java DuplicateKeyException已被捕获

Question

我正在使用HashMap并尝试生成DuplicateKeyException以防止覆盖现有密钥。

我试图根据某些用户输入抛出此异常，但是收到异常已经被捕获错误。

try {
  Student student = map.get(uid); // uid is a user defined input.
  if (student != null) { // test for key existence
    key = true;
  } else {
    key = false;
  }
  if (key = true) { // throw duplicate key exception
    throw new DuplicateKeyException();
  } else { // Put new uid into map
    map.put(uid, new Student(name, major));
  }
  JOptionPane.showMessageDialog(null, "It worked...");
} catch (NullPointerException z) { // If the key doesn't exist
  JOptionPane.showMessageDialog(null, "That record does not exist.");
} catch (DuplicateKeyException x) { // If the key does exist
  JOptionPane.showMessageDialog(null, "That record already exists.");
} catch (NumberFormatException y) { // If user input is other than expected
  JOptionPane.showMessageDialog(null, "You must enter integer.");
}

NullPointerException和NumberFormatException按预期工作，直到实现DuplicateKeyException。

“catch DuplicateKeyException（）”显示“异常DuplicateKeyException已被捕获。”

我试图修改例外顺序，以防它们过于宽泛。这似乎没有帮助解决这个问题。

我还是新手，非常感谢任何意见。