要按文档字段分组并统计出现次数，可以使用Elasticsearch的聚合功能。以下是一个包含代码示例的解决方案：

首先，你需要创建一个索引并插入一些示例数据：

// 创建索引
CreateIndexRequest request = new CreateIndexRequest("my_index");
client.indices().create(request, RequestOptions.DEFAULT);

// 插入示例数据
IndexRequest indexRequest1 = new IndexRequest("my_index").id("1").source("field", "A", "title", "Document A");
IndexRequest indexRequest2 = new IndexRequest("my_index").id("2").source("field", "B", "title", "Document B");
IndexRequest indexRequest3 = new IndexRequest("my_index").id("3").source("field", "A", "title", "Document C");
client.index(indexRequest1, RequestOptions.DEFAULT);
client.index(indexRequest2, RequestOptions.DEFAULT);
client.index(indexRequest3, RequestOptions.DEFAULT);

然后，你可以使用聚合查询来按字段分组并统计出现次数：

// 构建聚合查询
SearchRequest searchRequest = new SearchRequest("my_index");
SearchSourceBuilder searchSourceBuilder = new SearchSourceBuilder();
TermsAggregationBuilder aggregation = AggregationBuilders.terms("group_by_field").field("field");
searchSourceBuilder.aggregation(aggregation);
searchRequest.source(searchSourceBuilder);

// 执行查询
SearchResponse searchResponse = client.search(searchRequest, RequestOptions.DEFAULT);

// 解析聚合结果
Terms terms = searchResponse.getAggregations().get("group_by_field");
for (Terms.Bucket bucket : terms.getBuckets()) {
    String fieldValue = bucket.getKeyAsString();
    long count = bucket.getDocCount();
    System.out.println("Field: " + fieldValue + ", Count: " + count);
}

这将打印出每个字段值和对应的出现次数：

Field: A, Count: 2
Field: B, Count: 1

这是一个简单的示例，你可以根据自己的需求调整查询条件和结果解析方式。同时，你也可以使用其他类型的聚合（如sum、avg、max等）来执行更复杂的统计操作。