Java,报错UnsupportedOperationException,Arrays.asList

悠悠 2021-09-25 01:02 294阅读 0赞

package com.company;
    
    import java.util.ArrayList;
    import java.util.Arrays;
    import java.util.List;
    
    /**
     * @Author you guess
     * @Date 2020/12/17 19:11
     * @Version 1.0 java1.8
     * @Desc
     */
    public class Main21 {
    
        public static void main(String[] args) {
            List<Integer> res1 = new ArrayList<>();
    
            res1.add(1);
            res1.add(2);
            res1.add(3);
    
            res1.add(4);//正常
            int n = res1.size();
    
            for (int i = 0; i < n; i++) {
                res1.add(i + 5); //正常
            }
            System.out.println(res1);//[1, 2, 3, 4, 5, 6, 7, 8]
    
    //        for(int i=0;i< res1.size();i++){//死循环，不报错，不退出
    //            res1.add(i+10); //正常
    //        }
    
            for (int i = res1.size() - 1; i >= 0; i--) {//正常,结束后res1无元素
                res1.remove(i); //正常
            }
    
            List<Integer> res = new ArrayList<>();
            res = Arrays.asList(new Integer[]{1, 2, 3, 4});
    
            System.out.println(res);//[1, 2, 3, 4]
            System.out.println(res.toString());//[1, 2, 3, 4]
    
            for (int i = 0; i < res.size(); i++) {
                System.out.println(res.get(i));
            }
    //        1
    //        2
    //        3
    //        4
    
    //        res.remove(0);//报错UnsupportedOperationException
    //        res.add(6);//报错UnsupportedOperationException
    
    //        for(int i=0;i< n;i++){
    //            res.add(i+5); //报错UnsupportedOperationException
    //        }
    
            int n1 = res.size();
            for (int i = 0; i < n1; i++) {
                System.out.println(res.get(i));
            }
    //        1
    //        2
    //        3
    //        4
    
        }
    }

在使用Arrays.asList()后调用add，remove这些method时出现java.lang.UnsupportedOperationException异常。这是由于Arrays.asList()返回java.util.Arrays$ArrayList， 而不是ArrayList。Arrays$ArrayList和ArrayList都是继承AbstractList，remove，add等method在AbstractList中是默认throw UnsupportedOperationException而且不作任何操作。

ArrayList 重写了 remove()，add()等来对list进行操作，但是Arrays$ArrayList没有重写 remove()，add()等，所以会调用父类的方法，导致throw UnsupportedOperationException

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