63 lines
1.4 KiB
Java
63 lines
1.4 KiB
Java
## 空对象（Null）
|
||
|
||
### Intent
|
||
|
||
使用什么都不做
|
||
|
||
的空对象来代替 NULL。
|
||
|
||
一个方法返回 NULL，意味着方法的调用端需要去检查返回值是否是 NULL，这么做会导致非常多的冗余的检查代码。并且如果某一个调用端忘记了做这个检查返回值，而直接使用返回的对象，那么就有可能抛出空指针异常。
|
||
|
||
### Class Diagram
|
||
|
||
<div align="center"> <img src="https://cs-notes-1256109796.cos.ap-guangzhou.myqcloud.com/22870bbe-898f-4c17-a31a-d7c5ee5d1c10.png"/> </div><br>
|
||
|
||
### Implementation
|
||
|
||
```java
|
||
public abstract class AbstractOperation {
|
||
abstract void request();
|
||
}
|
||
```
|
||
|
||
```java
|
||
public class RealOperation extends AbstractOperation {
|
||
@Override
|
||
void request() {
|
||
System.out.println("do something");
|
||
}
|
||
}
|
||
```
|
||
|
||
```java
|
||
public class NullOperation extends AbstractOperation{
|
||
@Override
|
||
void request() {
|
||
// do nothing
|
||
}
|
||
}
|
||
```
|
||
|
||
```java
|
||
public class Client {
|
||
public static void main(String[] args) {
|
||
AbstractOperation abstractOperation = func(-1);
|
||
abstractOperation.request();
|
||
}
|
||
|
||
public static AbstractOperation func(int para) {
|
||
if (para < 0) {
|
||
return new NullOperation();
|
||
}
|
||
return new RealOperation();
|
||
}
|
||
}
|
||
```
|
||
|
||
|
||
|
||
|
||
|
||
|
||
<div align="center"><img width="320px" src="https://cs-notes-1256109796.cos.ap-guangzhou.myqcloud.com/githubio/公众号二维码-2.png"></img></div>
|