TreeMap通过值对象属性排序

和技术人员讨论TreeMap通过值对象排序的方法，记录一下

现在实现通过ReadData对象中的itemNo属性实现排序

public class ReadData {private String itemCatagory;private String itemNo;private String itemName;public String getItemCatagory() {return itemCatagory;}public void setItemCatagory(String itemCatagory) {this.itemCatagory = itemCatagory;}public String getItemNo() {return itemNo;}public void setItemNo(String itemNo) {this.itemNo = itemNo;}public String getItemName() {return itemName;}public void setItemName(String itemName) {this.itemName = itemName;}
}

第一种方法，初学者常写的方法，定义Comparator比较器，中间经过集合转换实现

public class DemoTest {public static void main(String[] args) {TreeMap<String,ReadData> treeMap = new TreeMap<String,ReadData>();ReadData readData1 = new ReadData();ReadData readData2 = new ReadData();readData1.setItemNo("002");readData2.setItemNo("001");treeMap.put("111",readData1);treeMap.put("222",readData2);Map<String, ReadData> resultMap = sortMapByValue(treeMap); //按Value进行排序for (Map.Entry<String, ReadData> entry : resultMap.entrySet()) {System.out.println(entry.getKey() + " " + entry.getValue().getItemNo());}}public static Map<String,ReadData> sortMapByValue(Map<String,ReadData> oriMap){if (oriMap == null || oriMap.isEmpty()) {return null;}Map<String, ReadData> sortedMap = new LinkedHashMap<String, ReadData>();List<Map.Entry<String, ReadData>> entryList = new ArrayList<Map.Entry<String, ReadData>>(oriMap.entrySet());Collections.sort(entryList, new MapValueComparator());Iterator<Map.Entry<String, ReadData>> iter = entryList.iterator();Map.Entry<String, ReadData> tmpEntry = null;while (iter.hasNext()) {tmpEntry = iter.next();sortedMap.put(tmpEntry.getKey(), tmpEntry.getValue());}return sortedMap;}
}class MapValueComparator implements Comparator<Map.Entry<String, ReadData>>{@Overridepublic int compare(Map.Entry<String, ReadData> me1, Map.Entry<String, ReadData> me2) {return me1.getValue().getItemNo().compareTo(me2.getValue().getItemNo());}
}

第二种方法，通过集合的stream流结合Comparator比较器实现（java8高级特性应用半成品）

public class DemoTest {public static void main(String[] args) {TreeMap<String,ReadData> treeMap = new TreeMap<String,ReadData>();ReadData readData1 = new ReadData();ReadData readData2 = new ReadData();readData1.setItemNo("002");readData2.setItemNo("001");treeMap.put("111",readData1);treeMap.put("222",readData2);Map<String, ReadData> sortedMap = treeMap.entrySet().stream().sorted(new MapValueComparator()).collect(Collectors.toMap(Map.Entry::getKey,Map.Entry::getValue,(oldVal,newVal) -> oldVal,LinkedHashMap::new));sortedMap.entrySet().forEach(System.out::println);}class MapValueComparator implements Comparator<Map.Entry<String, ReadData>>{@Overridepublic int compare(Map.Entry<String, ReadData> me1, Map.Entry<String, ReadData> me2) {return me1.getValue().getItemNo().compareTo(me2.getValue().getItemNo());}
}

第三种方法，和方法二类似

public class DemoTest {public static void main(String[] args) {TreeMap<String,ReadData> treeMap = new TreeMap<String,ReadData>();ReadData readData1 = new ReadData();ReadData readData2 = new ReadData();readData1.setItemNo("002");readData2.setItemNo("001");treeMap.put("111",readData1);treeMap.put("222",readData2);Map<String, ReadData> map =  new LinkedHashMap<>();treeMap.entrySet().stream().sorted(new MapValueComparator()).forEachOrdered(e -> map.put(e.getKey(),e.getValue()));map.entrySet().forEach(System.out::println);}
}class MapValueComparator implements Comparator<Map.Entry<String, ReadData>>{@Overridepublic int compare(Map.Entry<String, ReadData> me1, Map.Entry<String, ReadData> me2) {return me1.getValue().getItemNo().compareTo(me2.getValue().getItemNo());}
}

第四种方法，完全使用stream与functional方法实现，只需一行代码（java8高级特性应用成品）

public class DemoTest {public static void main(String[] args) {TreeMap<String,ReadData> treeMap = new TreeMap<String,ReadData>();ReadData readData1 = new ReadData();ReadData readData2 = new ReadData();readData1.setItemNo("002");readData2.setItemNo("001");treeMap.put("111",readData1);treeMap.put("222",readData2);Map<String, ReadData> map =  new LinkedHashMap<>();treeMap.entrySet().stream().sorted(Comparator.comparing(ReadData -> ReadData.getValue().getItemNo())).forEachOrdered(e -> map.put(e.getKey(),e.getValue()));map.entrySet().forEach(System.out::println);}
}

结论，java8高级特性真香！

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