我想为一个使用CompletableFuture.supplyAsync处理一些数据然后保存到数据库的公共方法编写JUnit测试。然而，传递的supplier方法包含一个私有方法，该方法包含处理和保存数据到数据库的主要逻辑。我想在测试父公共方法的过程中测试这个私有方法。我不知道如何进行。
下面是示例代码：

public void updateGrowthOverviewData(int startId, int endId){
        int limit = 10;
        int currentStartId = startId;
        List<CompletableFuture<Boolean>> futureList = new ArrayList<>();
        while (currentStartId <= endId) {
            long currentEndId = currentStartId + limit - 1;
            long finalCurrentStartId = currentStartId;
            futureList.add(this.getFuture(() -> {
                try {
                    return processGrowthOverviewData(finalCurrentStartId, currentEndId);
                } catch (Exception ex) {
                    log.error("Exception in updateGrowthOverviewData | while fetching data for updateGrowthOverviewData | startId : {}, endId : {} | Exception: {}", finalCurrentStartId, currentEndId, ex.getStackTrace());
                    return false;
                }
            }));
            currentStartId += limit;
        }
        
        futureList.forEach(obj -> {
            try {
                log.info("in updateGrowthOverviewData| value: {}", obj.get());
            } catch (InterruptedException | ExecutionException ex) {
                log.error("Exception | in updateGrowthOverviewData | while fetching data from Future Object | Exception {}", ExceptionUtils.getStackTrace(ex));
            }
        });
    }
    
    private Boolean processGrowthOverviewData(int startId, int endId){
        // code for processing data and saving it into the db -> includes several other private methods -> for better code readability
        return true;
    }

private <T> CompletableFuture<T> getFuture(Supplier<T> supplier) {
        return CompletableFuture.supplyAsync(supplier, this.serviceExecutor);
    }

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我不想通过使this.getFuture()成为受保护的方法来模拟它，因为我将无法测试为更好的代码可读性而创建的私有方法。请帮助我如何测试方法updateGrowthOverviewData。我是编写UT的新手。