56 lines
1.3 KiB
Java
56 lines
1.3 KiB
Java
## 空对象（Null）
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### Intent
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使用什么都不做
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的空对象来代替 NULL。
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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。
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### Class Diagram
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<div align="center"> <img src="https://cs-notes-1256109796.cos.ap-guangzhou.myqcloud.com/22870bbe-898f-4c17-a31a-d7c5ee5d1c10.png"/> </div><br>
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### Implementation
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```java
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public abstract class AbstractOperation {
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abstract void request();
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}
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```
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```java
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public class RealOperation extends AbstractOperation {
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@Override
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void request() {
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System.out.println("do something");
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}
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}
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```
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```java
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public class NullOperation extends AbstractOperation{
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@Override
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void request() {
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// do nothing
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}
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}
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```
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```java
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public class Client {
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public static void main(String[] args) {
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AbstractOperation abstractOperation = func(-1);
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abstractOperation.request();
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}
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public static AbstractOperation func(int para) {
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if (para < 0) {
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return new NullOperation();
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}
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return new RealOperation();
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}
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}
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```
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