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