记录一下常用的用法

定义测试对象

	@Datapublic class Employee {//idprivate Integer id;//姓名private String name;//年龄private Integer age;//身高private Double height;//存款private BigDecimal deposit;public Employee(Integer id, String name, Integer age, Double height, BigDecimal deposit) {this.id = id;this.name = name;this.age = age;this.height = height;this.deposit = deposit;}}

1. list操作如排序、求和、获取最大最小值、累加等

     List<Employee> listOne = new ArrayList<>();listOne.add(new Employee(1, "张三", 12, 160.23, new BigDecimal("5000")));listOne.add(new Employee(2, "李四", 18, 182.56, new BigDecimal("2000.15")));listOne.add(new Employee(3, "王五", 15, 185.65, new BigDecimal("300")));listOne.add(new Employee(4, "周扒皮", 55, 165.55, new BigDecimal("180654.57")));listOne.add(new Employee(5, "独眼龙", 18, 195.55, new BigDecimal("154.12")));//根据年龄从小到大排序 升序List<Employee> ages1 = listOne.stream().sorted(Comparator.comparing(Employee::getAge)).collect(Collectors.toList());ages1.forEach(System.out::println);System.out.println("---------------------------------------------------------");//根据年龄从大到小排序 降序List<Employee> ages2 = listOne.stream().sorted(Comparator.comparing(Employee::getAge, Comparator.reverseOrder())).collect(Collectors.toList());ages2.forEach(System.out::println);System.out.println("---------------------------------------------------------");//根据年龄从大到小排序 降序  根据身高从大到小排序 降序(仅针对于当一个属性相等时才有效)List<Employee> ages3 = listOne.stream().sorted(Comparator.comparing(Employee::getAge, Comparator.reverseOrder()).thenComparing(Employee::getHeight, Comparator.reverseOrder())).collect(Collectors.toList());ages3.forEach(System.out::println);System.out.println("---------------------------------------------------------");//获取年龄之和int ages = listOne.stream().mapToInt(Employee::getAge).sum();//获取身高之和double heights = listOne.stream().mapToDouble(Employee::getHeight).sum();//全部的存款BigDecimal reduce = listOne.stream().map(Employee::getDeposit).reduce(BigDecimal.ZERO, BigDecimal::add);
   

2. list转map

     //根据id转map Employee::getId作为key的属性 e作为map的value  (v1,v2)->v2 当key不唯一时的操作Map<Integer, Employee> map1 = listOne.stream().collect(Collectors.toMap(Employee::getId, e -> e, (v1, v2) -> v2));//根据某个属性(年龄)分组Map<Integer, List<Employee>> map2 = listOne.stream().collect(Collectors.groupingBy(Employee::getAge));
   

3. 两个map合并

    	Map<Integer, Employee> map3 = new HashMap<>();map3.put(2, new Employee(2, "李四2", 20, 182.56, new BigDecimal("2000.15")));map3.put(3, new Employee(3, "王五2", 16, 182.99, new BigDecimal("2222.15")));map3.put(4, new Employee(4, "周扒皮2", 60, 164.55, new BigDecimal("154.57")));map3.put(6, new Employee(6, "赵六", 18, 182.56, new BigDecimal("2000.15")));//合并 (v1, v2)key相同时value的值 v1为map3的value v2为map1的valuemap1.forEach((key1, value1) -> map3.merge(key1, value1, (v1, v2) -> {System.out.println("v1:"+v1);System.out.println("v2:"+v2);v2.setName(v1.getName());v2.setDeposit(v1.getDeposit().add(v2.getDeposit()));return v2;}));System.out.println("---------------------------------------------------------");map3.forEach((key,value)-> System.out.println(key+":"+value));
   

4. 结果

   Employee(id=1, name=张三, age=12, height=160.23, deposit=5000)
   Employee(id=3, name=王五, age=15, height=185.65, deposit=300)
   Employee(id=2, name=李四, age=18, height=182.56, deposit=2000.15)
   Employee(id=5, name=独眼龙, age=18, height=195.55, deposit=154.12)
   Employee(id=4, name=周扒皮, age=55, height=165.55, deposit=180654.57)
   ---------------------------------------------------------
   Employee(id=4, name=周扒皮, age=55, height=165.55, deposit=180654.57)
   Employee(id=2, name=李四, age=18, height=182.56, deposit=2000.15)
   Employee(id=5, name=独眼龙, age=18, height=195.55, deposit=154.12)
   Employee(id=3, name=王五, age=15, height=185.65, deposit=300)
   Employee(id=1, name=张三, age=12, height=160.23, deposit=5000)
   ---------------------------------------------------------
   Employee(id=4, name=周扒皮, age=55, height=165.55, deposit=180654.57)
   Employee(id=5, name=独眼龙, age=18, height=195.55, deposit=154.12)
   Employee(id=2, name=李四, age=18, height=182.56, deposit=2000.15)
   Employee(id=3, name=王五, age=15, height=185.65, deposit=300)
   Employee(id=1, name=张三, age=12, height=160.23, deposit=5000)
   ---------------------------------------------------------
   v1:Employee(id=2, name=李四2, age=20, height=182.56, deposit=2000.15)
   v2:Employee(id=2, name=李四, age=18, height=182.56, deposit=2000.15)
   v1:Employee(id=3, name=王五2, age=16, height=182.99, deposit=2222.15)
   v2:Employee(id=3, name=王五, age=15, height=185.65, deposit=300)
   v1:Employee(id=4, name=周扒皮2, age=60, height=164.55, deposit=154.57)
   v2:Employee(id=4, name=周扒皮, age=55, height=165.55, deposit=180654.57)
   ---------------------------------------------------------
   1:Employee(id=1, name=张三, age=12, height=160.23, deposit=5000)
   2:Employee(id=2, name=李四2, age=18, height=182.56, deposit=4000.30)
   3:Employee(id=3, name=王五2, age=15, height=185.65, deposit=2522.15)
   4:Employee(id=4, name=周扒皮2, age=55, height=165.55, deposit=180809.14)
   5:Employee(id=5, name=独眼龙, age=18, height=195.55, deposit=154.12)
   6:Employee(id=6, name=赵六, age=18, height=182.56, deposit=2000.15)