CodeGym/Help with Java Tasks/What's the idea here? I can't figure it out

Level 41

Vilnius

23.03.2020
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What's the idea here? I can't figure it out

Question about the task

AmigoSet (part 5)

Java Collections, Level 4, Lesson 15

Resolved

Pleaseeeeeee

Your AmigoSet set implements the Serializable interface. However, it doesn't serialize properly.

1. Implement your own logic for serializing and deserializing.
Remember which private methods you need to add in order to serialize correctly.

To serialize:
* serialize the set
* serialize the map object's capacity and loadFactor. You'll need them for deserialization.
Since this data is package-private, use the HashMapReflectionHelper utility class to get it.

To deserialize:
* read all of the data
* create a map using the constructor with capacity and loadFactor parameters

2. Remember what transient means?

Requirements:

The AmigoSet class must have a private writeObject method with one ObjectOutputStream parameter.
The AmigoSet class must have a private readObject method with one ObjectInputStream parameter.
The writeObject method must call the defaultWriteObject method on the ObjectOutputStream object passed as an argument.
The readObject method must call the defaultReadObject method on the ObjectInputStream object passed as an argument.
The object serialized using the writeObject method must equal the object deserialized using the readObject method.

package com.codegym.task.task37.task3707; import java.io.IOException; import java.io.ObjectInputStream; import java.io.ObjectOutputStream; import java.io.Serializable; import java.util.*; public class AmigoSet<E> extends AbstractSet implements Serializable, Cloneable, Set { private static final Object PRESENT = new Object(); private transient HashMap<E, Object> map; @Override public Iterator iterator() { Set keys = this.map.keySet(); return keys.iterator(); } @Override public int size() { return this.map.size(); } @Override public boolean isEmpty() { return map.isEmpty(); } @Override public boolean contains(Object o) { return map.containsKey(o); } @Override public void clear() { this.map.clear(); } @Override public boolean remove(Object o) { this.map.remove(o); return true; } public AmigoSet() { map = new HashMap<>(); } public AmigoSet(Collection<? extends E> collection) { int capacity = Math.max(16, (int) (Math.ceil(collection.size() / .75f))); this.map = new HashMap<>(capacity); for (E e : collection) { map.put(e, PRESENT); } } public boolean add (Object e) { if (map.containsKey(e)) { return false; } else { map.put((E) e, PRESENT); return true; } } public Object clone(){ AmigoSet<E> clone; try { clone = (AmigoSet<E>)super.clone(); clone.map = (HashMap<E,Object>)map.clone(); } catch (Exception e) { throw new InternalError(e); } return clone; } private void writeObject(ObjectOutputStream objoutstr) throws IOException { objoutstr.writeObject(this.map); } private void readObject(ObjectInputStream objectInputStream){ try { this.map = (HashMap<E, Object>) objectInputStream.readObject(); } catch (IOException e) { e.printStackTrace(); } catch (ClassNotFoundException e) { e.printStackTrace(); } } }

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yz Backend Developer

8 April 2020, 13:03

bro just use defultreadobject read about it if u have questions again text me

Jonas

Level 41 , Vilnius, Lithuania

9 April 2020, 17:15

Hello, thank you i passed requirements 3, 4 with this code: private void writeObject(ObjectOutputStream objoutstr) throws IOException { objoutstr.defaultWriteObject(); } private void readObject(ObjectInputStream objectInputStream) throws IOException, ClassNotFoundException { objectInputStream.defaultReadObject(); } but I'm still failing with last condition, it says "AmigoSet and deserialized set must be same size" Could you help, please?

yz Backend Developer

9 April 2020, 18:19

solution

yea sure, so u need 3 things: 1.global field to keep capacity of map ; 2.global filed to keep loadFactor(float); 3.Set to keep all keys from map(because map is trancient it cant be saved); before using defaultWriteObject method u need to get loadFactor capacity and copy all keys to set;(by copy i mean one by one add to set **dont just assign map.keySet to set( Set set = map.keySet() <- doesnt work)) and then use the defaultWriteObject method( it gonna save the data such as int capacity ,float loadFactor and Set; In void readObject method u should do this: use defaultReadObject , ( gets saved fields such as capacity,loaFactor and Set ) and then create new HashMap with custom loadFactor and capacity (new HashMap(cap,loadF) and then copy to this map data from Set as key and as value put PRESENT (it gonna solve last req) and then assign ur created map to map which is global(the transient one);

Jonas

Level 41 , Vilnius, Lithuania

11 April 2020, 15:18

Thanks, but could you please explain what is loadFactor? How should I get the value of it?

Jonas

Level 41 , Vilnius, Lithuania

11 April 2020, 15:39

I still don't quite get what is loadFactor but I took this getLoadFactor method from internet, but with this code it still says the maps' sizes must be the same: private void writeObject(ObjectOutputStream objoutstr) throws IOException { this.loadFactor = getLoadFactor(this.map); objoutstr.defaultWriteObject(); } public float getLoadFactor(HashMap map){ int count=0; float load; for(int i=0; i<map.size(); i++){ count += map.size(); } load = count/(float)map.size(); return load; } private void readObject(ObjectInputStream objectInputStream) throws IOException, ClassNotFoundException { objectInputStream.defaultReadObject(); HashMap<Object, E> returnMap = new HashMap(capacity,loadFactor); for (Iterator<E> it = setOfKeys.iterator(); it.hasNext(); ) { E e = it.next(); this.map.put(e, PRESENT); } }

yz Backend Developer

11 April 2020, 16:18

useful

loadFactor and capacity is private methods in HashMap claass to get them we should use java reflection(read about it); HashMapReflectionHelper is using it ( java reflection) this class provides u a method where u put map and method name( ex: map, "methodName" without "()" <- this) and this method invokes that method. to see what is the name methods( to get capacity and to ge loadfactor) check HashMap source; and then get two methods

yz Backend Developer

11 April 2020, 16:20

read about what is diff between capacity and size and also read what is loadfactor

Jonas

Level 41 , Vilnius, Lithuania

11 April 2020, 16:33

Thanks, read a bit about these things, but how my serialization and deserialization methods should look like? Could you give me a hint

Jonas

Level 41 , Vilnius, Lithuania

11 April 2020, 16:45

Thanks so much, solved it

yz Backend Developer

11 April 2020, 17:04

sure,

private Set<E> set = new HashMap<>(); // to save data from maps keys
private int capacity ; //to save cap;
private float loadFactor ; // to save loadF


private void writeObject(ObjectOutputStream out) throws IOException {
   capacity =   HashMapReflectionHelper.callHiddenMethod( map,"capacity" );
   loadFactor =   HashMapReflectionHelper.callHiddenMethod( map,"loadFactor" );
        for( E e : map.keySet()){
                set.add( e);                  // saving data to set which will be saved when we call writeDefaultObject method;
        }
       out.writeDefaultObject(); //this methods serialize all fields in ur class (set,capacity,loadFactor)
}
private void readObject(ObjectInputStream in){
 //here u should call readDefaultObject method first cause it gonna read from input to ur class field ( set,cap,loadF)
in.writeDefaultObject();// now after this method called u have capacity ,loadFactor and set,now u need to creat Hashmap with a cap loadF and put all data u from set remember))) ?
map = new HashMap(cap,loadF);
Iterator it = set.Iterator
while(it.hasNext){

map.put(it.next , PRESENT);

    }
}

JianQiu Hwang

Level 35 , Washington

12 April 2020, 04:35

I think your answer is:

private int capacity;
private float loadFactor;
private Set<E> set;

private void writeObject(ObjectOutputStream oos) throws IOException {
    capacity = HashMapReflectionHelper.callHiddenMethod(map, "capacity");
    loadFactor = HashMapReflectionHelper.callHiddenMethod(map, "loadFactor");
    set = new HashSet<>();
    set.addAll(map.keySet());
    oos.defaultWriteObject();
}

private void readObject(ObjectInputStream ois) throws IOException, ClassNotFoundException {
    ois.defaultReadObject();
    map = new HashMap<>(capacity, loadFactor);
    for (E e : set) {
        map.put(e, PRESENT);
    }
}