Anyone solved this?

Question about the task

Caching

Java Collections, Level 4, Lesson 8

Under discussion

I really don't understand what needs to be done here. Did someone successfully solved this?

The Cache class is a generic class for caching objects.
It works with generic classes that have:
a) a public constructor with one K type parameter;
b) a K getKey() method with any access modifier.

Task:
1. Choose the correct type for the cache field. The Map<K, V> cache field must store keys for which there are active references.
Keys with no active references must be automatically removed from the cache along with their values.
2. Implement the getByKey method:
2.1. Return an object from the cache that matches key.
2.2. If an object doesn't exist in the cache, then use reflection to add a new instance to the cache. See item a).
3. Implement the put method:
3.1. Using reflection, get a reference to the method described in item b).
3.2. Use reflection to allow access to it.
3.3. Using reflection, call the getKey method on obj to get key.
3.4. Add the <key, obj> pair to the cache.
3.5. Return true if the method finishes correctly, otherwise return false. Ignore exceptions.

Requirements:

The cache field must be initialized with a WeakHashMap object.
The getByKey method must return an object from the cache.
The getByKey method must add an object to the cache if it is not already there.
The put method must retrieve the key from the passed object and add a <key, obj> pair to the cache.
The put method must return true if it finishes correctly. Otherwise, it must return false.

package com.codegym.task.task34.task3408; /* Caching */ public class Solution { public static void main(String[] args) throws Exception { SomeKey someKey = new SomeKey(); someKey.name = "test"; SomeKey someKeyNew = new SomeKey(); someKeyNew.name = "testNew"; SomeValue value = new SomeValue(someKey); Cache<SomeKey, SomeValue> cache = new Cache<>(someKey); cache.put(value); SomeValue existingValueFromCache = cache.getByKey(someKey, SomeValue.class); System.out.println(existingValueFromCache); SomeValue newValueFromCache = cache.getByKey(someKeyNew, SomeValue.class); System.out.println(newValueFromCache); System.out.println(cache.size()); /* Expected output: SomeValue{myKey=SomeKey{name='test'}} SomeValue{myKey=SomeKey{name='testNew'}} 2 */ } public static class SomeKey { String name; @Override public String toString() { return "SomeKey{" + "name='" + name + '\'' + '}'; } } public static class SomeValue { private SomeKey myKey; public SomeValue() { } public SomeValue(SomeKey myKey) { // Use this constructor this.myKey = myKey; } private SomeKey getKey() { return myKey; } @Override public String toString() { return "SomeValue{" + "myKey=" + myKey + '}'; } } }

Comments (4)

Popular
New
Old

You must be signed in to leave a comment

Wei Cui

Level 39 , Newark, United States

15 April 2020, 02:44

public class Cache<K, V> {
    private Map<K, V> cache =new WeakHashMap<>();   //TODO add your code here

    public V getByKey(K key, Class<V> clazz) throws Exception {
        //TODO add your code here
        if (!cache.containsKey(key))
            cache.put(key, clazz.getConstructor(key.getClass()).newInstance(key));

        return cache.get(key);
    }

    public boolean put(V obj) {
        //TODO add your code here

        Method method = null;
        try {
            method = obj.getClass().getDeclaredMethod("getKey");
            method.setAccessible(true);

            cache.put((K) method.invoke(obj), obj);
            return cache.containsKey((K) method.invoke(obj));

        } catch (Exception ignore){

        }


        return false;
    }

    public int size() {
        return cache.size();
    }
}

BlueJavaBanana

Level 37

6 March 2021, 15:51

Beautiful code as always Wei Chu. My code was giving expected output and functioned the same as your, just was syntactically different. Wasn't validating annoyingly so I had to see who else was struggling. Thanks for your contributions!

Elvis Lee

Level 41 , Aurora, United States

26 February 2020, 20:13

For getByKey, create an object like this: V obj = (V) clazz.getConstructor(key.getClass()).newInstance(key);

Seb

Level 41 , Crefeld, Germany

21 February 2020, 10:35

You don't need lines 9-15 in your Cache class. Adjust line 22 so that it fulfills the requirement from point 2.2 (If an object doesn't exist in the cache, then use reflection to add a new instance to the cache.) Try: cache.put(key, clazz.getConstructor(key.getClass().newInstance(key)); Reflection also needs to be used to implement the put method: - Get a reference to the getKey method. - This method needs to be a method of the object, that is meant to be put into the cache - in this example the SomeValue class, actually has a getKey method. // Method method = null; - And then in a try catch block, you can access the getKey method using reflection. // method = obj.getClass().getDeclaredMethod("getKey"); - Set the modifier // method.setAccessible(true); - And now you can put the object into the cache // cache.put((K) method.invoke(obj), obj); That should do the trick. ;-) Cheers.