Variable Parameter Method for new features of JDK 5 (2) --- asList, JDK 5 --- aslist

Variable Parameter Method for new features of JDK 5 (2) --- asList, JDK 5 --- aslist

"Arrays. asList (T... A) use of methods

"UnsupportedOperationException Analysis

 

 

• Arrays. asList (T... A) use of methods

        
Package cn. itcast. day24.varparam; import java. util. arrays; import java. util. list;/***** public static <T> List <T> asList (T... a): convert an array to a set ** Note: * The essence of the converted set is an array, and the length is fixed. Therefore, the converted set cannot be added or deleted (the length of the array will be changed ), it can only be modified. **/Public class AsListDemo {public static void main (String [] args) {// define an array // String [] arrStr = {"Lin Qingxia", "Wu Song ", ""}; // List <String> list = Arrays. asList (arrStr); List <String> list = Arrays. asList ("java", "world", "hello"); // UnsupportedOperationException list. add ("javaee"); // error // list. remove (1); // Error list. set (1, "javaee"); // modified successfully // for (String s: list) {// System. out. println (s );//}}}
  

• UnsupportedOperationException Analysis

  
We can see that Arrays's private internal class Arrays $ ArrayList does not implement length-related methods such as add () and remove () from the parent class javasactlist,
  
Therefore, an UnsupportedOperationException is thrown, which means:Request operations (of course not supported)
  

      
Private static class ArrayList <E> extendsAbstractList<E> implements RandomAccess, java. io. serializable {private static final long serialVersionUID =-2764017481108945198L; private final E [] a; ArrayList (E [] array) {if (array = null) throw new NullPointerException (); a = array;} public int size () {return. length;} public Object [] toArray () {return. clone () ;}public <T> T [] toArray (T [] a) {int size = size (); if (. length <size) return Arrays. CopyOf (this. a, size, (Class <? Extends T []>). getClass (); System. arraycopy (this. a, 0, a, 0, size); if (. length> size) a [size] = null; return a;} public E get (int index) {return a [index];}// Rewrite the set Method of the parent class AbstractList, so no exception is thrown during set, and the add and remove methods are from the base class, so an exception is thrown.
    
    public E set(int index, E element) {        E oldValue = a[index];        a[index] = element;        return oldValue;    }        public int indexOf(Object o) {            if (o==null) {                for (int i=0; i<a.length; i++)                    if (a[i]==null)                        return i;            } else {                for (int i=0; i<a.length; i++)                    if (o.equals(a[i]))                        return i;            }            return -1;        }        public boolean contains(Object o) {            return indexOf(o) != -1;        }    }
  

  
AbstractList source code:

      
package java.util;public abstract class AbstractList<E> extends AbstractCollection<E> implements List<E> {    public E set(int index, E element) {    throw new UnsupportedOperationException();    }    public boolean add(E e) {    add(size(), e);    return true;    }    public void add(int index, E element) {    throw new UnsupportedOperationException();    }    public E remove(int index) {    throw new UnsupportedOperationException();    }    public void clear() {        removeRange(0, size());    }    public E set(int index, E element) {    throw new UnsupportedOperationException();    }    abstract public E get(int index);}