java lambda之ListHelper

小的第一次写博客，各位别见笑。

刚从C#转java不久，写代码各种不自在，各种不舒服。很多以前在C#很简单的写法，在java里却不行。特别是对于集合的处理，比如我要取集合里所有对象的Id，我还需要写一个循环，我无力吐槽，谁叫C#市场不行呢。 于是乎。。。。

后面知道java里其实也有lambda表达式这东西，所以自己便研究了一下，封装了一些以前在C#常用的一些处理集合的方法，看名字就知道几乎照着以前C# Linq里的取的。

废话少说上示例

package lambda;

import java.lang.reflect.Method;
import java.nio.file.ClosedWatchServiceException;
import java.util.*;

public class test {

    public static void main(String[] args) {

        List<Student> students = new ArrayList<>();
        students.add(new Student(1, "damao1", 13));
        students.add(new Student(2, "damao2", 12));
        students.add(new Student(3, "damao3", 13));
        students.add(new Student(4, "damao2", 12));
        students.add(new Student(5, "damao3", 13));

        //需求：得到所有id的集合
        //以前你得这样
        List<Integer> ids=new ArrayList<>();
        for (Student student: students){
             ids.add(student.getId());
        }
        //现在只需要这样
         ids=ListHelper.select(students,n->n.getId());


        //需求：查询一个id为1的Student
        //以前你得这样
        Student stu=null;
        for (Student student: students){
             if(student.getId().equals(1)){
                 stu=student;
                 break;
             }
        }
        //现在只需要这样
        stu=ListHelper.firstOrDefault(students,n->n.getId().equals(1));



        //需求：查询一个Id>2的Student 集合
        //以前你得这样
        List<Student> stus=new ArrayList<>();
        for (Student student: students){
            if(student.getId()>1){
               stus.add(student);
            }
        }
        //现在只需要这样
        stus=ListHelper.where(students,n->n.getId()>1);

        
        
        //需求：根据年龄分下组
        //以前你得这样
        //..........................我懒得写了，反正很麻烦
        //现在只需要这样
        HashMap<Integer, List<Student>> groups = ListHelper.group(students, n -> n.getAge());

       //需求：判断集合中有没有重复的姓名
        boolean repeated = ListHelper.isRepeated(students, n -> n.getName());

        //根据年龄排序
       students = ListHelper.orderBy(students, n -> n.getAge());

       //求年龄最大的学生
       Student max = ListHelper.max(students, n -> n.getAge());

      //求年龄最小的学生
       Student min = ListHelper.min(students, n -> n.getAge());

       //还有等等，就不一一列举了

  } }

 

下面是ListHelper的实现，你值得拥有

import java.util.*;
import java.util.function.Function;
import java.util.function.Predicate;
import java.util.stream.Collectors;

public class ListHelper {

    //example ListHelper.firstOrDefault(students, n -> n.getAge()==12)
    public static <T> T firstOrDefault(List<T> list, Predicate<? super T> predicate) {
        if (list != null) {
            Optional<T> first = list.stream().filter(predicate).findFirst();
            if (first.isPresent()) {
                return first.get();
            }
        }
        return null;
    }

    //example ListHelper.where(students, n -> n.getAge() >12)
    public static <T> List<T> where(List<T> list, Predicate<? super T> predicate) {
        if (list != null) {
            return list.stream().filter(predicate).collect(Collectors.toList());
        }
        return null;
    }

    //example ListHelper.select(students, n -> n.getAge())
    public static <T, E> List<E> select(List<T> list, Function<? super T, E> express) {
        List<E> eList = new ArrayList<>();
        for (T t : list) {
            E e = express.apply(t);
            eList.add(e);
        }
        return eList;
    }

    //是否重复
    // example ListHelper.isRepeated(students,n->n.getName()
    public static <T, E> boolean isRepeated(List<T> list, Function<? super T, E> express) {
        for (int i = 0; i < list.size()-1; i++) {
            E e = express.apply(list.get(i));
            for (int j = i + 1; j <list.size(); j++) {
                E e2 = express.apply(list.get(j));
                if (e.equals(e2)) {
                    return true;
                }
            }
        }
        return false;
    }


    //分组
    public  static  <T, E>  HashMap<E,List<T>> group(List<T> list, Function<? super T, E> express) {
        HashMap<E,List<T>>  groups=new HashMap<>();
        for (T t:list){
            E e = express.apply(t);
            if(groups.keySet().contains(e)){
                groups.get(e).add(t);
            }else
            {
                List<T> tempList=new ArrayList<>();
                tempList.add(t);
                groups.put(e,tempList);
            }
        }
        return groups;
    }

    // example ListHelper.orderBy(students,n->n.getAge())
    public static <T, U extends Comparable<? super U>> List<T> orderBy(List<T> list, Function<? super T, ? extends U> keyExtractor) {
        list.sort(Comparator.comparing(keyExtractor));
        return list;
    }


    // example ListHelper.orderBy(students,n->n.getAge())
    public static <T, U extends Comparable<? super U>> List<T> orderByDesc(List<T> list, Function<? super T, ? extends U> keyExtractor) {
        list.sort(Comparator.comparing(keyExtractor).reversed());
        return list;
    }

    // example  ListHelper.max(students,n->n.getAge())
    public static <T, U extends Comparable<? super U>> T max(List<T> list, Function<? super T, ? extends U> keyExtractor) {
        list.sort(Comparator.comparing(keyExtractor));
        if (list.size() > 0) {
            return list.get(list.size() - 1);
        }
        return null;
    }

    // example  ListHelper.min(student,n->n.getAge())
    public static <T, U extends Comparable<? super U>> T min(List<T> list, Function<? super T, ? extends U> keyExtractor) {
        list.sort(Comparator.comparing(keyExtractor));
        if (list.size() > 0) {
            return list.get(0);
        }
        return null;
    }

}

 

 

为了方便大家测试，我把Student类也贴出来

public class Student {
    Integer id;
    String name;
    Integer age;


    public Student() {
    }

    public Student(Integer id, String name, Integer age) {
        this.id = id;
        this.name = name;
        this.age = age;
    }

    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 getAge() {
        return age;
    }

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


}