在应用开发的时候，我们有时候会遇到需要对一个给定集合进行分组的情况。如果该集合的数据记录是从数据库的获取，通常我们最简单的方案是直接在sql层面select后group by完成。 但是，假如某些场景下该数据不是从数据库获取呢？那就需要在应用程序层面进行分组。
假设给定一个集合，{A:5,A:4,A:3,B:6,B:7}， 需要对其进行分组，使其结果是 {A:[5,4,3],B:[6,7]}。
下面是常规的写法：
//1)对集合进行分组 public Map<String,List<TspPefScoreCalTotal>> groupListByFields(List<TspPefScoreCalTotal> list){ Map<String,List<TspPefScoreCalTotal>> resultMap = new HashMap<String,List<TspPefScoreCalTotal>>(); String groupKey = null; for(TspPefScoreCalTotal obj : list){ groupKey = formatGroupKeyByFields(obj); if (resultMap.containsKey(groupKey)) { resultMap.get(groupKey).add(obj); } else { List<TspPefScoreCalTotal> pefScoreCalTotalList = new ArrayList<TspPefScoreCalTotal>(); pefScoreCalTotalList.add(obj); resultMap.put(groupKey, pefScoreCalTotalList); } } return resultMap; } //2)构造分组的关键字 private String formatGroupKeyByFields(TspPefScoreCalTotal obj) { return new StringBuilder().append(obj.getBuyerCompanyId()).append("-").append(obj.getOrganizationId()).toString(); }上面的例子1）方法就hardcode了List集合里的对象是TspPefScoreCalTotal，这样就使得程序不具有通用性。试试想，下一次是其它数据对象呢？那岂不是重复写上面相似的代码？ 还有，2）方法里的分组的关键字随业务变化而调整呢？ 那岂不是要改方法里的组合字段？
基于Java设计模式的开放封闭原则。如果要写通用程序，尝试使用java的反射机制，使其可以让调用者输入任意的对象，且可以任意指定按照哪些字段进行分组。下面给出具体实现例子。
public class GroupUtil<T> { private String formatGroupKeyByFields(T obj, List<String> fields) { StringBuilder strBu = new StringBuilder(); for(String field : fields){ strBu.append(getFieldValueByName(field,obj)).append("-"); } return strBu.toString(); } public Map<String,List<T>> groupListByFields(List<T> list, Class ObjClazz, List<String> fields){ Map<String,List<T>> resultMap = new HashMap<String,List<T>>(); String groupKey = null; for(T obj : list){ groupKey = formatGroupKeyByFields(obj,fields); if (resultMap.containsKey(groupKey)) { resultMap.get(groupKey).add(obj); } else { List<T> objectList = new ArrayList<T>(); objectList.add(obj); resultMap.put(groupKey, objectList); } } return resultMap; } /** * 根据属性名获取属性值 * */ public Object getFieldValueByName(String fieldName, Object o) { try { String firstLetter = fieldName.substring(0, 1).toUpperCase(); String getter = "get" + firstLetter + fieldName.substring(1); Method method = o.getClass().getMethod(getter, new Class[] {}); Object value = method.invoke(o, new Object[] {}); return value; } catch (Exception e) { e.printStackTrace(); return null; } } /** * @param args */ public static void main(String[] args) { //构造测试例子start GroupUtil<TspPefScoreCalTotal> testGroup = new GroupUtil<TspPefScoreCalTotal>(); TspPefScoreCalTotal t1 = new TspPefScoreCalTotal(); TspPefScoreCalTotal t2 = new TspPefScoreCalTotal(); TspPefScoreCalTotal t3 = new TspPefScoreCalTotal(); TspPefScoreCalTotal t4 = new TspPefScoreCalTotal(); t1.setPefTotalScoreId(1L); t1.setBuyerCompanyId(1L); t1.setOrganizationId(9L); t1.setTotalScore(10L); t2.setPefTotalScoreId(2L); t2.setBuyerCompanyId(2L); t2.setOrganizationId(9L); t2.setTotalScore(5L); t3.setPefTotalScoreId(3L); t3.setBuyerCompanyId(1L); t3.setOrganizationId(9L); t3.setTotalScore(20L); t4.setPefTotalScoreId(4L); t4.setBuyerCompanyId(1L); t4.setOrganizationId(9L); t4.setTotalScore(20L); List<TspPefScoreCalTotal> list = new ArrayList<TspPefScoreCalTotal>(); list.add(t1); list.add(t2); list.add(t3); list.add(t4); //构造测试例子end System.out.println("===================通用分组======================="); List<String> fields = new LinkedList<String>(); fields.add("buyerCompanyId");//指定分组字段 fields.add("organizationId");//指定分组字段 Map<String, List<TspPefScoreCalTotal>> rstMap2 = testGroup.groupListByFields(list,TspPefScoreCalTotal.class,fields); for( Entry<String,List<TspPefScoreCalTotal>> entryTemp : rstMap2.entrySet()){ System.out.println(entryTemp.getKey()); for(TspPefScoreCalTotal obj : entryTemp.getValue()){ System.out.println(testGroup.getFieldValueByName("pefTotalScoreId", obj)); } } } } public class TspPefScoreCalTotal { private Long pefTotalScoreId; //主键ID private Long totalScore; //得分 private Long totalScoreRanking; //排列名次 private Long buyerCompanyId; //公司 private Long organizationId; //工厂 public Long getPefTotalScoreId() { return this.pefTotalScoreId; } public void setPefTotalScoreId (Long pefTotalScoreId) { this.pefTotalScoreId = pefTotalScoreId; } public Long getTotalScore() { return this.totalScore; } public void setTotalScore (Long totalScore) { this.totalScore = totalScore; } public Long getTotalScoreRanking() { return this.totalScoreRanking; } public void setTotalScoreRanking (Long totalScoreRanking) { this.totalScoreRanking = totalScoreRanking; } public Long getBuyerCompanyId() { return this.buyerCompanyId; } public void setBuyerCompanyId (Long buyerCompanyId) { this.buyerCompanyId = buyerCompanyId; } public Long getOrganizationId() { return this.organizationId; } public void setOrganizationId (Long organizationId) { this.organizationId = organizationId; } }当然，上面的程序还没有考虑对于大数据量时候的分组处理是否有性能问题。下回找个时间测试一下。
