在Java中，我们如何显式地调用垃圾回收（GC）呢？

public class GarbageCollectionTest {
   public static void main(String args[]) {
      System.out.println(Runtime.getRuntime().freeMemory());
      for (int i=0; i<= 100000; i++) {
         Double d = new Double(300);
      }
      System.out.println(Runtime.getRuntime().freeMemory());
      System.gc();
      System.out.println(Runtime.getRuntime().freeMemory());
   }
}

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