使用面向对象技术替代switch-case和if-else
在日常开发中，常常会作一些状态判断，用到swich-case与if-else。在面向对象的环境里，有两种方式可以替代它们。一种是使用继承子类的多态，另一种是使用state模式。它们使用对象的间接性有效地摆脱了传统的状态判断。
举个例子。
Method.java
|
package
com.zj.original;
import
com.zj.utils.NoMethodTypeException;
publicclass
Method {
privateint
_type
;
publicstaticfinalint
POST
= 0;
publicstaticfinalint
GET
= 1;
publicstaticfinalint
PUT
= 2;
publicstaticfinalint
DELETE
= 3;
public
Method(
int
type) {
_type
= type;
}
public
String getMethod()
throws
NoMethodTypeException {
switch
(
_type
) {
case
POST
:
return
"This is POST method"
;
case
GET
:
return
"This is GET method"
;
case
PUT
:
return
"This is PUT method"
;
case
DELETE
:
return
"This is DELETE method"
;
default
:
thrownew
NoMethodTypeException();
}
}
publicboolean
safeMethod() {
if
(
_type
==
GET
)
returntrue
;
else
returnfalse
;
}
publicboolean
passwordRequired() {
if
(
_type
==
POST
)
returnfalse
;
else
returntrue
;
}
}
|
类Method中，存在四个状态Post、Get、Put和Delete。有一个switch-case判断，用于输出四种方法的描述信息；两个if-else判断，分别判断方法是否安全(只有Get方法是安全的)，方法是否需要密码(只有Post方法不需要密码)。
1.使用继承子类多态
使用继承子类多态的方式，通常对于某个具体对象，它的状态是不可改变的(在对象的生存周期中)。
现在使用四个子类分别代表四种类型的方法。这样就可以使用多态将各个方法的具体逻辑分置到子类中去了。
在抽象基类Method中可以提供创建子类实例的静态方法，当然也可以使用SimpleFactory模式。对于
getMethod()方法，延迟到子类中实现；对于
safeMethod()方法和
passwordRequired()方法，提供一个默认的实现，这个实现应该符合绝大部分子类的要求，这样的话，对于少数不符合默认实现的子类只需override相应方法即可。
<>Method.java
|
package
com.zj.subclass;
publicabstractclass
Method {
publicfinalstatic
Method createPostMethod() {
returnnew
PostMethod();
}
publicfinalstatic
Method createGetMethod() {
returnnew
GetMethod();
}
publicfinalstatic
Method createPutMethod() {
returnnew
PutMethod();
}
publicfinalstatic
Method createDeleteMethod() {
returnnew
DelMethod();
}
abstractpublic
String getMethod();
publicboolean
safeMethod() {
returnfalse
;
}
publicboolean
passwordRequired() {
returntrue
;
}
}
|
四个子类分别继承和override相应的方法。
PostMethod.java
|
package
com.zj.subclass;
publicclass
PostMethod
extends
Method {
@Override
public
String getMethod() {
return
"This is POST method"
;
}
@Override
publicboolean
passwordRequired() {
returnfalse
;
}
}
|
GetMethod.java
|
package
com.zj.subclass;
publicclass
GetMethod
extends
Method{
@Override
public
String getMethod() {
return
"This is GET method"
;
}
@Override
publicboolean
safeMethod() {
returntrue
;
}
}
|
PutMethod.java
|
package
com.zj.subclass;
publicclass
PutMethod
extends
Method {
@Override
public
String getMethod() {
return
"This is PUT method"
;
}
}
|
DelMethod.java
|
package
com.zj.subclass;
publicclass
DelMethod
extends
Method{
@Override
public
String getMethod(){
return
"This is DELETE method"
;
}
}
|
2.使用state模式
如果希望对象在生存周期内，可以变化自己的状态，则可以选择state模式。
这里抽象状态为一个接口MethodType，四种不同的状态实现该接口。
<>MethodType.java
|
package
com.zj.state;
publicinterface
MethodType {
String getTypeDescription();
String getMethodDescription();
boolean
isSafe();
boolean
isRequired();
}
|
Post.java
|
package
com.zj.state;
publicclass
Post
implements
MethodType{
public
String getMethodDescription() {
return
"This is POST method"
;
}
public
String getTypeDescription() {
return
"===POST==="
;
}
publicboolean
isRequired() {
returnfalse
;
}
publicboolean
isSafe() {
returnfalse
;
}
}
|
Get.java
|
package
com.zj.state;
publicclass
Get
implements
MethodType{
public
String getMethodDescription() {
return
"This is GET method"
;
}
public
String getTypeDescription() {
return
"===GET==="
;
}
publicboolean
isRequired() {
returntrue
;
}
publicboolean
isSafe() {
returntrue
;
}
}
|
Put.java
|
package
com.zj.state;
publicclass
Put
implements
MethodType{
public
String getMethodDescription() {
return
"This is PUT method"
;
}
public
String getTypeDescription() {
return
"===PUT==="
;
}
publicboolean
isRequired() {
returntrue
;
}
publicboolean
isSafe() {
returnfalse
;
}
}
|
Delete.java
|
package
com.zj.state;
publicclass
Delete
implements
MethodType{
public
String getMethodDescription() {
return
"This is DELETE method"
;
}
public
String getTypeDescription() {
return
"===DELETE==="
;
}
publicboolean
isRequired() {
returntrue
;
}
publicboolean
isSafe() {
returnfalse
;
}
}
|
此时，在类Method中保存一个field表示MethodType，在某对象中，可以随时变化四种已知的状态(具体见
runAllMethods()方法)。
Method.java
|
package
com.zj.state;
publicclass
Method {
private
MethodType
_type
;
public
Method() {
_type
=
null
;
}
public
Method(MethodType type) {
_type
= type;
}
public
String getMethod() {
return
_type
.getMethodDescription();
}
publicboolean
safeMethod() {
return
_type
.isSafe();
}
publicboolean
passwordRequired() {
return
_type
.isRequired();
}
publicvoid
changeType(MethodType type) {
_type
= type;
}
publicvoid
runAllMethods() {
MethodType[] types =
new
MethodType[] {
new
Post(),
new
Get(),
new
Put(),
new
Delete() };
for
(MethodType type : types) {
System.
out
.println(type.getTypeDescription());
changeType(type);
System.
out
.println(getMethod());
System.
out
.println(safeMethod());
System.
out
.println(passwordRequired());
}
}
}
|
3.测试
在测试类中，分别使用上面3中机制展示结果。它们的结果应该是一致的。
Client.java
|
package
com.zj.utils;
publicclass
Client {
staticvoid
print(String s) {
System.
out
.println(s);
}
staticvoid
print(Boolean b) {
System.
out
.println(b);
}
publicstaticvoid
main(String[] args)
throws
NoMethodTypeException {
print(
"===original==="
);
print(
"===POST==="
);
com.zj.original.Method post1 =
new
com.zj.original.Method(
com.zj.original.Method.
POST
);
print(post1.getMethod());
print(post1.safeMethod());
print(post1.passwordRequired());
print(
"===GET==="
);
com.zj.original.Method get1 =
new
com.zj.original.Method(
com.zj.original.Method.
GET
);
print(get1.getMethod());
print(get1.safeMethod());
print(get1.passwordRequired());
print(
"===PUT==="
);
com.zj.original.Method put1 =
new
com.zj.original.Method(
com.zj.original.Method.
PUT
);
print(put1.getMethod());
print(put1.safeMethod());
print(put1.passwordRequired());
print(
"===DELETE==="
);
com.zj.original.Method del1 =
new
com.zj.original.Method(
com.zj.original.Method.
DELETE
);
print(del1.getMethod());
print(del1.safeMethod());
print(del1.passwordRequired());
print(
"===subclass==="
);
print(
"===POST==="
);
com.zj.subclass.Method post2 = com.zj.subclass.Method
.
createPostMethod();
print(post2.getMethod());
print(post2.safeMethod());
print(post2.passwordRequired());
print(
"===GET==="
);
com.zj.subclass.Method get2 = com.zj.subclass.Method.
createGetMethod();
print(get2.getMethod());
print(get2.safeMethod());
print(get2.passwordRequired());
print(
"===PUT==="
);
com.zj.subclass.Method put2 = com.zj.subclass.Method.
createPutMethod();
print(put2.getMethod());
print(put2.safeMethod());
print(put2.passwordRequired());
print(
"===DELETE==="
);
com.zj.subclass.Method del2 = com.zj.subclass.Method
.
createDeleteMethod();
print(del2.getMethod());
print(del2.safeMethod());
print(del2.passwordRequired());
print(
"===state==="
);
new
com.zj.state.Method().runAllMethods();
}
}
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本文出自 “子 孑” 博客，请务必保留此出处http://zhangjunhd.blog.51cto.com/113473/68070