Java 中的弱引用是什么

Object c = new Car();  
c=null;

Object c = new Car(); //只要c还指向car object, car object就不会被回收 -->（强引用）

public class Car {
  private double   price;
  private String  color;

  public Car(double price, String color)
  {
    this.price = price;
    this.color = color;
  }

  public double getPrice()
  {
    return price;
  }

  public String getColor()
  {
    return color;
  }

  public String toString()
  {
    return "This car is a " + this.color + " car, costs $" + price;
  }
}

public class WeakReferenceCar extends WeakReference<Car>
{
  public WeakReferenceCar(Car car)
  {
    super(car);
  }
}

public static void main(String[] args)
{
  Car car = new Car(2000.0, "red");
  WeakReferenceCar wrc = new WeakReferenceCar(car);
  wrc.setStr("111");
  int i = 0;
  while (true)
  {
    if (wrc.get() != null)
    {
      i++;
      System.out.println("WeakReferenceCar's Car is alive for " + i + ", loop - " + wrc);
    }
    else
    {
      System.out.println("WeakReferenceCar's Car has bean collected");
      break;
    }
  }
}

WeakReferenceCar's Car is alive for 68450, loop - interview.WeakReferenceCar@776ec8df
WeakReferenceCar's Car is alive for 68451, loop - interview.WeakReferenceCar@776ec8df
WeakReferenceCar's Car is alive for 68452, loop - interview.WeakReferenceCar@776ec8df
WeakReferenceCar's Car is alive for 68453, loop - interview.WeakReferenceCar@776ec8df
[GC (Allocation Failure) [PSYoungGen: 34304K->1000K(38400K)] 34320K->1016K(125952K), 0.0015129 secs] [Times: user=0.02 sys=0.02, real=0.00 secs] 
WeakReferenceCar's Car is alive for 68454, loop - interview.WeakReferenceCar@776ec8df
WeakReferenceCar's Car has bean collected
Heap
PSYoungGen   total 38400K, used 1986K [0x00000000d5e00000, 0x00000000da900000, 0x0000000100000000)
eden space 33280K, 2% used [0x00000000d5e00000,0x00000000d5ef6b70,0x00000000d7e80000)
from space 5120K, 19% used [0x00000000d7e80000,0x00000000d7f7a020,0x00000000d8380000)
to  space 5120K, 0% used [0x00000000da400000,0x00000000da400000,0x00000000da900000)
ParOldGen    total 87552K, used 16K [0x0000000081a00000, 0x0000000086f80000, 0x00000000d5e00000)
object space 87552K, 0% used [0x0000000081a00000,0x0000000081a04000,0x0000000086f80000)
Metaspace    used 3547K, capacity 4564K, committed 4864K, reserved 1056768K
class space  used 381K, capacity 388K, committed 512K, reserved 1048576K