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notes/设计模式 - 生成器.md

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+## 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)
+
+
+
+
+
+
+<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>