目的

封装一个对象的构造过程，并允许按步骤构造。

类图

生成器类图.png

实现

以下是一个简易的 StringBuilder 实现，参考了 JDK1.8 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.         ensuseCapacityInternal(count + 1);
  13.         value[count++] = c;
  14.         return this;
  15.     }
  17.     private void ensuseCapacityInternal(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.             new Capacity = Integer.MAX_VALUE;
  31.         }
  32.         value = Arrays.copyOf(value, newCapacity);
  33.     }
  34. }
  37. public class StringBuilder extends AbstractStringBuilder{
  38.     public StringBuilder(){
  39.         super(16);
  40.     }
  42.     @Override
  43.     public String toString(){
  44.         //Create a copy, don't share the array
  45.         return new String(value, 0, count);
  46.     }
  47. }
  50. public class Client{
  51.     public static void main(String[] args){
  52.         StringBuilder sb = new StringBuilder();
  53.         final int count = 26;
  54.         for(int i = 0; i < count; i++){
  55.             sb.append((char) ('a' + i));
  56.         }
  57.         System.out.println(sb.toString());
  58.     }
  59. }
  62. 运行结果
  63. abcdefghijklmnopqrstuvwxyz

JDK中的体现