-4242", "161 cm");
    public Saver() {

        Set<Object> obj = new HashSet<Object>();
       
        obj.add(new Integer(4910));
        obj.add(new Double(3.1415));
        obj.add(new Boolean(true));                                        //Adding the objects to the HashSet
        obj.add(new Integer(491*10));
        obj.add(peter);
        obj.add(poul);
        obj.add(mary);
       
       

        //printing the obj
    System.out.println("----Printing set----");
    for(Object o : obj)
        System.out.println(o.toString());
//        Iterator<Object> itr = obj.iterator();
//        while (itr.hasNext()) {
//            Object o = itr.next();
//           
//            System.out.println(o);
//        }
        if (obj.contains(peter)&& obj.contains(mary)){
            System.out.println("Person found");
        }else{
            System.out.println("Cannot find person");
        }
        try {
            FileOutputStream fos = new FileOutputStream("object", false);

            ObjectOutputStream oos = new ObjectOutputStream(fos);
            oos.writeObject(obj);
            oos.close();
        } catch (FileNotFoundException e) {
            System.out.println("Uable to write to Objects");
            e.printStackTrace();
        } catch (IOException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }
    }
}

package The_Use_Of_Hashset;

import java.io.EOFException;
import java.io.FileInputStream;
import java.io.FileNotFoundException;
import java.io.IOException;
import java.io.ObjectInputStream;
import java.util.HashSet;
import java.util.Iterator;
import java.util.Set;

public class Loader {

    /**
    * @param args
    */
    public static void main(String[] args) {
        // TODO Auto-generated method stub

        Saver save = new Saver();

        ObjectInputStream ois;
        try {
            ois = new ObjectInputStream(new FileInputStream("object"));
            try {
                @SuppressWarnings("unchecked")
                Set<Object> load = (HashSet<Object>) ois.readObject();
                System.out.println();
                System.out.println("----Print the Loadet file <object.txt>----");
                System.out.println("Number of Elements in HashSet " + load.size());

                Iterator<Object> itr = load.iterator();
                while (itr.hasNext()) {
                    System.out.println(itr.next());
                }
               
           

            } catch (ClassNotFoundException e) {
                // TODO Auto-generated catch block
                e.printStackTrace();
            }
        } catch (FileNotFoundException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        } catch (IOException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }

    }

}
Min Output ser mærkelig ud?
---Printing set----
The_Use_Of_Hashset.Person@12dacd1
The_Use_Of_Hashset.Person@e48e1b
3.1415
true
4910
The_Use_Of_Hashset.Person@1ad086a
Person found

----Print the Loadet file <object.txt>----
Number of Elements in HashSet 6
The_Use_Of_Hashset.Person@1754ad2
3.1415
true
The_Use_Of_Hashset.Person@1833955
4910
The_Use_Of_Hashset.Person@e86da0

Hvad gør jeg forkert?

Hvis det er dem her:

The_Use_Of_Hashset.Person@12dacd1
The_Use_Of_Hashset.Person@e48e1b

du ikke kan lide, saa lav en toString metode i Person klassen.

Jeg fandt fejlen! Jeg mangler min toString i Person.. Men ellers mange mange tak Arne_v

saa vil jeg smide et svar