java中List转Map以及map遍历的四种方式
原文地址:https://www.cnblogs.com/damoblog/p/9124937.html
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Java8List转map分组
Java8List转map分组 此处是根据名称作为key 分组
public Map<String, List<Student>> groupList(List<Student> students) {
Map<String, List<Student>> map = students.stream().collect(Collectors.groupingBy(Student::getName));
return map;
}
在java中所有的map都实现了Map接口，因此所有的Map（如HashMap, TreeMap, LinkedHashMap, Hashtable等）都可以用以下的方式去遍历。
方法一：在for循环中使用entries实现Map的遍历：
/**
* 最常见也是大多数情况下用的最多的，一般在键值对都需要使用
*/
Map <String,String>map = new HashMap<String,String>();
map.put("熊大", "棕色");
map.put("熊二", "黄色");
for(Map.Entry<String, String> entry : map.entrySet()){
String mapKey = entry.getKey();
String mapValue = entry.getValue();
System.out.println(mapKey+":"+mapValue);
}
方法二：在for循环中遍历key或者values，一般适用于只需要map中的key或者value时使用，在性能上比使用entrySet较好；
Map <String,String>map = new HashMap<String,String>();
map.put("熊大", "棕色");
map.put("熊二", "黄色");
//key
for(String key : map.keySet()){
System.out.println(key);
}
//value
for(String value : map.values()){
System.out.println(value);
}
方法三：通过Iterator遍历:
Iterator<Entry<String, String>> entries = map.entrySet().iterator();
while(entries.hasNext()){
Entry<String, String> entry = entries.next();
String key = entry.getKey();
String value = entry.getValue();
System.out.println(key+":"+value);
}
方法四：通过键找值遍历，这种方式的效率比较低，因为本身从键取值是耗时的操作:
for(String key : map.keySet()){
String value = map.get(key);
System.out.println(key+":"+value);
}
list转map在Java8中stream的应用
- 利用Collectors.toMap方法进行转换 <k,v>
public Map<Long, String> getIdNameMap(List<Account> accounts) {
//key不会重复时候
return accounts.stream().collect(Collectors.toMap(Account::getId, Account::getUsername));
//key重复的时
return accounts.stream().collect(Collectors.toMap(Account::getId, Account::getUsername, (key1, key2) -> key2));
}
- 收集对象实体本身
- 在开发过程中我们也需要有时候对自己的list中的实体按照其中的一个字段进行分组（比如 id ->List）这时候要设置map的value值是实体本身。
public Map<Long, Account> getIdAccountMap(List<Account> accounts) {
return accounts.stream().collect(Collectors.toMap(Account::getId, account -> account));
}
account -> account是一个返回本身的lambda表达式，其实还可以使用Function接口中的一个默认方法 Function.identity()，这个方法返回自身对象，更加简洁
重复key的情况。在list转为map时，作为key的值有可能重复，这时候流的处理会抛出个异常：Java.lang.IllegalStateException:Duplicate key。这时候就要在toMap方法中指定当key冲突时key的选择。(这里是选择第二个key覆盖第一个key)
public Map<String, Account> getNameAccountMap(List<Account> accounts) {
return accounts.stream().collect(Collectors.toMap(Account::getUsername, Function.identity(), (key1, key2) -> key2));
}
用groupingBy 或者 partitioningBy进行分组 根据一个字段或者属性分组也可以直接用groupingBy方法，很方便
Map<Integer, List<Person>> personGroups = Stream.generate(new PersonSupplier()).
limit(100).
collect(Collectors.groupingBy(Person::getAge));
Iterator it = personGroups.entrySet().iterator();
while (it.hasNext()) {
Map.Entry<Integer, List<Person>> persons = (Map.Entry) it.next();
System.out.println("Age " + persons.getKey() + " = " + persons.getValue().size());
}
统计
Map<Stirng,Long> overflowMap = ?
if (CommonUtil.isNotEmpty(overflow.getList())){
overflowMap =overflow.getList().stream().collect(Collectors.groupingBy(SmartDeviceDTO::getNzbm, Collectors.counting()));
}
BigDdecimal 的求和
BigDecimal result2 = userList.stream()
// 将user对象的age取出来map为Bigdecimal
.map(User::getAge)
// 使用reduce()聚合函数,实现累加器
.reduce(BigDecimal.ZERO,BigDecimal::add);
获取json中的key
/**
* 递归读取所有的key
*
* @param jsonObject
*/
public static StringBuffer getAllKey(JSONObject jsonObject) {
StringBuffer stringBuffer = new StringBuffer();
Iterator<String> keys = jsonObject.keySet().iterator();// jsonObject.keys();
while (keys.hasNext()) {
String key = keys.next();
stringBuffer.append(key.toString()).append("|");
if (jsonObject.get(key) instanceof JSONObject) {
JSONObject innerObject = (JSONObject) jsonObject.get(key);
stringBuffer.append(getAllKey(innerObject));
} else if (jsonObject.get(key) instanceof JSONArray) {
JSONArray innerObject = (JSONArray) jsonObject.get(key);
stringBuffer.append(getAllKey(innerObject));
}
}
return stringBuffer;
}
java 遍历Map及Map转化为二维数组的实例
int a = 0, b = 0, c = 0;
// 第一种:通过Map.keySet()遍历Map及将Map转化为二维数组
Map<String, String> map1 = new HashMap<String, String>();
map1.put("012013012013", "张三");
map1.put("012013012014", "张四");
String[][] group1 = new String[map1.size()][2];
System.out.println("第一种:通过Map.keySet()遍历map1的key和value");
for (String key : map1.keySet()) {
System.out.println("key = " + key + " and value = " + map1.get(key));
group1[a][0] = key;
group1[a][1] = map1.get(key);
a++;
}
System.out.println("map1.size()为：" + map1.size() + ",a为：" + a + "，group1数组的长度为：" + group1.length);
System.out.println("----------------------------------------------------");
for(int n = 0; n < group1.length; n++) {
System.out.println("key = " + group1[n][0] + " and value = " + group1[n][1]);
}
// 第二种:通过Map.entrySet()使用iterator()遍历Map及将Map转化为二维数组
Map<String, String> map2 = new HashMap<String, String>();
map2.put("112013012013", "李三");
map2.put("112013012014", "李四");
System.out.println("
" + "第二种:通过Map.entrySet()使用iterator()遍历map2的key和value");
Iterator<Map.Entry<String, String>> iterator = map2.entrySet().iterator();
String[][] group2 = new String[map2.size()][2];
while (iterator.hasNext()) {
Map.Entry<String, String> entry = iterator.next();
System.out.println("key = " + entry.getKey() + " and value = " + entry.getValue());
group2[b][0] = entry.getKey();
group2[b][1] = entry.getValue();
b++;
}
System.out.println("map2.size()为：" + map2.size() + ",b为：" + b + "，group2数组的长度为：" + group2.length);
System.out.println("----------------------------------------------------");
for(int n = 0; n < group2.length; n++) {
System.out.println("key = " + group2[n][0] + " and value = " + group2[n][1]);
}
// 第三种:通过Map.entrySet()遍历遍历Map及将Map转化为二维数组
Map<String, String> map = new HashMap<String, String>();
map.putAll(map1);
map.putAll(map2);
String[][] group3 = new String[map.size()][2];
System.out.println("
" + "第三种:通过Map.entrySet()遍历map的key和value ");
for (Map.Entry<String, String> entry : map.entrySet()) {
System.out.println("key = " + entry.getKey() + " and value = " + entry.getValue());
group3[c][0] = entry.getKey();
group3[c][1] = entry.getValue();
c++;
}
System.out.println("map.size()为：" + map.size() + ",c为：" + c + "，group3数组的长度为：" + group3.length);
System.out.println("----------------------------------------------------");
for(int n = 0; n < group3.length; n++) {
System.out.println("key = " + group3[n][0] + " and value = " + group3[n][1]);
}
}
}
多字段分组
public static void main(String[] args) {
User user1 = new User("zhangsan", "beijing", 10);
User user2 = new User("zhangsan", "beijing", 20);
User user3 = new User("lisi", "shanghai", 30);
List<User> list = new ArrayList<User>();
list.add(user1);
list.add(user2);
list.add(user3);
Map<String, List<User>> collect = list.stream().collect(Collectors.groupingBy(e -> fetchGroupKey(e)));
//{zhangsan#beijing=[User{age=10, name='zhangsan', address='beijing'}, User{age=20, name='zhangsan', address='beijing'}],
// lisi#shanghai=[User{age=30, name='lisi', address='shanghai'}]}
System.out.println(collect);
}
private static String fetchGroupKey(User user){
return user.getName() +"#"+ user.getAddress();
}
JSONObject与对象互转
//Javabean对象转换成String类型的JSON字符串
JSONObject.toJSONString(Javabean对象)
//String类型的JSON字符串转换成Javabean对象
JSONObject.toJavaObject(JSON字符串,Javabean.class)
//Json字符串转换成JSONObject对象
JSONObject.parseObject(JSON字符串)
//JSON字符串转换成Javabean对象
JSONObject.parseObject(JSON字符串,Javabean.class)
例如
Refund r = new Refund();
String jsonStr = JSONObject.toJSONString(r);
String jsonStr = "{"msg":"ZhangSan"}";
Refund r = JSONObject.toJavaObject(jsonStr,Refund.class);
JSONObject jsonObject = JSONObject.parseObject(jsonStr);
Refund r = JSONObject.parseObject(jsonStr,Refund.class);