## 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
### 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)
