超详细利用Lambda处理List集合
遍历
public static void main(String[] args) {
List userList = Lists.newArrayList();
User user1 = new User(1L, "张三", 24);
User user2 = new User(2L, "李四", 27);
User user3 = new User(3L, "王五", 21);
userList.add(user1);
userList.add(user2);
userList.add(user3);
userList.stream().forEach(user ->{
System.out.println(user.getName());
});
} List转为Map
常用：
public Map getIdNameMap(List accounts) {
return accounts.stream().collect(Collectors.toMap(Account::getId, Account::getUsername));//第一个参数为map键值对中的key，第二个为value
} 或
public Map getIdNameMap(List accounts) {
return accounts.stream().collect(Collectors.toMap(a->a.getId(), a->a.getUsername()));//第一个参数为map键值对中的key，第二个为value
} a为形参名
将自身对象作为value
public Map getIdAccountMap(List 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，account为形参，可任意替换，还可以使用Function接口中的一个默认方法代替，使整个方法更简洁优雅：
public Map getIdAccountMap(List accounts) {
return accounts.stream().collect(Collectors.toMap(Account::getId, Function.identity()));
} key值重复问题
上述方法可能报错（java.lang.IllegalStateException: Duplicate key），因为name是有可能重复的。toMap有个重载方法，可以传入一个合并的函数来解决key冲突问题：
public Map getNameAccountMap(List accounts) {
return accounts.stream().collect(Collectors.toMap(Account::getUsername, Function.identity(), (key1, key2) -> key2));
} (key1,key2)->key2的意思为，当key1,key2两个键值对的key值重复时，保留第二个，->key1则保留第一个，同样key1,key2只是形参名
完整代码
public class Test{
public static void main(String[] args){
List userList = new ArrayList();//存放user对象集合
User user1 = new User(1L, "张三", 24);
User user2 = new User(2L, "李四", 27);
User user3 = new User(3L, "王五", 21);
User user4 = new User(3L, "aaa",33);
userList.add(user1);
userList.add(user2);
userList.add(user3);
userList.add(user4);
//ID为key，转为Map
Map userMap = userList.stream().collect(Collectors.toMap(User::getId, Function.identity(),(k1, k2)->k1));
for (Entry entry : userMap.entrySet()) {
System.out.println("key = " + entry.getKey() + ", value = " + entry.getValue().getName());
}
}
}
class User{
long id;
String name;
int age;
public User(long a, String b, int c) {
this.id = a;
this.name = b;
this.age = c;
}
public long getId() {
return id;
}
public String getName() {
return name;
}
public int getAge() {
return age;
}
} List中以对象某个属性分组
public static void main(String[] args) {
List userList = Lists.newArrayList();//存放user对象集合
User user1 = new User(1L, "张三", 24);
User user2 = new User(2L, "李四", 27);
User user3 = new User(3L, "王五", 21);
User user4 = new User(4L, "张三", 22);
User user5 = new User(5L, "李四", 20);
User user6 = new User(6L, "王五", 28);
userList.add(user1);
userList.add(user2);
userList.add(user3);
userList.add(user4);
userList.add(user5);
userList.add(user6);
//根据name来将userList分组
Map> groupBy = userList.stream().collect(Collectors.groupingBy(User::getName));
System.out.println(groupBy);
} 过滤：从List中过滤出符合条件的元素
public static void main(String[] args) {
List userList = Lists.newArrayList();//存放user对象集合
User user1 = new User(1L, "张三", 24);
User user2 = new User(2L, "李四", 27);
User user3 = new User(3L, "王五", 21);
User user4 = new User(4L, "张三", 22);
User user5 = new User(5L, "李四", 20);
User user6 = new User(6L, "王五", 28);
userList.add(user1);
userList.add(user2);
userList.add(user3);
userList.add(user4);
userList.add(user5);
userList.add(user6);
//取出名字为张三的用户
List filterList = userList.stream().filter(user -> user.getName().equals("张三")).collect(Collectors.toList());
filterList.stream().forEach(user ->{
System.out.println(user.getName());
});
} 求和：将List中的数据按某个属性求和
public static void main(String[] args) {
List userList = Lists.newArrayList();//存放user对象集合
User user1 = new User(1L, "张三", 24);
User user2 = new User(2L, "李四", 27);
User user3 = new User(3L, "王五", 21);
User user4 = new User(4L, "张三", 22);
User user5 = new User(5L, "李四", 20);
User user6 = new User(6L, "王五", 28);
userList.add(user1);
userList.add(user2);
userList.add(user3);
userList.add(user4);
userList.add(user5);
userList.add(user6);
//取出名字为张三的用户
int totalAge = userList.stream().mapToInt(User::getAge).sum();
System.out.println("和：" + totalAge);
} 实战：计算标准差
类中存在一个 public ArrayList
public double getStandard_Deviation() {
// 精度控制
DecimalFormat df = new DecimalFormat();
String style = "0.#####";// 定义要显示的数字的格式
df.applyPattern(style);// 将格式应用于格式化器
// 计算标准差
int numOfCell = classifier_cell.size();
if (numOfCell == 1) {
return 0;
} else {
double sum = 0;//和
double nvariance = 0;//n倍方差
for (int j = 0; j < numOfCell; j++) {//求和
sum += classifier_cell.get(j);
}
double average = sum / numOfCell;//算平均值
for (int j = 0; j < numOfCell; j++) {//算n倍方差
nvariance += Math.pow((classifier_cell.get(j) - average), 2);
}
double standardDeviation = Math.sqrt(nvariance / numOfCell);//求标准差
return Double.parseDouble(df.format(standardDeviation));
}
}改为lambda
public double getStandard_Deviation() {
// 精度控制
DecimalFormat df = new DecimalFormat();
String style = "0.#####";// 定义要显示的数字的格式
df.applyPattern(style);// 将格式应用于格式化器
// 计算标准差
int numOfCell = classifier_cell.size();
if (numOfCell == 1) {
return 0;
} else {
double average = classifier_cell.stream().mapToDouble(num->num).sum() / numOfCell;//平均数
double variance = classifier_cell.stream().mapToDouble(num->Math.pow(num-average,2)).sum() / numOfCell;//方差
double standardDeviation = Math.sqrt(variance);//标准差
return Double.parseDouble(df.format(standardDeviation));
}
}