Create an immutable collection
What is an immutable collection
- An immutable collection is a collection that cannot be modified.
- A collection's data items are provided at creation time and are immutable throughout their lifetime. Otherwise report an error
Why create immutable collections
- If a certain number cannot be modified, it is good practice to preventively copy it to an immutable collection
- Or the immutable form is safe when the collection object is not a trusted library utility
How to create immutable collections
- In the List, Set, and Map interfaces, there is an of method, which can create an immutable collection
public static void main(String[] args) {
//只需要对数据进行观看，并不需要修改数据
List<Double> lists= List.of(569.5,700.5,523.0,570.5);
//lists.add(12.0); 会报错 不可修改
System.out.println(lists);
//2.不可变的Set集合
Set<String> names= Set.of("迪丽热巴","迪丽热九");
System.out.println(names);
//2.不可变的Map集合
Map<String,Integer> maps= Map.of("迪丽热巴",1);
System.out.println(names);
}
Stream
What is Stream
Purpose: API for simplifying collection and array operations, combined with Lambda expressions
public static void main(String[] args) {
List<String> names=new ArrayList<>();
Collections.addAll(names,"张小红","小敏","笑话","张敏","张涛");
System.out.println(names);
//1.从集合中找出姓张的放到新集合
List<String> zhanglist=new ArrayList<>();
for(String name:names){
if(name.startsWith("张")){
zhanglist.add(name);
}
}
System.out. println(zhanglist);
//名称长度为3的
List<String> zhangThreelist=new ArrayList<>();
for(String name:zhanglist){
if(name.length()==3){
zhangThreelist.add(name);
}
}
System.out.println(zhangThreelist);
And using Stream one line of code can
names.stream().filter(s ->s.startsWith("张")).filter(s ->s.length()==3).forEach(s -> System.out.println(s));
The core of the stream thinking:
- First get the Stream stream of the collection or array (that is, a conveyor belt)
- put the element on
- Then use this streamlined API to conveniently manipulate elements.
Get the Stream stream
Get Stream stream
- Create a pipeline and put data on the pipeline to prepare for operation.
Intermediate method - operations on the pipeline. After one operation is completed, you can continue to perform other operations.
Termination method - A Stream can only have one termination method, which is the last operation on the pipeline. The
first step in a Stream operation collection or array is to obtain the Stream first, and then use the function
set of the Stream to obtain the Stream. - You can use the default method stream() in the Collaboration interface to produce stream
arrays to obtain the Stream stream
//Collection集合获取流
Collection<String> list=new ArrayList<>();
Stream<String> s=list.stream();
//Map集合获取流
Map<String,Integer> maps=new HashMap<>();
//键流
Stream<String> keyStream=maps.keySet().stream();
//值流
Stream<Integer> valueStream=maps.values().stream();
//键值对流（拿整体）
Stream<Map.Entry<String,Integer>> keyAndValueStream=maps.entrySet().stream();
//数组获取流
String[] names={
"赵敏","小昭","灭绝","周芷若"};
//拿到Stream流方法一
Stream<String> nameStream= Arrays.stream(names);
//方法二
Stream<String> nameStream2=Stream.of(names);
Stream common API (intermediate operation method)
- The intermediate method is also called a non-terminal method. After the call is completed, the returned Stream can continue to be used, and chain programming is supported.
- Data in collections and arrays cannot be directly modified in Stream.
Common terminal operation methods in Stream
Note: After the operation method is terminated, the stream cannot be used after the call is completed, because the Stream will not be returned
Comprehensive Application of Stream
package com.itheima.stream_test;
import java.math.BigDecimal;
import java.math.RoundingMode;
import java.util.ArrayList;
import java.util.List;
import java.util.stream.Stream;
public class StreamDemo04 {
public static double allMoney ;
public static double allMoney2 ; // 2个部门去掉最高工资，最低工资的总和
public static void main(String[] args) {
List<Employee> one = new ArrayList<>();
one.add(new Employee("猪八戒",'男',30000 , 25000, null));
one.add(new Employee("孙悟空",'男',25000 , 1000, "顶撞上司"));
one.add(new Employee("沙僧",'男',20000 , 20000, null));
one.add(new Employee("小白龙",'男',20000 , 25000, null));
List<Employee> two = new ArrayList<>();
two.add(new Employee("武松",'男',15000 , 9000, null));
two.add(new Employee("李逵",'男',20000 , 10000, null));
two.add(new Employee("西门庆",'男',50000 , 100000, "被打"));
two.add(new Employee("潘金莲",'女',3500 , 1000, "被打"));
two.add(new Employee("武大郎",'女',20000 , 0, "下毒"));
// 1、开发一部的最高工资的员工。（API）
// 指定大小规则了
// Employee e = one.stream().max((e1, e2) -> Double.compare(e1.getSalary() + e1.getBonus(), e2.getSalary() + e2.getBonus()))
// .get();
// System.out.println(e);
Topperformer t = one.stream().max((e1, e2) -> Double.compare(e1.getSalary() + e1.getBonus(), e2.getSalary() + e2.getBonus()))
.map(e -> new Topperformer(e.getName(), e.getSalary() + e.getBonus())).get();
System.out.println(t);
// 2、统计平均工资，去掉最高工资和最低工资
one.stream().sorted((e1, e2) -> Double.compare(e1.getSalary() + e1.getBonus(), e2.getSalary() + e2.getBonus()))
.skip(1).limit(one.size() - 2).forEach(e -> {
// 求出总和：剩余员工的工资总和
allMoney += (e.getSalary() + e.getBonus());
});
System.out.println("开发一部的平均工资是：" + allMoney / (one.size() - 2));
// 3、合并2个集合流，再统计
Stream<Employee> s1 = one.stream();
Stream<Employee> s2 = two.stream();
Stream<Employee> s3 = Stream.concat(s1 , s2);
s3.sorted((e1, e2) -> Double.compare(e1.getSalary() + e1.getBonus(), e2.getSalary() + e2.getBonus()))
.skip(1).limit(one.size() + two.size() - 2).forEach(e -> {
// 求出总和：剩余员工的工资总和
allMoney2 += (e.getSalary() + e.getBonus());
});
// BigDecimal
BigDecimal a = BigDecimal.valueOf(allMoney2);
BigDecimal b = BigDecimal.valueOf(one.size() + two.size() - 2);
System.out.println("开发部的平均工资是：" + a.divide(b,2, RoundingMode.HALF_UP));
}
}
package com.itheima.stream_test;
public class Employee {
private String name;
private char sex;
private double salary;
private double bonus;
private String punish; // 处罚信息
public Employee(){
}
public Employee(String name, char sex, double salary, double bonus, String punish) {
this.name = name;
this.sex = sex;
this.salary = salary;
this.bonus = bonus;
this.punish = punish;
}
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
public char getSex() {
return sex;
}
public void setSex(char sex) {
this.sex = sex;
}
public double getSalary() {
return salary;
}
public void setSalary(double salary) {
this.salary = salary;
}
public double getBonus() {
return bonus;
}
public void setBonus(double bonus) {
this.bonus = bonus;
}
public String getPunish() {
return punish;
}
public void setPunish(String punish) {
this.punish = punish;
}
public double getTotalSalay(){
return salary * 12 + bonus;
}
@Override
public String toString() {
return "Employee{" +
"name='" + name + '\'' +
", sex=" + sex +
", salary=" + salary +
", bonus=" + bonus +
", punish='" + punish + '\'' +
'}'+"\n";
}
}
package com.itheima.stream_test;
public class Topperformer {
private String name;
private double money; // 月薪
public Topperformer() {
}
public Topperformer(String name, double money) {
this.name = name;
this.money = money;
}
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
public double getMoney() {
return money;
}
public void setMoney(double money) {
this.money = money;
}
@Override
public String toString() {
return "Topperformer{" +
"name='" + name + '\'' +
", money=" + money +
'}';
}
}