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.UnsupportedOperationException
removeAll(): java.lang.UnsupportedOperationException
clear(): java.lang.UnsupportedOperationException
add(): java.lang.UnsupportedOperationException
addAll(): java.lang.UnsupportedOperationException
remove(): 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();
}