List<String>转换成以逗号分隔的字符串
public static void convertListMapToStringJoiningComma(){
List<String> list = new ArrayList<>();
list.add("1");
list.add("2");
list.stream().map(String::valueOf).collect(Collectors.joining(","));
// 输出结果：1,2
}
List<Map>中的的某个key字段值转换成以逗号分隔的字符串
public static void convertListMapToStringJoiningComma(){
List<Map<String,String>> list = new ArrayList<>();
Map name1 = new HashMap();
name1.put("name","张三");
Map name2 = new HashMap();
name2.put("name","李四");
list.add(name1);
list.add(name2);
list.stream().map(name->name.get("name")).collect(Collectors.joining(","));
// 输出结果：张三,李四
}
将List<Map>中的某个key字段值相同的Map去重
方法1：
public static void distinctListMapByKey(){
List<Map<String, String>> list = new ArrayList<>() ;
Map<String,String> map1 = new HashMap<>();
map1.put("key1", "val1");
Map<String,String> map2 = new HashMap<>();
map2.put("key1", "val1");
Map<String,String> map3 = new HashMap<>();
map3.put("key1", "val2");
list.add(map1);
list.add(map2);
list.add(map3);
List<Map<String, String>> data = list.stream().collect(
Collectors.collectingAndThen(Collectors.toCollection(
() -> new TreeSet<>(Comparator.comparing(m -> m.get("key1")))), ArrayList::new));
System.out.println(data.toString());
// 输出结果：[{key1=val1}, {key1=val2}]
}
方法2：
public static void distinctListMapByKey(){
List<Map<String, String>> list = new ArrayList<>() ;
Map<String,String> map1 = new HashMap<>();
map1.put("key1", "val1");
Map<String,String> map2 = new HashMap<>();
map2.put("key1", "val1");
Map<String,String> map3 = new HashMap<>();
map3.put("key1", "val2");
list.add(map1);
list.add(map2);
list.add(map3);
List<Map<String, String>> data = list.stream().filter(distinctByKey(map->map.get("key1"))).collect(Collectors.toList());
System.out.println(data.toString());
// 输出结果：[{key1=val1}, {key1=val2}]
}
private static <T> Predicate<T> distinctByKey(Function<? super T, ?> keyExtractor) {
Set<Object> seen = ConcurrentHashMap.newKeySet();
return t -> seen.add(keyExtractor.apply(t));
}
将List<Map>中的某个key字段值相同的Map对象分组
public static void groupListMapByKey(){
List<Map<String, String>> list = new ArrayList<>() ;
Map<String,String> map1 = new HashMap<>();
map1.put("key1", "val1");
map1.put("key2", "val2");
map1.put("key3", "val3");
Map<String,String> map2 = new HashMap<>();
map2.put("key1", "val2");
map2.put("key2", "val2");
map2.put("key3", "val3");
Map<String,String> map3 = new HashMap<>();
map3.put("key1", "val2");
map3.put("key2", "val4");
map3.put("key3", "val4");
list.add(map1);
list.add(map2);
list.add(map3);
Map<String,List<Map>> mapGroup= list.stream().collect(Collectors.groupingBy(e -> e.get("key1").toString()));
System.out.println(mapGroup.toString());
// 输出结果：{val2=[{key1=val2, key2=val2, key3=val3}, {key1=val2, key2=val4, key3=val4}], val1=[{key1=val1, key2=val2, key3=val3}]}
}
提取List<Map>中的某个key字段值转换成List<String>并去重
public static void convertListMapToListWithDistinct (){
List<Map<String,String>> list = new ArrayList<>();
Map name1 = new HashMap();
name1.put("name","张三");
Map name2 = new HashMap();
name2.put("name","李四");
Map name3 = new HashMap();
name3.put("name","张三");
list.add(name1);
list.add(name2);
list.add(name3);
List<String> listResult = list.stream().map(project->project.get("name")).distinct().collect(Collectors.toList());
System.out.println(listResult);
// 输出结果：[张三, 李四]
}
对List<Map>中的某个key字段值求和
public static void listMapKeySum (){
List<Map<String,Integer>> list = new ArrayList<>();
Map name1 = new HashMap();
name1.put("sum",4);
Map name2 = new HashMap();
name2.put("sum",5);
Map name3 = new HashMap();
name3.put("sum",6);
list.add(name1);
list.add(name2);
list.add(name3);
int sum = list.stream().mapToInt(map->map.get("sum")).sum();
System.out.println(sum);
// 输出结果：15
}