弱引用get出來賦值後又會變成強引用 因此通常儘可能 get後面直接用

public static void main(String[] args) {

A a = new A();

a.name="dingjiahui";

WeakReference<A> sr = new WeakReference<A>(a);

// A ddd = sr.get();

System.out.println(sr.get().name);

a=null;

System.gc();

System.out.println(sr.get());

}