Java集合(4)：未获支持的操作及UnsupportedOperationException

执行各种添加和移除的方法在Collection中都是可选操作的，这意味着实现类并不需要为这些方法提供实现。当我们调用这些方法时，将不会执行有意义的行为，而是通常抛出UnsupportedOperationException。

下面例子我们可以看出Collection的各种实现类中：

(1) java.util.ArrayList实现了所有Collection接口。

(2) java.util.Arrays.ArrayList不支持对对象的add，remove，clear，但是支持set修改元素值。这是因为Arrays.asList返回固定尺寸的List。

(3) java.util.Collections.UnmodifiableList产生的是read-only的List，所以不能做任何修改。

import java.util.ArrayList;
import java.util.Arrays;
import java.util.Collection;
import java.util.Collections;
import java.util.List;

class Unsupported {
    static void test(String msg, List<String> list) {
        System.out.println("--- " + msg + " ---");
        Collection<String> c = list;
        Collection<String> subList = list.subList(1, 8);
        Collection<String> c2 = new ArrayList<String>(subList);
        try {
            c.retainAll(c2);
            System.out.println("retainAll(): SUCCESS!");
        } catch (Exception e) {
            System.out.println("retainAll(): " + e);
        }
        try {
            c.removeAll(c2);
            System.out.println("removeAll(): SUCCESS!");
        } catch (Exception e) {
            System.out.println("removeAll(): " + e);
        }
        try {
            c.clear();
            System.out.println("clear(): SUCCESS!");
        } catch (Exception e) {
            System.out.println("clear(): " + e);
        }
        try {
            c.add("X");
            System.out.println("add(): SUCCESS!");
        } catch (Exception e) {
            System.out.println("add(): " + e);
        }
        try {
            c.addAll(c2);
            System.out.println("addAll(): SUCCESS!");
        } catch (Exception e) {
            System.out.println("addAll(): " + e);
        }
        try {
            c.remove("C");
            System.out.println("remove(): SUCCESS!");
        } catch (Exception e) {
            System.out.println("remove(): " + e);
        }
        try {
            list.set(0, "X");
            System.out.println("List.set(): SUCCESS!");
        } catch (Exception e) {
            System.out.println("List.set(): " + e);
        }
    }
}

public class Test6 {
    public static void main(String[] args) {
        List<String> list = Arrays.asList("A B C D E F G H I J K L".split(" "));
        Unsupported.test("Modifiable Copy", new ArrayList<String>(list));
        Unsupported.test("Arrays.asList()", list);
        Unsupported.test("unmodifiableList()", Collections.unmodifiableList(new ArrayList<String>(list)));
        // Output:
        // --- Modifiable Copy ---
        // retainAll(): SUCCESS!
        // removeAll(): SUCCESS!
        // clear(): SUCCESS!
        // add(): SUCCESS!
        // addAll(): SUCCESS!
        // remove(): SUCCESS!
        // List.set(): SUCCESS!
        // --- Arrays.asList() ---
        // retainAll(): java.lang.UnsupportedOperationException
        // removeAll(): java.lang.UnsupportedOperationException
        // clear(): java.lang.UnsupportedOperationException
        // add(): java.lang.UnsupportedOperationException
        // addAll(): java.lang.UnsupportedOperationException
        // remove(): java.lang.UnsupportedOperationException
        // List.set(): SUCCESS!
        // --- unmodifiableList() ---
        // retainAll(): java.lang.UnsupportedOperationException
        // removeAll(): java.lang.UnsupportedOperationException
        // clear(): java.lang.UnsupportedOperationException
        // add(): java.lang.UnsupportedOperationException
        // addAll(): java.lang.UnsupportedOperationException
        // remove(): java.lang.UnsupportedOperationException
        // List.set(): java.lang.UnsupportedOperationException
    }
}