lamda -- 两个集合的各种问题处理

ゝ一纸荒年。 2023-02-22 12:11 11阅读 0赞

引入jar文件

<dependency>
    	<groupId>com.google.guava</groupId>
    	<artifactId>guava</artifactId>
    	<version>20.0</version>
    </dependency>
    
    <dependency>
    	<groupId>commons-lang</groupId>
    	<artifactId>commons-lang</artifactId>
    	<version>2.6</version>
    </dependency>

### LamdaMoreList.java ###

package com.example.demo.util;
    
    import com.example.demo.entity.Student;
    import com.example.demo.entity.User;
    import com.example.demo.entity.UserDTO;
    import com.example.demo.entity.UserPO;
    import com.google.common.collect.Lists;
    import org.apache.commons.lang.StringUtils;
    
    import java.util.ArrayList;
    import java.util.Arrays;
    import java.util.List;
    import java.util.function.Function;
    import java.util.stream.Collectors;
    
    public class LamdaMoreList { 
    
        /** * List<PO> 转 List<DTO> */
        public static void test1() { 
            //1. 初始化数据
            UserPO user1 = new UserPO(1, "java");
            UserPO user2 = new UserPO(2, "C");
            UserPO user3 = new UserPO(3, "PHP");
            List<UserPO> users = new ArrayList<>();
            users.add(user1);
            users.add(user2);
            users.add(user3);
    
            //2. List<PO> 转 List<DTO>
            List<UserDTO> dtoList = users.stream().map(getParamConverter()).collect(Collectors.toList());
            System.out.println(dtoList);
        }
    
        private static Function<UserPO, UserDTO> getParamConverter(){ 
            return userPO -> { 
                UserDTO dto = new UserDTO();
                dto.setId(userPO.getId());
                dto.setName(userPO.getName());
                return dto;
            };
        }
    
        /** * List<String> 转 List<Integer> */
        public static void test2() { 
            //1. 初始化List<String>
            String nums = "100,101,102,103,104";
            List<String> numStrList = Arrays.asList(nums.split(","));
    
            //2. List<String> 转 List<Integer>
            List<Integer> numList = numStrList.stream().map(Integer::valueOf)
                    .collect(Collectors.toList());
    
            //3. 输出
            numList.stream().forEach(num ->{ 
                System.out.println(num.intValue());
            });
        }
    
        /** * 集合A的某个字段数据 copy 到集合B * 例如： * 集合A：users = [User{id=1, name='java'}, User{id=2, name='C'}, User{id=3, name='GO'}] * 把集合A的id值复制给集合B，如： * 集合B：integersList = [1, 2, 3] */
        public static void test3() { 
            //1. 初始化集合A
            List<User> users = Lists.newArrayListWithExpectedSize(3);
            User u1 = new User(1,"java");
            User u2 = new User(2,"C");
            User u3 = new User(3,"GO");
            users.add(u1);
            users.add(u2);
            users.add(u3);
            System.out.println("users = " + users.toString());
    
            //2. 集合A的数据 copy 到集合B
            List<Integer> integersList = users.stream()
                    .map(user -> { 
                        return user.getId();
                    }).collect(Collectors.toList());
    
            //3. 输出
            System.out.println("integersList = " + integersList.toString());
        }
    
        /** * 获取集合A的数据是集合B中年没有的 * 例如：全集U={2,3} A={1，2，3} B={1，4，5} */
        public static void test4() { 
            //1. 初始化集合A
            List<Integer> listA = Lists.newArrayListWithExpectedSize(3);
            listA.add(1);
            listA.add(2);
            listA.add(3);
            System.out.println("listA = " + listA.toString());
    
            //2. 初始化集合B
            List<Integer> listB = Lists.newArrayListWithExpectedSize(3);
            listB.add(1);
            listB.add(4);
            listB.add(5);
            System.out.println("listB = " + listB.toString());
    
            //3. 获取集合A的数据是集合B中年没有的
            List<Integer> ids = listA.stream()
                    .filter(item -> !listB.contains(item))
                    .collect(Collectors.toList());
    
            //4. 输出
            System.out.println(ids.toString());
        }
    
        /** * 把集合B中为null的数据赋值，赋值数据来源于集合A */
        public static void test5() { 
            //1. 初始化集合A
            List<User> users = Lists.newArrayListWithExpectedSize(3);
            User u1 = new User(1,"java");
            User u2 = new User(2,"C");
            User u3 = new User(3,"GO");
            users.add(u1);
            users.add(u2);
            users.add(u3);
            System.out.println("users = " + users.toString());
    
            //2. 初始化集合B
            List<Student> students = Lists.newArrayListWithExpectedSize(3);
            Student s1 = new Student("1","java");
            Student s2 = new Student("2","C");
            Student s3 = new Student(null,"GO");
            students.add(s1);
            students.add(s2);
            students.add(s3);
            System.out.println("students before: " + students.toString());
    
            //3. 把集合B中为null的数据赋值，赋值来源于集合A
            students.stream().filter(student -> StringUtils.isEmpty(student.getId())).forEach(student -> { 
                users.stream().filter(user -> StringUtils.equals(user.getName(),student.getName()) && user.getId() != 0).forEach(user -> { 
                    student.setId(String.valueOf(user.getId()));
                });
            });
    
            //4. 输出
            System.out.println("students after: " + students.toString());
        }
    
        public static void main(String[] args) { 
            System.out.println("List<PO> 转 List<DTO>: ");
            test1();
    
            System.out.println("\nList<String> 转 List<Integer>:");
            test2();
    
            System.out.println("\n集合A的某个字段数据 copy 到集合B:");
            test3();
    
            System.out.println("\n获取集合A的数据是集合B中年没有的:");
            test4();
    
            System.out.println("\n把集合B中为null的数据赋值，赋值来源于集合A:");
            test5();
        }
    }

### User.java ###

package com.example.demo.entity;
    
    public class User { 
        private Integer id;
        private String name;
    
        public User(Integer id, String name) { 
            this.id = id;
            this.name = name;
        }
    
        public Integer getId() { 
            return id;
        }
    
        public void setId(Integer id) { 
            this.id = id;
        }
    
        public String getName() { 
            return name;
        }
    
        public void setName(String name) { 
            this.name = name;
        }
    
        @Override
        public String toString() { 
            return "User{" +
                    "id=" + id +
                    ", name='" + name + '\'' +
                    '}';
        }
    }

### UserPO.java ###

package com.example.demo.entity;
    
    public class UserPO { 
        private Integer id;
        private String name;
    
        public UserPO(Integer id, String name) { 
            this.id = id;
            this.name = name;
        }
    
        public Integer getId() { 
            return id;
        }
    
        public void setId(Integer id) { 
            this.id = id;
        }
    
        public String getName() { 
            return name;
        }
    
        public void setName(String name) { 
            this.name = name;
        }
    
        @Override
        public String toString() { 
            return "User{" +
                    "id=" + id +
                    ", name='" + name + '\'' +
                    '}';
        }
    }

### UserDTO.java ###

package com.example.demo.entity;
    
    public class UserDTO { 
        private Integer id;
        private String name;
    
        public UserDTO() { 
        }
    
        public UserDTO(Integer id, String name) { 
            this.id = id;
            this.name = name;
        }
    
        public Integer getId() { 
            return id;
        }
    
        public void setId(Integer id) { 
            this.id = id;
        }
    
        public String getName() { 
            return name;
        }
    
        public void setName(String name) { 
            this.name = name;
        }
    
        @Override
        public String toString() { 
            return "User{" +
                    "id=" + id +
                    ", name='" + name + '\'' +
                    '}';
        }
    }

### Student.java ###

package com.example.demo.entity;
    
    public class Student { 
        private String id;
        private String name;
    
        public Student(String id, String name) { 
            this.id = id;
            this.name = name;
        }
    
        public String getId() { 
            return id;
        }
    
        public void setId(String id) { 
            this.id = id;
        }
    
        public String getName() { 
            return name;
        }
    
        public void setName(String name) { 
            this.name = name;
        }
    
        @Override
        public String toString() { 
            return "Student{" +
                    "id='" + id + '\'' +
                    ", name='" + name + '\'' +
                    '}';
        }
    }

### 输出 ###

List<PO> 转 List<DTO>: 
    [User{ id=1, name='java'}, User{ id=2, name='C'}, User{ id=3, name='PHP'}]
    
    List<String> 转 List<Integer>:
    100
    101
    102
    103
    104
    
    集合A的某个字段数据 copy 到集合B:
    users = [User{ id=1, name='java'}, User{ id=2, name='C'}, User{ id=3, name='GO'}]
    integersList = [1, 2, 3]
    
    获取集合A的数据是集合B中年没有的:
    listA = [1, 2, 3]
    listB = [1, 4, 5]
    [2, 3]
    
    把集合B中为null的数据赋值，赋值来源于集合A:
    users = [User{ id=1, name='java'}, User{ id=2, name='C'}, User{ id=3, name='GO'}]
    students before: [Student{ id='1', name='java'}, Student{ id='2', name='C'}, Student{ id='null', name='GO'}]
    students after: [Student{ id='1', name='java'}, Student{ id='2', name='C'}, Student{ id='3', name='GO'}]