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

设计模式 - 生成器 - 图1

Implementation

以下是一个简易的 StringBuilder 实现，参考了 JDK 1.8 源码。

  1. public class AbstractStringBuilder {
  2.     protected char[] value;
  4.     protected int count;
  6.     public AbstractStringBuilder(int capacity) {
  7.         count = 0;
  8.         value = new char[capacity];
  9.     }
  11.     public AbstractStringBuilder append(char c) {
  12.         ensureCapacityInternal(count + 1);
  13.         value[count++] = c;
  14.         return this;
  15.     }
  17.     private void ensureCapacityInternal(int minimumCapacity) {
  18.         // overflow-conscious code
  19.         if (minimumCapacity - value.length > 0)
  20.             expandCapacity(minimumCapacity);
  21.     }
  23.     void expandCapacity(int minimumCapacity) {
  24.         int newCapacity = value.length * 2 + 2;
  25.         if (newCapacity - minimumCapacity < 0)
  26.             newCapacity = minimumCapacity;
  27.         if (newCapacity < 0) {
  28.             if (minimumCapacity < 0) // overflow
  29.                 throw new OutOfMemoryError();
  30.             newCapacity = Integer.MAX_VALUE;
  31.         }
  32.         value = Arrays.copyOf(value, newCapacity);
  33.     }
  34. }
  1. public class StringBuilder extends AbstractStringBuilder {
  2.     public StringBuilder() {
  3.         super(16);
  4.     }
  6.     @Override
  7.     public String toString() {
  8.         // Create a copy, don't share the array
  9.         return new String(value, 0, count);
  10.     }
  11. }
  1. public class Client {
  2.     public static void main(String[] args) {
  3.         StringBuilder sb = new StringBuilder();
  4.         final int count = 26;
  5.         for (int i = 0; i < count; i++) {
  6.             sb.append((char) ('a' + i));
  7.         }
  8.         System.out.println(sb.toString());
  9.     }
  10. }
  1. abcdefghijklmnopqrstuvwxyz

JDK

设计模式 - 生成器 - 图2