问题描述
不可变的课程很棒,但有一个大问题我无法想出一个明智的解决方法 - 周期。
Immutable classes are great but there is one big problem i cant think of a sensible way to solve - cycles.
class Friend {
Set<Friend> friends();
}
我是如何模仿我作为朋友而反过来拥有我的朋友朋友回来?
How does one model Me having You as a friend who in turn has me as a Friend back ?
IMMUTABILITY
来自外部世界的这个类绝对应该是不可变的。为了进行相等检查,内部持有的值应该是常量。
IMMUTABILITYThis class from the outside world should definitely be immutable. The value held internally should be constant for the purposes of equality checks.
推荐答案
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这就是为什么构建者对于不可变的东西非常好 - 它们允许在构造期间的可变性在你冻结它之前设置所有东西。在这种情况下,我猜你需要一个支持创建周期的朋友构建器。
That's why builders are so nice for immutables - they allow mutability during construction to get everything set before you "freeze" it. In this case, I guess you need a Friend builder that supports creating cycles.
final FriendBuilder john = new FriendBuilder().setName("john");
final FriendBuilder mary = new FriendBuilder().setName("mary");
final FriendBuilder susan = new FriendBuilder().setName("susan");
john
.likes(mary)
.likes(susan);
mary
.likes(susan)
.likes(john);
susan
.likes(john);
// okay lets build the immutable Friends
Map<Friend> friends = FriendsBuilder.createCircleOfFriends(john, mary, susan);
Friend immutableJohn = friends.get("john");
编辑:在下面添加了不可变示例以演示方法:
Added immutable example below to demonstrate approach:
-
评论中有一些关于是否可以使用不可变版本的讨论。
There was some discussion in the comments about whether an immutable version was possible.
字段是最终的且不可变的。在构造函数中使用了一个可修改的集合,但它只在构造之后保留了不可修改的引用。
Fields are final and immutable. A modifiable set is used in the constructor, but it only the unmodifiable reference is kept after construction.
我有另一个版本使用Guava ImmutableSet来实现真正不可变的设置而不是JDK的不可修改的包装器。它的工作方式相同,但使用了Guava的漂亮设置构建器。
I have another version that uses Guava ImmutableSet for a truly immutable set rather than JDK's unmodifiable wrapper. It works the same, but uses Guava's nice set builder.
代码:
import java.util.Collections;
import java.util.HashMap;
import java.util.HashSet;
import java.util.IdentityHashMap;
import java.util.Map;
import java.util.Set;
/**
* Note: potentially cycle graph - be careful of deep equals/hashCode/toString/etc.
* Immutable
*/
public class Friend {
public static class Builder {
private final String name;
private final Set<Builder> friends =
new HashSet<Builder>();
Builder(final String name) {
this.name = name;
}
public String getName() {
return name;
}
public Set<Builder> getFriends() {
return friends;
}
void likes(final Builder... newFriends) {
for (final Builder newFriend : newFriends)
friends.add(newFriend);
}
public Map<String, Friend> createCircleOfFriends() {
final IdentityHashMap<Builder, Friend> existing =
new IdentityHashMap<Builder, Friend>();
// Creating one friend creates the graph
new Friend(this, existing);
// after the call existingNodes contains all the nodes in the graph
// Create map of the all nodes
final Map<String, Friend> map =
new HashMap<String, Friend>(existing.size(), 1f);
for (final Friend current : existing.values()) {
map.put(current.getName(), current);
}
return map;
}
}
final String name;
final Set<Friend> friends;
private Friend(
final Builder builder,
final Map<Builder, Friend> existingNodes) {
this.name = builder.getName();
existingNodes.put(builder, this);
final IdentityHashMap<Friend, Friend> friends =
new IdentityHashMap<Friend, Friend>();
for (final Builder current : builder.getFriends()) {
Friend immutableCurrent = existingNodes.get(current);
if (immutableCurrent == null) {
immutableCurrent =
new Friend(current, existingNodes);
}
friends.put(immutableCurrent, immutableCurrent);
}
this.friends = Collections.unmodifiableSet(friends.keySet());
}
public String getName() {
return name;
}
public Set<Friend> getFriends() {
return friends;
}
/** Create string - prints links, but does not traverse them */
@Override
public String toString() {
final StringBuffer sb = new StringBuffer();
sb.append("Friend ").append(System.identityHashCode(this)).append(" {\n");
sb.append(" name = ").append(getName()).append("\n");
sb.append(" links = {").append("\n");
for (final Friend friend : getFriends()) {
sb
.append(" ")
.append(friend.getName())
.append(" (")
.append(System.identityHashCode(friend))
.append(")\n");
}
sb.append(" }\n");
sb.append("}");
return sb.toString();
}
public static void main(final String[] args) {
final Friend.Builder john = new Friend.Builder("john");
final Friend.Builder mary = new Friend.Builder("mary");
final Friend.Builder susan = new Friend.Builder("susan");
john
.likes(mary, susan);
mary
.likes(susan, john);
susan
.likes(john);
// okay lets build the immutable Friends
final Map<String, Friend> friends = john.createCircleOfFriends();
for(final Friend friend : friends.values()) {
System.out.println(friend);
}
final Friend immutableJohn = friends.get("john");
}
}
输出:
Node 11423854 {
value = john
links = {
susan (19537476)
mary (2704014)
}
}
Node 2704014 {
value = mary
links = {
susan (19537476)
john (11423854)
}
}
Node 19537476 {
value = susan
links = {
john (11423854)
}
}
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