?在使用Arrays.asList()后調用add，remove這些method時出現 java.lang.UnsupportedOperationException異常。這是由于Arrays.asList() 返回java.util.Arrays$ArrayList， 而不是ArrayList。Arrays$ArrayList和ArrayList都是繼承AbstractList，remove，add等 method在AbstractList中是默認throw UnsupportedOperationException而且不作任何操作。ArrayList override這些method來對list進行操作，但是Arrays$ArrayList沒有override remove()，add()等，所以throw UnsupportedOperationException。

????? 例子：

package com.test;

????import java.util.Arrays;
import java.util.List;

public class TestUnsupported {
? public static void main(String[] args) {
??????? String[] s = {
??????????? "one", "two", "three", "four", "five",
??????????? "six", "seven", "eight", "nine", "ten",
????????? };

??????? List a = Arrays.asList(s);
??????? System.out.println(
????????? "a.contains(" + s[0] + ") = " +
????????? a.contains(s[0]));
??????? a.add("eleven"); // Unsupported
??????? a.remove(s[0]); // Unsupported
????? }
}

運行后，拋出異常如下：

Exception in thread "main" java.lang.UnsupportedOperationException
?at java.util.AbstractList.add(AbstractList.java:151)
?at java.util.AbstractList.add(AbstractList.java:89)
?at com.test.TestUnsupported.main(TestUnsupported.java:28)

?

解決方法是轉換為ArrayList

List?arrayList?=?new?ArrayList(a);

或者直接這么寫? List?arrayList?=?new?ArrayList(Arrays.asList(s));

參考

When you call Arrays.asList it does not return a java.util.ArrayList. It returns a java.util.Arrays$ArrayList which is an immutable list. You cannot add to it and you cannot remove from it.

If you want a mutable list built from your array you will have to loop over the array yourself and add each element into the list in turn.

Even then your code won't work because you'll get an IndexOutOfBoundsException as you remove the elements from the list in the for loop. There are two options: use an Iterator which allows you to remove from the list as you iterate over it (my recommendation as it makes the code easier to maintain) or loop backwards over the loop removing from the last one downwards (harder to read).

You are using AbstractList. ArrayList and Arrays$ArrayList are both types of AbstractList. That's why you get UnsupportedOperationException: Arrays$ArrayList does not override remove(int) so the method is called on the superclass, AbstractList, which is what is throwing the exception because this method is not implemented on that class (the reason being to allow you to build immutable subclasses).

轉載于:https://www.cnblogs.com/toSeeMyDream/p/5253794.html

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