jdk1.8比之前的1.7/1/6/1/5增加了不少新特性

1.stream().map() 將物件轉換成另一個物件

@Test
    public  void text1() {
          List<Staff> staff = Arrays.asList(

                    new Staff("mkyong", 30, new BigDecimal(10000)),

                    new Staff("jack", 27, new BigDecimal(20000)),

                    new Staff("lawrence", 33, new
 
 BigDecimal(30000))

            );

     

            // convert inside the map() method directly.

            List<StaffPublic> result = staff.stream().map(temp -> {

                StaffPublic obj = new StaffPublic();

                obj.setName(temp.getName());

                obj.setAge(temp.getAge());

                
 
if ("mkyong".equals(temp.getName())) {

                    obj.setExtra("this field is for mkyong only!");

                }

                return obj;

            }).collect(Collectors.toList());

     

            System.out.println("1:"+result);

    }

public class Staff {
     private String name;

        
 
private int age;

        private BigDecimal salary;

        public String getName() {
            return name;
        }

        public void setName(String name) {
            this.name = name;
        }

        public int getAge() {
            return age;
        }

        public void setAge(int age) {
            this.age = age;
        }

        public BigDecimal getSalary() {
            return salary;
        }

        public void setSalary(BigDecimal salary) {
            this.salary = salary;
        }

        public Staff(String name, int age, BigDecimal salary) {
            super();
            this.name = name;
            this.age = age;
            this.salary = salary;
        }

        @Override
        public String toString() {
            return "Staff [name=" + name + ", age=" + age + ", salary="
                    + salary + "]";
        }
        
        
}

public class StaffPublic {
     private String name;

        private int age;

        private String extra;

        public String getName() {
            return name;
        }

        public void setName(String name) {
            this.name = name;
        }

        public int getAge() {
            return age;
        }

        public void setAge(int age) {
            this.age = age;
        }

        public String getExtra() {
            return extra;
        }

        public void setExtra(String extra) {
            this.extra = extra;
        }

        public StaffPublic() {
            super();
            // TODO Auto-generated constructor stub
        }

        public StaffPublic(String name, int age, String extra) {
            super();
            this.name = name;
            this.age = age;
            this.extra = extra;
        }
}

反饋結果：

1:[zjjysjz2019_model.StaffPublic@7f77e91b, zjjysjz2019_model.StaffPublic@44a664f2, zjjysjz2019_model.StaffPublic@7f9fcf7f]

2 Streams Collectors groupingBy 進行分組，並顯示每組的個數

@Test
    public  void text2() {
          //3 apple, 2 banana, others 1
        List<String> items =
                Arrays.asList("apple", "apple", "banana",
                        "apple", "orange", "banana", "papaya");
        Map<String, Long> result =items
                .stream()
                .collect(Collectors.groupingBy(Function.identity(), Collectors.counting()));
        System.out.println(result);

        Map<String, Long> finalMap = new LinkedHashMap<>();
        //reversed 反排序
        result.entrySet().stream()
                .sorted(Map.Entry.<String, Long>comparingByValue()
                        .reversed()).forEachOrdered(e -> finalMap.put(e.getKey(), e.getValue()));
        System.out.println(finalMap);
    }

反饋結果：

{papaya=1, orange=1, banana=2, apple=3}
{apple=3, banana=2, papaya=1, orange=1}