Java编程思想第4版[中文版](PDF格式)-第67部分
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print(c);
c。remove(〃3〃); // Removes the first one
print(c);
c。remove(〃3〃); // Removes the second one
print(c);
// Remove all ponents that are in the
// argument collection:
c。removeAll(newCollection());
print(c);
c。addAll(newCollection());
print(c);
// Is an element in this Collection?
System。out。println(
〃c。contains(”4”) = 〃 + c。contains(〃4〃));
// Is a Collection in this Collection?
System。out。println(
〃c。containsAll(newCollection()) = 〃 +
c。containsAll(newCollection()));
Collection c2 = newCollection(5; 3);
// Keep all the elements that are in both
// c and c2 (an intersection of sets):
c。retainAll(c2);
print(c);
// Throw away all the elements in c that
// also appear in c2:
c。removeAll(c2);
System。out。println(〃c。isEmpty() = 〃 +
c。isEmpty());
c = newCollection();
print(c);
c。clear(); // Remove all elements
System。out。println(〃after c。clear():〃);
print(c);
}
} ///:~
通过第一个方法,我们可用测试数据填充任何集合。在当前这种情况下,只是将 int 转换成 String。第二个
方法将在本章其余的部分经常采用。
newCollection()的两个版本都创建了ArrayList,用于包含不同的数据集,并将它们作为集合对象返回。所
以很明显,除了Collection 接口之外,不会再用到其他什么。
print()方法也会在本节经常用到。由于它用一个反复器(Iterator )在一个集合内遍历,而任何集合都可以
产生这样的一个反复器,所以它适用于List 和Set,也适用于由一个Map 生成的Collection。
main()用简单的手段显示出了集合内的所有方法。
在后续的小节里,我们将比较 List ,Set 和 Map 的不同实现方案,同时指出在各种情况下哪一种方案应成为
首选(带有星号的那个)。大家会发现这里并未包括一些传统的类,如Vector,Stack 以及Hashtable 等。
因为不管在什么情况下,新集合内都有自己首选的类。
8。7。2 使用 Lists
L i s t Order is the most important feature of a L i s t ; it promises to maintain elements
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(interface) in a particular sequence。 L i s t adds a number of methods to Collection that allow
insertion and removal of elements in the middle of a L i s t 。 (This is remended
only for a L i n k e d L i s t 。 ) A List will produce a ListIterator ; and using this you
can traverse the L i s t in both directions; as well as insert and remove elements
in the middle of the list (again; remended only for a L i n k e d L i s t )。
ArrayList* A L i s t backed by an array。 Use instead of V e c t o r as a general…purpose object
holder。 Allows rapid random access to elements; but is slow when inserting and
removing elements from the middle of a list。 ListIterator should be used only for
back…and…forth traversal of an A r r a y L i s t ; but not for inserting and removing
elements; which is expensive pared to L i n k e d L i s t 。
LinkedList Provides optimal sequential access; with inexpensive insertions and deletions from
the middle of the list。 Relatively slow for random access。 (Use A r r a y L i s t
instead。) Also has a d d F i r s t ( ) ; a d d L a s t ( ) ; g e t F i r s t ( ) ; g e tLast( );
r e m o v e F i r s t ( ) ; and r e m o v e L a s t ( ) (which are not defined in any interfaces or
base classes) to allow it to be used as a stack; a queue; and a dequeue。
List (接口) 顺序是 List 最重要的特性;它可保证元素按照规定的顺序排列。List 为 Collection 添加了
大量方法,以便我们在 List 中部插入和删除元素(只推荐对LinkedList 这样做)。List 也会生成一个
ListIterator (列表反复器),利用它可在一个列表里朝两个方向遍历,同时插入和删除位于列表中部的元
素(同样地,只建议对 LinkedList 这样做)
ArrayList * 由一个数组后推得到的List 。作为一个常规用途的对象容器使用,用于替换原先的Vector。允
许我们快速访问元素,但在从列表中部插入和删除元素时,速度却嫌稍慢。一般只应该用ListIterator 对一
个ArrayList 进行向前和向后遍历,不要用它删除和插入元素;与 LinkedList 相比,它的效率要低许多
LinkedList 提供优化的顺序访问性能,同时可以高效率地在列表中部进行插入和删除操作。但在进行随机访
问时,速度却相当慢,此时应换用ArrayList。也提供了addFirst(),addLast(),getFirst(),
getLast(),removeFirst() 以及removeLast() (未在任何接口或基础类中定义),以便将其作为一个规格、
队列以及一个双向队列使用
下面这个例子中的方法每个都覆盖了一组不同的行为:每个列表都能做的事情(basicTest()),通过一个反
复器遍历(iterMotion())、用一个反复器改变某些东西(iterManipulation())、体验列表处理的效果
(testVisual())以及只有LinkedList 才能做的事情等:
//: List1。java
// Things you can do with Lists
package c08。newcollections;
import java。util。*;
public class List1 {
// Wrap Collection1。fill() for convenience:
public static List fill(List a) {
return (List)Collection1。fill(a);
}
// You can use an Iterator; just as with a
// Collection; but you can also use random
// access with get():
public static void print(List a) {
for(int i = 0; i 《 a。size(); i++)
System。out。print(a。get(i) + 〃 〃);
System。out。println();
}
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static boolean b;
static Object o;
static int i;
static Iterator it;
static ListIterator lit;
public static void basicTest(List a) {
a。add(1; 〃x〃); // Add at location 1
a。add(〃x〃); // Add at end
// Add a collection:
a。addAll(fill(new ArrayList()));
// Add a collection starting at location 3:
a。addAll(3; fill(new ArrayList()));
b = a。contains(〃1〃); // Is it in there?
// Is the entire collection in there?
b = a。containsAll(fill(new ArrayList()));
// Lists allow random access; which is cheap
// for ArrayList; expensive for LinkedList:
o = a。get(1); // Get object at location 1
i = a。indexOf(〃1〃); // Tell index of object
// indexOf; starting search at location 2:
i = a。indexOf(〃1〃; 2);
b = a。isEmpty(); // Any elements inside?
it = a。iterator(); // Ordinary Iterator
lit = a。listIterator(); // ListIterator
lit = a。listIterator(3); // Start at loc 3
i = a。lastIndexOf(〃1〃); // Last match
i = a。lastIndexOf(〃1〃; 2); // 。。。after loc 2
a。remove(1); // Remove location 1
a。remove(〃3〃); // Remove this object
a。set(1; 〃y〃); // Set location 1 to 〃y〃
// Keep everything that's in the argument
// (the intersection of the two sets):
a。retainAll(fill(new ArrayList()));
// Remove elements in this range:
a。removeRange(0; 2);
// Remove everything that's in the argument:
a。removeAll(fill(new ArrayList()));
i = a。size(); // How big is it?
a。clear(); // Remove all elements
}
public static void iterMotion(List a) {
ListIterator it = a。listIterator();
b = it。hasNext();
b = it。hasPrevious();
o = it。next();
i = it。nextIndex();
o = it。previous();
i = it。previousIndex();
}
public static void iterManipulation(List a) {
ListIterator it = a。listIterator();
it。add(〃47〃);
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// Must move to an element after add():
it。next();
// Remove the element that was just produced:
it。remove();
// Must move to an element after remove():
it。next();
// Change the element that was just produced:
it。set(〃47〃);
}
public static void testVisual(List a) {
print(a);
List b = new ArrayList();
fill(b);
System。out。print(〃b = 〃);
print(b);
a。addAll(b);
a。addAll(fill(new ArrayList()));
print(a);
// Shrink the list by removing all the
// elements beyond the first 1/2 of the list
System。out。println(a。size());
System。out。println(a。size()/2);
a。removeRange(a。size()/2; a。size()/2 + 2);
print(a);
// Insert; remove; and replace elements
// using a ListIterator:
ListIterator x = a。listIterator(a。size()/2);
x。add(〃one〃);
print(a);
System。out。println(x。next());
x。remove();
System。out。println(x。next());
x。set(〃47〃);
print(a);
// Traverse the list backwards:
x = a。listIterator(a。size());
while(x。hasPrevious())
System。out。print(x。previous() + 〃 〃);
System。out。println();
System。out。println(〃testVisual finished〃);
}
// There are some things that only
// LinkedLists can do:
public static void testLinkedList() {
LinkedList ll = new LinkedList();
Collection1。fill(ll; 5);
print(ll);
// Treat it like a stack; pushing:
ll。addFirst(〃one〃);
ll。addFirst(〃two〃);
print(ll);
// Like 〃peeking〃 at the top of a stack:
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System。out。println(ll。getFirst());
// Like popping a stack:
System。out。println(ll。removeFirst());
System。out。println(ll。removeFirst());
// Treat it like a queue; pulling elements
// off the tail end:
System。out。println