模拟静态访问和全局访问是一个棘手的问题。幸运的是，每个问题都可以通过另一层间接来解决：

interface DisplayWrapper {
  int resultDisplay(List<Integer> list);
}
class DisplayWrapperImpl implements DisplayWrapper {
  @Override int resultDisplay(final List<Integer> list) {
    return Display.resultDisplay(list);
  }
}
public class Summation {
    private static final List<Integer> list = new ArrayList<>();

    private final DisplayWrapper display;
    
    public Summation(final DisplayWrapper display) {
      this.display = display;
    }

    public int testCode(int... a) {
        Collections.addAll(list, a);
        return display.resultDisplay(list);
    }
}

现在，您的类可以轻松地进行测试。
生产代码：

new Summation(new DisplayWrapperImpl())

测试代码：

final DisplayWrapper display = mock(DisplayWrapper.class);
final Summation summation = new Summation(display);
// ...