think in java容器深入研究（未获取支持的操作）

分析一个简单的代码

package containers;//: containers/Unsupported.java// Unsupported operations in Java containers.import java.util.*;public 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);    // Copy of the sublist:    Collection<String> c2 = new ArrayList<String>(subList);    try { c.retainAll(c2); } catch(Exception e) {      System.out.println("retainAll(): " + e);    }    try { c.removeAll(c2); } catch(Exception e) {      System.out.println("removeAll(): " + e);    }    try { c.clear(); } catch(Exception e) {      System.out.println("clear(): " + e);    }    try { c.add("X"); } catch(Exception e) {      System.out.println("add(): " + e);    }    try { c.addAll(c2); } catch(Exception e) {      System.out.println("addAll(): " + e);    }    try { c.remove("C"); } catch(Exception e) {      System.out.println("remove(): " + e);    }  }  public static void main(String[] args) {    List<String> list =      Arrays.asList("A B C D E F G H I J K L".split(" "));    test("Modifiable Copy", new ArrayList<String>(list));    test("Arrays.asList()", list);  }} /* Output:--- Modifiable Copy ------ Arrays.asList() ---retainAll(): java.lang.UnsupportedOperationExceptionremoveAll(): java.lang.UnsupportedOperationExceptionclear(): java.lang.UnsupportedOperationExceptionadd(): java.lang.UnsupportedOperationExceptionaddAll(): java.lang.UnsupportedOperationExceptionremove(): java.lang.UnsupportedOperationException*///:~

在main方法中test("Modifiable Copy", new ArrayList<String>(list));这行代码执行没有任何问题。

test("Arrays.asList()", list);代码在执行后，抛出了一堆的异常"java.lang.UnsupportedOperationException”。

 

分析结果：

List<String> list = Arrays.asList("A B C D E F G H I J K L".split(" "));

这行代码通过Arrays.asList生成Stirng[]数组，数组固定的长度，不允许对数组进行增加、删除、等操作。

这行代码test("Modifiable Copy", new ArrayList<String>(list));，将生成的数组，采用ArrayList容器进行封装，经查询源码ArrayList类继承AbstractList并实现List接口，并可以进行增加、删除、等操作。

public class ArrayList extends AbstractList        implements List, RandomAccess, Cloneable, java.io.Serializable

查看AbstractList源码会发现，如果单单继承AbstractList但未重写add方法，则会抛出UnsupportedOperationException异常。

public abstract class AbstractList extends AbstractCollection implements List {    /**     * Sole constructor.  (For invocation by subclass constructors, typically     * implicit.)     */    protected AbstractList() {    }    public void add(int index, Object element) {throw new UnsupportedOperationException();    }