java8新特性:为什么要用lambda

最近刚开始学习java8的新特性，发现lambda真是好用。

场景设计，你有一个list集合，需要对list进行过滤取值，有哪些做法？

public class Student {
    private String name;
    private int age;
    private int score;
    //省略get set方法
}

 List studentList =null;
    @Before
   public void befor(){
       studentList = Arrays.asList(
                new Student("zhangsan",15,93),
                new Student("lisi",16,100),
                new Student("wangwu",17,86),
                new Student("zhaoliu",13,71),
                new Student("yuanqi",14,65)
        );
   }

原始做法：

 /**
     * 查找成绩>70的学生
     */
   @Test
   public void test1(){
       List dataList = new ArrayList<>();
       for(Student student :studentList){
           if(student.getScore()>70){
               dataList.add(student);
           }
       }
       for(Student student :dataList){
           System.out.println(student);
       }
   }

    /**
     * 查找年龄>14的学生
     */
    @Test
    public void test2(){
        List dataList = new ArrayList<>();
        for(Student student :studentList){
            if(student.getAge()>14){
                dataList.add(student);
            }
        }
        for(Student student :dataList){
            System.out.println(student);
        }
    }

缺点显而易见，每次有新的过滤方法，都得写一堆重复代码。

优化一：策略模式

 /**
     * 优化1 策略模式
     */
    public List getStudentByFilter(List studentList ,StudentFilter studentFilter){
        List data = new ArrayList<>();
        for(Student student:studentList){
            if(studentFilter.compare(student)){
                data.add(student);
            }
        }
        return data;
    }

    public void printStudentList(List studentList){
        for(Student student:studentList){
            System.out.println(student);
        }
    }

public interface StudentFilter {
    boolean compare(Student student);
}
public class StudentFilterByAge implements StudentFilter{
    @Override
    public boolean compare(Student student) {
        return student.getAge()>14;
    }
}
public class StudentFilterByScore  implements StudentFilter{
    @Override
    public boolean compare(Student student) {
        return student.getScore()>70;
    }
}

    /**
     * 优化1之后 查找成绩>70的学生
     */
    @Test
    public void test3(){
        List studentByFilter = getStudentByFilter(studentList, new StudentFilterByScore());
        printStudentList(studentByFilter);
    }

    /**
     * 优化1之后 查找年龄>14的学生
     */
    @Test
    public void test4(){
        List studentByFilter = getStudentByFilter(studentList, new StudentFilterByAge());
        printStudentList(studentByFilter);
    }

使用策略模式以后重复代码减少了，但是同样有个问题，每次新增过滤方法得写一个新类。

优化二：匿名内部类

/**
     * 继续优化2 采用匿名内部类查找成绩>70的学生
     */
    @Test
    public void test5(){
        List studentByFilter = getStudentByFilter(studentList, new StudentFilter(){
            @Override
            public boolean compare(Student student) {
                return student.getScore()>70;
            }
        });
        printStudentList(studentByFilter);
    }

    /**
     * 继续优化2 采用匿名内部类查找年龄>14的学生
     */
    @Test
    public void test6(){
        List studentByFilter = getStudentByFilter(studentList, new StudentFilter(){
            @Override
            public boolean compare(Student student) {
                return student.getAge()>14;
            }
        });
        printStudentList(studentByFilter);
    }

这种做法也是java7的常规做法，有没有更好的方式呢，这个时候lambda表达式就诞生了

优化三：lambda

   /**
     * 优化3 lambda 查找年龄>14的学生
     */
    @Test
    public void test7(){
        List studentByFilter =getStudentByFilter(studentList,(e)->e.getAge()>14);
        studentByFilter.forEach(System.out::println);
    }

    /**
     * 优化3 lambda 查找成绩>70的学生
     */
    @Test
    public void test8(){
        List studentByFilter =getStudentByFilter(studentList,(e)->e.getScore()>70);
        studentByFilter.forEach(System.out::println);
    }

除了lambda表达式还有别的优化方式吗，java8有StreamAPI

优化四：StreamAPI

 /**
     * 优化4 StreamAPI 查找年龄>14的学生
     */
    @Test
    public void test9(){
        studentList.stream()
                .filter(student -> student.getAge()>14)
                .forEach(System.out::println);
    }

    /**
     * 优化4 StreamAPI 查找成绩>70的学生
     */
    @Test
    public void test10(){
        studentList.stream()
                .filter(student -> student.getScore()>70)
                .forEach(System.out::println);
    }