返回信息流Haskell
module Main where
import Data.List
data Tree = Leaf
| Branch Tree Int Tree
theTree = Branch
(Branch Leaf 1 Leaf)
2
(Branch
(Branch Leaf 3 Leaf)
4
(Branch Leaf 5 Leaf)
)
ppLine node indent =
case node of
Leaf -> ""
Branch l n r ->
(ppLine l (indent+1)) ++
(concat $ replicate indent " ") ++ (show n) ++ "\n" ++
(ppLine r (indent+1))
main = do
putStr $ ppLine theTree 0
Scala
trait Tree
case object Leaf extends Tree
case class Branch(val l: Tree, val num: Int, val r: Tree) extends Tree
object TreeDemo extends App {
println("Hello world!")
val t = Branch(
Branch(Leaf, 1, Leaf),
2,
Branch(
Branch(Leaf, 3, Leaf),
4,
Branch(Leaf, 5, Leaf)))
def visitNode(t: Tree, indent: Int = 0): Unit = {
t match {
case Leaf => {}
case Branch(l, n, r) => {
visitNode(l, indent + 1)
println(" " * indent + n)
visitNode(r, indent + 1)
}
}
}
visitNode(t)
}
有臭味的Java代码
package cn.byr.nuanyangyang.visitor0;
interface Tree {
}
class Leaf implements Tree {
public static Leaf INSTANCE = new Leaf();
}
class Branch implements Tree {
public Tree left;
public int num;
public Tree right;
public Branch(Tree left, int num, Tree right) {
this.left = left;
this.num = num;
this.right = right;
}
}
public class VisitorDemo {
public static void visitTree(Tree tree, int indent) {
if (tree instanceof Leaf) {
// do nothing
} else if (tree instanceof Branch) {
Branch branch = (Branch) tree;
visitTree(branch.left, indent + 1);
for (int i = 0; i < indent; i++) {
System.out.print(" ");
}
System.out.println(branch.num);
visitTree(branch.right, indent + 1);
} else {
throw new RuntimeException("Unknown branch");
}
}
public static void main(String[] args) {
Tree tree = new Branch( //
new Branch(Leaf.INSTANCE, 1, Leaf.INSTANCE), //
2, //
new Branch( //
new Branch(Leaf.INSTANCE, 3, Leaf.INSTANCE), //
4, //
new Branch(Leaf.INSTANCE, 5, Leaf.INSTANCE)));//
visitTree(tree, 0);
}
}
“正统”的Java代码
package cn.byr.nuanyangyang.visitor;
interface TreeVisitor {
void visitLeaf(Leaf leaf);
void visitBranch(Branch branch);
}
interface Tree {
void accept(TreeVisitor visitor);
}
class Leaf implements Tree {
public static Leaf INSTANCE = new Leaf();
@Override
public void accept(TreeVisitor visitor) {
visitor.visitLeaf(this);
}
}
class Branch implements Tree {
public Tree left;
public int num;
public Tree right;
public Branch(Tree left, int num, Tree right) {
this.left = left;
this.num = num;
this.right = right;
}
@Override
public void accept(TreeVisitor visitor) {
visitor.visitBranch(this);
}
}
public class VisitorDemo {
static class MyTreeVisitor implements TreeVisitor {
private int indent;
public MyTreeVisitor(int indent) {
this.indent = indent;
}
@Override
public void visitLeaf(Leaf leaf) {
}
@Override
public void visitBranch(Branch branch) {
branch.left.accept(new MyTreeVisitor(indent + 1));
for (int i = 0; i < indent; i++) {
System.out.print(" ");
}
System.out.println(branch.num);
branch.right.accept(new MyTreeVisitor(indent + 1));
}
}
public static void main(String[] args) {
Tree tree = new Branch( //
new Branch(Leaf.INSTANCE, 1, Leaf.INSTANCE), //
2, //
new Branch( //
new Branch(Leaf.INSTANCE, 3, Leaf.INSTANCE), //
4, //
new Branch(Leaf.INSTANCE, 5, Leaf.INSTANCE)));//
TreeVisitor myTreeVisitor = new MyTreeVisitor(0);
tree.accept(myTreeVisitor);
}
}
这是一条镜像帖。来源：北邮人论坛 / java / #29346同步于 2014/4/16
该镜像源已超过 30 天没有更新，可能在源站已被删除。
Java机器人发帖
[黑Java系列]累死Java程序员之4: 访客
nuanyangyang
2014/4/16镜像同步5 回复
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5 条回复
instanceof不够漂亮，类型情况多的时候效率也不一定比visitor模式高。但也难说，没有测量过性能。
老的c++没有rtti，只能用visitor。
【 在 poiuasd 的大作中提到: 】
: 什么叫有臭味？
: 来自「北邮人论坛手机版」
来自「北邮人论坛手机版」
主要是和scala比较一下java对模式匹配没有专门的语法，大量的instanceof和类型转换很难看。
【 在 poiuasd 的大作中提到: 】
: 什么叫有臭味？
: 来自「北邮人论坛手机版」
来自「北邮人论坛手机版」