90 lines
2.5 KiB
Java
90 lines
2.5 KiB
Java
## 5. 生成器（Builder）
|
||
|
||
### Intent
|
||
|
||
封装一个对象的构造过程，并允许按步骤构造。
|
||
|
||
### Class Diagram
|
||
|
||
<div align="center"> <img src="https://cs-notes-1256109796.cos.ap-guangzhou.myqcloud.com/db5e376d-0b3e-490e-a43a-3231914b6668.png"/> </div><br>
|
||
|
||
### Implementation
|
||
|
||
以下是一个简易的 StringBuilder 实现，参考了 JDK 1.8 源码。
|
||
|
||
```java
|
||
public class AbstractStringBuilder {
|
||
protected char[] value;
|
||
|
||
protected int count;
|
||
|
||
public AbstractStringBuilder(int capacity) {
|
||
count = 0;
|
||
value = new char[capacity];
|
||
}
|
||
|
||
public AbstractStringBuilder append(char c) {
|
||
ensureCapacityInternal(count + 1);
|
||
value[count++] = c;
|
||
return this;
|
||
}
|
||
|
||
private void ensureCapacityInternal(int minimumCapacity) {
|
||
// overflow-conscious code
|
||
if (minimumCapacity - value.length > 0)
|
||
expandCapacity(minimumCapacity);
|
||
}
|
||
|
||
void expandCapacity(int minimumCapacity) {
|
||
int newCapacity = value.length * 2 + 2;
|
||
if (newCapacity - minimumCapacity < 0)
|
||
newCapacity = minimumCapacity;
|
||
if (newCapacity < 0) {
|
||
if (minimumCapacity < 0) // overflow
|
||
throw new OutOfMemoryError();
|
||
newCapacity = Integer.MAX_VALUE;
|
||
}
|
||
value = Arrays.copyOf(value, newCapacity);
|
||
}
|
||
}
|
||
```
|
||
|
||
```java
|
||
public class StringBuilder extends AbstractStringBuilder {
|
||
public StringBuilder() {
|
||
super(16);
|
||
}
|
||
|
||
@Override
|
||
public String toString() {
|
||
// Create a copy, don't share the array
|
||
return new String(value, 0, count);
|
||
}
|
||
}
|
||
```
|
||
|
||
```java
|
||
public class Client {
|
||
public static void main(String[] args) {
|
||
StringBuilder sb = new StringBuilder();
|
||
final int count = 26;
|
||
for (int i = 0; i < count; i++) {
|
||
sb.append((char) ('a' + i));
|
||
}
|
||
System.out.println(sb.toString());
|
||
}
|
||
}
|
||
```
|
||
|
||
```html
|
||
abcdefghijklmnopqrstuvwxyz
|
||
```
|
||
|
||
### JDK
|
||
|
||
- [java.lang.StringBuilder](http://docs.oracle.com/javase/8/docs/api/java/lang/StringBuilder.html)
|
||
- [java.nio.ByteBuffer](http://docs.oracle.com/javase/8/docs/api/java/nio/ByteBuffer.html#put-byte-)
|
||
- [java.lang.StringBuffer](http://docs.oracle.com/javase/8/docs/api/java/lang/StringBuffer.html#append-boolean-)
|
||
- [java.lang.Appendable](http://docs.oracle.com/javase/8/docs/api/java/lang/Appendable.html)
|
||
- [Apache Camel builders](https://github.com/apache/camel/tree/0e195428ee04531be27a0b659005e3aa8d159d23/camel-core/src/main/java/org/apache/camel/builder)
|