java 8 ????????????Stream?????????/??????

Posted 2021-01-25

?????????reac   ??????   ring   ted   Collector   col   equals   ?????????   strong   

 

Stream??????

Stream???Java8?????????????????????API????????????java.util.stream?????????Stream API?????????????????????????????????Java?????????????????????????????????????????????????????????Java?????????????????????????????????????????????????????????????????????????????????
????????????????????????????????????????????? ???????????????????????? ???????????????????????????????????????????????? ??????????????? ????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????

Stream??????

• ????????????

class Employee {
    private Long empno; //?????????
    private String ename; //????????????
    private Integer salary; //??????
    private Integer deptno; //???????????????
    //????????????get/set???????????????????????????toString??????
}
Employee e1 = new Employee(7369L, "SMITH", 800, 20);
Employee e2 = new Employee(7499L, "ALLEN", 1600, 30);
Employee e3 = new Employee(7521L, "WARD", 1250, 30);
Employee e4 = new Employee(7782L, "CLARK", 2450, 10);
Employee e5 = new Employee(7876L, "ADAMS", 1100, 20);

List<Employee> employees = Arrays.asList(e1, e2, e3, e4, e5);

• forEach??????
  forEach??????????????????stream????????????????????????

employees.stream().forEach(System.out::println);

???????????????

Employee{empno=7369, ename=???SMITH???, salary=800, deptno=20}
Employee{empno=7499, ename=???ALLEN???, salary=1600, deptno=30}
Employee{empno=7521, ename=???WARD???, salary=1250, deptno=30}
Employee{empno=7782, ename=???CLARK???, salary=2450, deptno=10}
Employee{empno=7876, ename=???ADAMS???, salary=1100, deptno=20}

• map??????
  map???????????????????????????????????????stream????????????????????????????????????

//???????????????????????????
List<String> enames = employees.stream().map(employee -> employee.getEname()).collect(Collectors.toList());
enames.stream().forEach(System.out::println);

???????????????

SMITH
ALLEN
WARD
CLARK
ADAMS

• mapToInt/mapToLong/mapToDouble??????
  ??????????????????????????????stream??????????????????????????????????????????

 //?????????????????????????????????
int totalSalary = employees.stream().mapToInt(employee -> employee.getSalary()).sum();
System.out.println("???????????????" + totalSalary);

???????????????

???????????????7200

• filter??????
  filter????????????????????????????????????stream??????????????????????????????

//??????????????????1500?????????
List<Employee> filterEmp = employees.stream().filter(employee -> employee.getSalary()>1500).collect(Collectors.toList());
filterEmp.stream().forEach(System.out::println);

???????????????

Employee{empno=7499, ename=???ALLEN???, salary=1600, deptno=30}
Employee{empno=7782, ename=???CLARK???, salary=2450, deptno=10}

• sorted??????
  sorted??????????????????????????????????????????

//????????????????????????????????????
List<Employee> sortedEmp = employees.stream().sorted(Comparator.comparing(Employee::getSalary)).collect(Collectors.toList());
sortedEmp.stream().forEach(System.out::println);

???????????????

Employee{empno=7369, ename=???SMITH???, salary=800, deptno=20}
Employee{empno=7876, ename=???ADAMS???, salary=1100, deptno=20}
Employee{empno=7521, ename=???WARD???, salary=1250, deptno=30}
Employee{empno=7499, ename=???ALLEN???, salary=1600, deptno=30}
Employee{empno=7782, ename=???CLARK???, salary=2450, deptno=10}

• Collectors???
  Collectors ??????????????????????????????????????????????????????????????????????????????Collectors ?????????????????????????????????

//????????????????????????????????????
Map<Integer, List<Employee>> map = employees.stream().collect(Collectors.groupingBy(employee -> employee.getDeptno()));
for(Map.Entry<Integer, List<Employee>> entry : map.entrySet()) {
    System.out.println("key: " + entry.getKey() + "  value???" + entry.getValue());
}

System.out.println();

//????????????????????????","????????????
String enameString = employees.stream().map(employee -> employee.getEname()).collect(Collectors.joining(","));
System.out.println(enameString);

???????????????

key: 20  value???[Employee{empno=7369, ename=???SMITH???, salary=800, deptno=20}, Employee{empno=7876, ename=???ADAMS???, salary=1100, deptno=20}]
key: 10  value???[Employee{empno=7782, ename=???CLARK???, salary=2450, deptno=10}]
key: 30  value???[Employee{empno=7499, ename=???ALLEN???, salary=1600, deptno=30}, Employee{empno=7521, ename=???WARD???, salary=1250, deptno=30}]

SMITH,ALLEN,WARD,CLARK,ADAMS

• ????????????
  Stream API???????????????????????????????????????????????????????????????

//??????20?????????????????????,???????????????????????????
List<String> names = employees.stream().filter(employee -> employee.getDeptno().equals(20)).sorted(Comparator.comparing(Employee::getSalary).reversed()).map(employee -> employee.getEname()).collect(Collectors.toList());
names.stream().forEach(System.out::println);

???????????????

ADAMS
SMITH

• ??????
  Stream API?????????????????????????????????????????????????????????????????????????????????Java7?????????????????????????????????????????????Stream API???????????????????????????????????????