map集合解决双重for循环的性能问题

叁歲伎倆 2023-06-18 10:56 10阅读 0赞

现在有个需求，需要将student表和class表的所有数据返回回去，关联条件时class\_id。如果用sql很容易实现，直接连表查询就可以了，但是现在项目是微服务，不能连表查询了，所以得在代码里面修改。

定义三个类，student类，class类，还有返回所有数据的studentAndClass类。

import java.util.ArrayList;
    import java.util.List;
    
    /**
     * Created by 郝雨烁 on 2019/12/1.
     */
    public class Student {
    
        private Integer id;
        private  String 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;
        }
    
        public Integer getClassId() {
            return classId;
        }
    
        public void setClassId(Integer classId) {
            this.classId = classId;
        }
    
        private  Integer classId;
    
        public Student(Integer id, String name, Integer classId) {
            this.id = id;
            this.name = name;
            this.classId = classId;
        }
    
        @Override
        public String toString() {
            return super.toString();
        }
    }

/**
     * Created by 郝雨烁 on 2019/12/2.
     */
    public class Class {
        private Integer classId;
        private String className;
    
        public Integer getClassId() {
            return classId;
        }
    
        public void setClassId(Integer classId) {
            this.classId = classId;
        }
    
        public String getClassName() {
            return className;
        }
    
        public void setClassName(String className) {
            this.className = className;
        }
    
        public Class(Integer classId, String className) {
            this.classId = classId;
            this.className = className;
        }
    
        @Override
        public String toString() {
            return super.toString();
        }
    }

数据组装代码如下

import java.util.ArrayList;
    import java.util.List;
    
    public class Main {
    
        public static void main(String[] args) {
            List<Student> studentList = new ArrayList<>();
            Student student = new Student(1,"小明",1);
            Student student1= new Student(2,"小红",2);
            studentList.add(student);
            studentList.add(student1);
            List<Class> classList = new ArrayList<>();
           Class cl = new Class(1,"1班");
           Class c2 = new Class(2,"2班");
    
            classList.add(cl);
            classList.add(c2);
            List<StudentAndClass> list = new ArrayList<>();
            for (Student studentArray : studentList) {
                for (Class aClass : classList) {
                    if(studentArray.getClassId().equals(aClass.getClassId())){
                        StudentAndClass studentAndClass = new StudentAndClass();
                        studentAndClass.setClassId(studentArray.getClassId());
                        studentAndClass.setClassName(aClass.getClassName());
                        studentAndClass.setId(studentArray.getId());
                        studentAndClass.setName(studentArray.getName());
                        list.add(studentAndClass);
                    }
                }
    
            }
    
        }
    }

双层for循环，此时时间复杂度是n2.

import java.util.ArrayList;
    import java.util.HashMap;
    import java.util.List;
    import java.util.Map;
    
    public class Main {
    
        public static void main(String[] args) {
            List<Student> studentList = new ArrayList<>();
            Student student = new Student(1,"小明",1);
            Student student1= new Student(2,"小红",2);
            studentList.add(student);
            studentList.add(student1);
            List<Class> classList = new ArrayList<>();
           Class cl = new Class(1,"1班");
           Class c2 = new Class(2,"2班");
            classList.add(cl);
            classList.add(c2);
            // list 转成map，map存放的是key和value的映射关系
            Map<Integer,Class> map = new HashMap<>();
            for (Class aClass : classList) {
                map.put(aClass.getClassId(),aClass);
            }
            List<StudentAndClass> list = new ArrayList<>();
            if(studentList.size()>0){
                for (Student studentArray : studentList) {
                    Class c1= map.get(studentArray.getClassId());
                    if(c1!=null){
                        StudentAndClass studentAndClass = new StudentAndClass();
                        studentAndClass.setClassId(studentArray.getClassId());
                        studentAndClass.setClassName(c1.getClassName());
                        studentAndClass.setId(studentArray.getId());
                        studentAndClass.setName(studentArray.getName());
                        list.add(studentAndClass);
                    }
    
                }
            }
    
    
        }

将其中一个list转成map之后就成了并列for循环。此时时间复杂度成了n。

lamda表达式解决list转map问题更加方便了，一句代码解决三句代码才解决的问题。

import java.util.ArrayList;
    import java.util.List;
    import java.util.Map;
    import java.util.stream.Collectors;
    
    public class Main {
    
        public static void main(String[] args) {
            List<Student> studentList = new ArrayList<>();
            Student student = new Student(1,"小明",1);
            Student student1= new Student(2,"小红",2);
            studentList.add(student);
            studentList.add(student1);
            List<Class> classList = new ArrayList<>();
           Class cl = new Class(1,"1班");
           Class c2 = new Class(2,"2班");
            classList.add(cl);
            classList.add(c2);
            // list 转成map，map存放的是key和value的映射关系
    //        Map<Integer,Class> map = new HashMap<>();
    //        for (Class aClass : classList) {
    //            map.put(aClass.getClassId(),aClass);
    //        }
            Map<Integer, Class> map = classList.stream().collect(Collectors.toMap(Class::getClassId, e -> e));
            List<StudentAndClass> list = new ArrayList<>();
            if(studentList.size()>0){
                for (Student studentArray : studentList) {
                    Class c1= map.get(studentArray.getClassId());
                    if(c1!=null){
                        StudentAndClass studentAndClass = new StudentAndClass();
                        studentAndClass.setClassId(studentArray.getClassId());
                        studentAndClass.setClassName(c1.getClassName());
                        studentAndClass.setId(studentArray.getId());
                        studentAndClass.setName(studentArray.getName());
                        list.add(studentAndClass);
                    }
    
                }
            }
    
        }
    }

最近项目在拆分微服务，所以此类操作更加多了，也就有了更多的可以练手的机会，开心。