浅谈java中unmodifiableList方法的应用场景

java对象中primitive类型变量可以通过不提供set方法保证不被修改,但对象的List成员在提供get方法后,就可以随意add、remove改变其结构,这不是希望的结果。网上看了下,发现Collections的静态方法unmodifiableList可以达到目的。方法原型为:public static <T> List<T> unmodifiableList(List<? extends T> list);用法也很简单,传入一个List实例la,返回这个list的只读视图lb,类型依然是List。之后对lb进行add、remove等改变其内容的操作将导致编译不通过。

 

Student.java

 1 package com.test;
 2 
 3 import java.util.ArrayList;
 4 import java.util.Collections;
 5 import java.util.List;
 6 
 7 /**
 8  * Created by Liulei on 2017/5/31.
 9  */
10 public class Student {
11   private String name;
12   private int age;
13   private List<String> courses;
14 
15   public Student(){
16     courses = new ArrayList<String>();
17   }
18   public Student(String name,int age,List<String> courses){
19     this.name = name;
20     this.age = age;
21     this.courses = courses;
22   }
23 
24   public List<String> getCourses(){
25     return this.courses;
26   }
27 
28   public void setName(String name) {
29     this.name = name;
30   }
31 
32   public void setAge(int age) {
33     this.age = age;
34   }
35 
36   public String getName() {
37     return name;
38   }
39 
40   public int getAge() {
41     return age;
42   }
43 
44   public void describe(){
45     System.out.println(this.name);
46     System.out.println(this.age);
47     for (String course:courses){
48       System.out.println(course);
49     }
50   }
51 
52 
53 }

 

 

App.java

 1 package com.liulei.test;
 2  
 3 import java.util.ArrayList;
 4 import java.util.List;
 5  
 6 /**
 7  * Hello world!
 8  *
 9  */
10 public class App
11 {
12   public static void main( String[] args )
13   {
14     ArrayList<String> courses = new ArrayList<String>();
15     courses.add("Math");
16     courses.add("Chinese");
17  
18     Student student = new Student("Alice",18,courses);
19     student.describe();
20     List<String> myCourses = student.getCourses();
21     myCourses.add("English");
22     student.describe();
23  
24   }
25 }

 

结果:

 1 Alice
 2 
 3 18
 4 
 5 Math
 6 
 7 Chinese
 8 
 9 Alice
10 
11 18
12 
13 Math
14 
15 Chinese
16 
17 English

 

虽然只有getCourse,但依然可以被加上1门English。使用unmodifiableList可以解决这个问题,将Student的getCourses改写:

1 public List<String> getCourses(){
2     return Collections.unmodifiableList(this.courses);
3   }

再次执行,编译器提示出错:

Exception in thread "main" java.lang.UnsupportedOperationException

at java.util.Collections$UnmodifiableCollection.add(Collections.java:1055)

总结,使用unmodifiableList可以保证对象的list内容不被意料之外地修改,保证对象的封装性。

 

此博文转自https://www.jb51.net/article/115065.htm

 

posted @ 2020-05-12 14:57  xajh00789  阅读(618)  评论(0编辑  收藏  举报