Car car = new Car(22000,"silver");
WeakReference<Car> weakCar = new WeakReference<Car>(car);

package org.apache.tomcat.util.collections;

import java.util.Map;
import java.util.WeakHashMap;
import java.util.concurrent.ConcurrentHashMap;

public final class ConcurrentCache<K,V> {

  private final int size;

  private final Map<K,V> eden;

  private final Map<K,V> longterm;

  public ConcurrentCache(int size) {
    this.size = size;
    this.eden = new ConcurrentHashMap<>(size);
    this.longterm = new WeakHashMap<>(size);
  }

  public V get(K k) {
    V v = this.eden.get(k);
    if (v == null) {
      synchronized (longterm) {
        v = this.longterm.get(k);
      }
      if (v != null) {
        this.eden.put(k, v);
      }
    }
    return v;
  }

  public void put(K k, V v) {
    if (this.eden.size() >= size) {
      synchronized (longterm) {
        this.longterm.putAll(this.eden);
      }
      this.eden.clear();
    }
    this.eden.put(k, v);
  }
}