我所知道设计模式之建造者模式

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前言需求


接下里介绍的是Java 的设计模式之一:建造者模式

需要建房子:这一过程为打桩、砌墙、封顶

房子有各种各样的,比如普通房,高楼,别墅

各种房子的过程虽然一样,但是要求不要相同的

请问你会怎么制作呢?

一、什么是建造者模式

建造者模式(Builder Pattern):又叫生成器模式,是一种对象构建模式

它可以将复杂对象的建造过程抽象出来(抽象类别),使这个抽象过程的不同实现方法可以构造出不同表现(属性)的对象

建造者模式是一步一步创建一个复杂的对象,它允许用户只通过指定复杂对象的类型和内容就可以构建它们, 用户不需要知道内部的具体构建细节

图片.png

二、通过示例说明情况

我们使用传统的方式解决盖房子问题,一般抽象一个父类出来创建方式

图片.png

abstract class AbstractHouse {
    //打地基
    public abstract void buildBasic();
    //砌墙
    public abstract void buildWalls();
    //封顶
    public abstract void roofed();
    //按照顺序建房子
    public void build() { buildBasic(); buildWalls(); roofed();}
}

假如我们是普通的房子就是继承这个父类

class CommonHouse extends AbstractHouse {
     @Override
     public void buildBasic() {
            System.out.println(" 普通房子打地基 ");
     }
        @Override
     public void buildWalls() {
            System.out.println(" 普通房子砌墙 ");
     }
        @Override
     public void roofed() {
            System.out.println(" 普通房子封顶 ");
     }
}

当我们创建普通房子的时候,即可直接调用普通房子的实现类即可

public static void main(String[] args) {
    CommonHouse commonHouse = new CommonHouse();
    commonHouse.build();
}

运行结果如下:
 普通房子打地基 
 普通房子砌墙 
 普通房子封顶

传统方式的问题分析

  • 优点是比较好理解,简单易操作
  • 缺点是设计的程序结构,过于简单,没有设计缓存层对象,程序的扩展和维护不好.

也就是说,这种设计方案,把产品(即:房子) 和 创建产品的过程(即:建房子流程) 封装在一起,耦合性增强了。

解决方案:将产品和产品建造过程解耦 => 建造者模式

三、建造者模式的四个角色

  • Product(产品角色): 一个具体的产品对象
  • Builder(抽象建造者): 创建一个 Product 对象的各个部件指定的 接口/抽象类
  • ConcreteBuilder(具体建造者): 实现接口,构建和装配各个部件
  • Director(指挥者): 构建一个使用 Builder 接口的对象

它主要是用于创建一个复杂的对象。

它主要有两个作用:
1:隔离了客户与对象的生产过程
2:负责控制产品对象的生产过程

图片.png

比如说这辆车是一个产品,我们只需要将方向盘、轮胎、发动机进行组装

四、建造者模式原理类图

图片.png

将对象构造代码从产品类中抽取出来, 并将其放在为生成器的独立对象中

图片.png

五、建造者模式解决盖房子问题

使用建造者模式进行优化,我们先根据四个进行类图画一画

图片.png

现在我们创建一个产品:房子

// 产 品 ->Product
class House {
    private String baise;//地基
    private String wall;//墙
    private String roofed;//屋顶
    public String getBaise() {
           return baise;
    }
       public void setBaise(String baise) {
           this.baise = baise;
    }
       public String getWall() {
           return wall;
    }
       public void setWall(String wall) {
           this.wall = wall;
    }
       public String getRoofed() {
           return roofed;
    }
       public void setRoofed(String roofed) {
           this.roofed = roofed;
    }
}

接着我们需要一个建造者,提供建造的流程、组合产品所需方法

// 抽象的建造者
abstract class HouseBuilder {
    protected House house = new House();
    //将建造的流程所需写好, 抽象的方法
    public abstract void buildBasic();
    public abstract void buildWalls();
    public abstract void roofed();
    //建造房子好, 将产品(房子) 返回
    public House buildHouse() {return house;}
}

接下里我们进行普通房子的具体建造者实现

class CommonHouse extends HouseBuilder {
    @Override
    public void buildBasic() {
        System.out.println(" 普通房子打地基 5 米 ");
    }
    @Override
    public void buildWalls() {
        System.out.println(" 普通房子砌墙 10cm ");
    }
    @Override
    public void roofed() {
        System.out.println(" 普通房子屋顶 ");
    }
}

至于具体的建造流程,是先砌墙、先弄地基、还是屋顶交给指挥者来决定

//指挥者,这里去指定制作流程,返回产品
class HouseDirector {


    HouseBuilder houseBuilder = null;

    //构造器传入 houseBuilder
    public HouseDirector(HouseBuilder houseBuilder) {
        this.houseBuilder = houseBuilder;
    }

    //通过 setter 传入 houseBuilder
    public void setHouseBuilder(HouseBuilder houseBuilder) {
        this.houseBuilder = houseBuilder;
    }

    //如何处理建造房子的流程,交给指挥者
    public House constructHouse() {
        houseBuilder.buildBasic();
        houseBuilder.buildWalls();
        houseBuilder.roofed();
        return houseBuilder.buildHouse();
    }
}

接下里一起使用demo 来体会一下建造者模式下盖普通房、盖高楼有何区别

public static void main(String[] args) {

    //盖普通房子
    CommonHouse commonHouse = new CommonHouse();

    //准备创建房子的指挥者
    HouseDirector houseDirector = new HouseDirector(commonHouse);

    //完成盖房子,返回产品(普通房子)
    House house = houseDirector.constructHouse();

    //System.out.println(" 输 出 流 程 ");
    System.out.println("--------------------------");
}

运行结果:
普通房子打地基 5 米 
普通房子砌墙 10cm 
普通房子屋顶 
--------------------------

那么这时我们要盖高楼呢?盖别墅呢?

public static void main(String[] args) {

    //盖高楼
    HighBuilding highBuilding = new HighBuilding();

    //准备创建房子的指挥者
    HouseDirector houseDirector = new HouseDirector(highBuilding);
    //完成盖房子,返回产品(高楼)
    House house = houseDirector.constructHouse();

    //System.out.println(" 输 出 流 程 ");
    System.out.println("--------------------------");
}
运行结果:
高楼的打地基 100 米 
高楼的砌墙 20cm 
高楼的透明屋顶

相比之前的抽取父类,普通房子与高楼会有一些不同的建造细节

而抽取父类在砌墙、盖屋顶、打地基还可以,若高楼一些别的需求与父类是没有的,那么就可能又要抽取一个高楼的父类

建造者模式呢,只需建造者里提供建造的流程、组合、细节的方法

六、建造者模式在 JDK 的应用和源码分析

我们一起来看看java.lang.StringBuilder中的建造者模式

public static void main(String[] args) {
    StringBuilder stringBuilder = new StringBuilder("hello,world");
    System.out.println(stringBuilder);
}

运行结果:
hello,world

那么我们一起来看看StringBuilder 这个类是怎么样的呢?

public final class StringBuilder
    extends AbstractStringBuilder
    implements java.io.Serializable, CharSequence
{

    
    static final long serialVersionUID = 4383685877147921099L;

    public StringBuilder() {
        super(16);
    }

    public StringBuilder(int capacity) {
        super(capacity);
    }

    public StringBuilder(String str) {
        super(str.length() + 16);
        append(str);
    }

    public StringBuilder(CharSequence seq) {
        this(seq.length() + 16);
        append(seq);
    }

    @Override
    public StringBuilder append(Object obj) {
        return append(String.valueOf(obj));
    }

    @Override
    public StringBuilder append(String str) {
        super.append(str);
        return this;
    }

    public StringBuilder append(StringBuffer sb) {
        super.append(sb);
        return this;
    }

    @Override
    public StringBuilder append(CharSequence s) {
        super.append(s);
        return this;
    }
    
    @Override
    public StringBuilder append(CharSequence s, int start, int end) {
        super.append(s, start, end);
        return this;
    }

    @Override
    public StringBuilder append(char[] str) {
        super.append(str);
        return this;
    }

    @Override
    public StringBuilder append(char[] str, int offset, int len) {
        super.append(str, offset, len);
        return this;
    }

    @Override
    public StringBuilder append(boolean b) {
        super.append(b);
        return this;
    }

    @Override
    public StringBuilder append(char c) {
        super.append(c);
        return this;
    }

    @Override
    public StringBuilder append(int i) {
        super.append(i);
        return this;
    }

    @Override
    public StringBuilder append(long lng) {
        super.append(lng);
        return this;
    }

    @Override
    public StringBuilder append(float f) {
        super.append(f);
        return this;
    }

    @Override
    public StringBuilder append(double d) {
        super.append(d);
        return this;
    }

    @Override
    public StringBuilder appendCodePoint(int codePoint) {
        super.appendCodePoint(codePoint);
        return this;
    }

    @Override
    public StringBuilder delete(int start, int end) {
        super.delete(start, end);
        return this;
    }

    @Override
    public StringBuilder deleteCharAt(int index) {
        super.deleteCharAt(index);
        return this;
    }

    @Override
    public StringBuilder replace(int start, int end, String str) {
        super.replace(start, end, str);
        return this;
    }

    @Override
    public StringBuilder insert(int index, char[] str, int offset,
                                int len)
    {
        super.insert(index, str, offset, len);
        return this;
    }
    @Override
    public StringBuilder insert(int offset, Object obj) {
            super.insert(offset, obj);
            return this;
    }
    @Override
    public StringBuilder insert(int offset, String str) {
        super.insert(offset, str);
        return this;
    }
    
    @Override
    public StringBuilder insert(int offset, char[] str) {
        super.insert(offset, str);
        return this;
    }

    @Override
    public StringBuilder insert(int dstOffset, CharSequence s) {
            super.insert(dstOffset, s);
            return this;
    }

    @Override
    public StringBuilder insert(int dstOffset, CharSequence s,
                                int start, int end)
    {
        super.insert(dstOffset, s, start, end);
        return this;
    }

    @Override
    public StringBuilder insert(int offset, boolean b) {
        super.insert(offset, b);
        return this;
    }

    @Override
    public StringBuilder insert(int offset, char c) {
        super.insert(offset, c);
        return this;
    }
    @Override
    public StringBuilder insert(int offset, int i) {
        super.insert(offset, i);
        return this;
    }
    @Override
    public StringBuilder insert(int offset, long l) {
        super.insert(offset, l);
        return this;
    }

    @Override
    public StringBuilder insert(int offset, float f) {
        super.insert(offset, f);
        return this;
    }

    @Override
    public StringBuilder insert(int offset, double d) {
        super.insert(offset, d);
        return this;
    }

    @Override
    public int indexOf(String str) {
        return super.indexOf(str);
    }

    @Override
    public int indexOf(String str, int fromIndex) {
        return super.indexOf(str, fromIndex);
    }

    @Override
    public int lastIndexOf(String str) {
        return super.lastIndexOf(str);
    }

    @Override
    public int lastIndexOf(String str, int fromIndex) {
        return super.lastIndexOf(str, fromIndex);
    }

    @Override
    public StringBuilder reverse() {
        super.reverse();
        return this;
    }

    @Override
    public String toString() {
        // Create a copy, don't share the array
        return new String(value, 0, count);
    }
    private void writeObject(java.io.ObjectOutputStream s)
        throws java.io.IOException {
        s.defaultWriteObject();
        s.writeInt(count);
        s.writeObject(value);
    }

    private void readObject(java.io.ObjectInputStream s)
        throws java.io.IOException, ClassNotFoundException {
        s.defaultReadObject();
        count = s.readInt();
        value = (char[]) s.readObject();
    }

}

我们发现这个类是final类并继承了AbstractStringBuilder,进去父类看看


abstract class AbstractStringBuilder implements Appendable, CharSequence {
    
    char[] value;

    int count;

    AbstractStringBuilder() {}

    AbstractStringBuilder(int capacity) {
        value = new char[capacity];
    }

    @Override
    public int length() {
        return count;
    }

    public int capacity() {
        return value.length;
    }
    public void ensureCapacity(int minimumCapacity) {
        if (minimumCapacity > 0)
            ensureCapacityInternal(minimumCapacity);
    }

    private void ensureCapacityInternal(int minimumCapacity) {
        if (minimumCapacity - value.length > 0) {
            value = Arrays.copyOf(value,
                    newCapacity(minimumCapacity));
        }
    }
    private static final int MAX_ARRAY_SIZE = Integer.MAX_VALUE - 8;

    private int newCapacity(int minCapacity) {
        int newCapacity = (value.length << 1) + 2;
        if (newCapacity - minCapacity < 0) {
            newCapacity = minCapacity;
        }
        return (newCapacity <= 0 || MAX_ARRAY_SIZE - newCapacity < 0)
            ? hugeCapacity(minCapacity)
            : newCapacity;
    }

    private int hugeCapacity(int minCapacity) {
        if (Integer.MAX_VALUE - minCapacity < 0) { // overflow
            throw new OutOfMemoryError();
        }
        return (minCapacity > MAX_ARRAY_SIZE)
            ? minCapacity : MAX_ARRAY_SIZE;
    }
    
    public void trimToSize() {
        if (count < value.length) {
            value = Arrays.copyOf(value, count);
        }
    }

    public void setLength(int newLength) {
        if (newLength < 0)
            throw new StringIndexOutOfBoundsException(newLength);
        ensureCapacityInternal(newLength);

        if (count < newLength) {
            Arrays.fill(value, count, newLength, '\0');
        }

        count = newLength;
    }
    @Override
    public char charAt(int index) {
        if ((index < 0) || (index >= count))
            throw new StringIndexOutOfBoundsException(index);
        return value[index];
    }

    public int codePointAt(int index) {
        if ((index < 0) || (index >= count)) {
            throw new StringIndexOutOfBoundsException(index);
        }
        return Character.codePointAtImpl(value, index, count);
    }

    public int codePointBefore(int index) {
        int i = index - 1;
        if ((i < 0) || (i >= count)) {
            throw new StringIndexOutOfBoundsException(index);
        }
        return Character.codePointBeforeImpl(value, index, 0);
    }

    public int codePointCount(int beginIndex, int endIndex) {
        if (beginIndex < 0 || endIndex > count || beginIndex > endIndex) {
            throw new IndexOutOfBoundsException();
        }
        return Character.codePointCountImpl(value, beginIndex, endIndex-beginIndex);
    }

    public int offsetByCodePoints(int index, int codePointOffset) {
        if (index < 0 || index > count) {
            throw new IndexOutOfBoundsException();
        }
        return Character.offsetByCodePointsImpl(value, 0, count,
                                                index, codePointOffset);
    }
    public void getChars(int srcBegin, int srcEnd, char[] dst, int dstBegin)
    {
        if (srcBegin < 0)
            throw new StringIndexOutOfBoundsException(srcBegin);
        if ((srcEnd < 0) || (srcEnd > count))
            throw new StringIndexOutOfBoundsException(srcEnd);
        if (srcBegin > srcEnd)
            throw new StringIndexOutOfBoundsException("srcBegin > srcEnd");
        System.arraycopy(value, srcBegin, dst, dstBegin, srcEnd - srcBegin);
    }
    public void setCharAt(int index, char ch) {
        if ((index < 0) || (index >= count))
            throw new StringIndexOutOfBoundsException(index);
        value[index] = ch;
    }
    public AbstractStringBuilder append(Object obj) {
        return append(String.valueOf(obj));
    }

    public AbstractStringBuilder append(String str) {
        if (str == null)
            return appendNull();
        int len = str.length();
        ensureCapacityInternal(count + len);
        str.getChars(0, len, value, count);
        count += len;
        return this;
    }

    public AbstractStringBuilder append(StringBuffer sb) {
        if (sb == null)
            return appendNull();
        int len = sb.length();
        ensureCapacityInternal(count + len);
        sb.getChars(0, len, value, count);
        count += len;
        return this;
    }

    AbstractStringBuilder append(AbstractStringBuilder asb) {
        if (asb == null)
            return appendNull();
        int len = asb.length();
        ensureCapacityInternal(count + len);
        asb.getChars(0, len, value, count);
        count += len;
        return this;
    }

    @Override
    public AbstractStringBuilder append(CharSequence s) {
        if (s == null)
            return appendNull();
        if (s instanceof String)
            return this.append((String)s);
        if (s instanceof AbstractStringBuilder)
            return this.append((AbstractStringBuilder)s);

        return this.append(s, 0, s.length());
    }

    private AbstractStringBuilder appendNull() {
        int c = count;
        ensureCapacityInternal(c + 4);
        final char[] value = this.value;
        value[c++] = 'n';
        value[c++] = 'u';
        value[c++] = 'l';
        value[c++] = 'l';
        count = c;
        return this;
    }

    @Override
    public AbstractStringBuilder append(CharSequence s, int start, int end) {
        if (s == null)
            s = "null";
        if ((start < 0) || (start > end) || (end > s.length()))
            throw new IndexOutOfBoundsException(
                "start " + start + ", end " + end + ", s.length() "
                + s.length());
        int len = end - start;
        ensureCapacityInternal(count + len);
        for (int i = start, j = count; i < end; i++, j++)
            value[j] = s.charAt(i);
        count += len;
        return this;
    }
    public AbstractStringBuilder append(char[] str) {
        int len = str.length;
        ensureCapacityInternal(count + len);
        System.arraycopy(str, 0, value, count, len);
        count += len;
        return this;
    }

    public AbstractStringBuilder append(char str[], int offset, int len) {
        if (len > 0)                // let arraycopy report AIOOBE for len < 0
            ensureCapacityInternal(count + len);
        System.arraycopy(str, offset, value, count, len);
        count += len;
        return this;
    }

    public AbstractStringBuilder append(boolean b) {
        if (b) {
            ensureCapacityInternal(count + 4);
            value[count++] = 't';
            value[count++] = 'r';
            value[count++] = 'u';
            value[count++] = 'e';
        } else {
            ensureCapacityInternal(count + 5);
            value[count++] = 'f';
            value[count++] = 'a';
            value[count++] = 'l';
            value[count++] = 's';
            value[count++] = 'e';
        }
        return this;
    }

    @Override
    public AbstractStringBuilder append(char c) {
        ensureCapacityInternal(count + 1);
        value[count++] = c;
        return this;
    }

    public AbstractStringBuilder append(int i) {
        if (i == Integer.MIN_VALUE) {
            append("-2147483648");
            return this;
        }
        int appendedLength = (i < 0) ? Integer.stringSize(-i) + 1
                                     : Integer.stringSize(i);
        int spaceNeeded = count + appendedLength;
        ensureCapacityInternal(spaceNeeded);
        Integer.getChars(i, spaceNeeded, value);
        count = spaceNeeded;
        return this;
    }

    public AbstractStringBuilder append(long l) {
        if (l == Long.MIN_VALUE) {
            append("-9223372036854775808");
            return this;
        }
        int appendedLength = (l < 0) ? Long.stringSize(-l) + 1
                                     : Long.stringSize(l);
        int spaceNeeded = count + appendedLength;
        ensureCapacityInternal(spaceNeeded);
        Long.getChars(l, spaceNeeded, value);
        count = spaceNeeded;
        return this;
    }

    public AbstractStringBuilder append(float f) {
        FloatingDecimal.appendTo(f,this);
        return this;
    }

    public AbstractStringBuilder append(double d) {
        FloatingDecimal.appendTo(d,this);
        return this;
    }

    public AbstractStringBuilder delete(int start, int end) {
        if (start < 0)
            throw new StringIndexOutOfBoundsException(start);
        if (end > count)
            end = count;
        if (start > end)
            throw new StringIndexOutOfBoundsException();
        int len = end - start;
        if (len > 0) {
            System.arraycopy(value, start+len, value, start, count-end);
            count -= len;
        }
        return this;
    }

    public AbstractStringBuilder appendCodePoint(int codePoint) {
        final int count = this.count;

        if (Character.isBmpCodePoint(codePoint)) {
            ensureCapacityInternal(count + 1);
            value[count] = (char) codePoint;
            this.count = count + 1;
        } else if (Character.isValidCodePoint(codePoint)) {
            ensureCapacityInternal(count + 2);
            Character.toSurrogates(codePoint, value, count);
            this.count = count + 2;
        } else {
            throw new IllegalArgumentException();
        }
        return this;
    }

    public AbstractStringBuilder deleteCharAt(int index) {
        if ((index < 0) || (index >= count))
            throw new StringIndexOutOfBoundsException(index);
        System.arraycopy(value, index+1, value, index, count-index-1);
        count--;
        return this;
    }

    public AbstractStringBuilder replace(int start, int end, String str) {
        if (start < 0)
            throw new StringIndexOutOfBoundsException(start);
        if (start > count)
            throw new StringIndexOutOfBoundsException("start > length()");
        if (start > end)
            throw new StringIndexOutOfBoundsException("start > end");

        if (end > count)
            end = count;
        int len = str.length();
        int newCount = count + len - (end - start);
        ensureCapacityInternal(newCount);

        System.arraycopy(value, end, value, start + len, count - end);
        str.getChars(value, start);
        count = newCount;
        return this;
    }
    public String substring(int start) {
        return substring(start, count);
    }

    @Override
    public CharSequence subSequence(int start, int end) {
        return substring(start, end);
    }

    public String substring(int start, int end) {
        if (start < 0)
            throw new StringIndexOutOfBoundsException(start);
        if (end > count)
            throw new StringIndexOutOfBoundsException(end);
        if (start > end)
            throw new StringIndexOutOfBoundsException(end - start);
        return new String(value, start, end - start);
    }

    public AbstractStringBuilder insert(int index, char[] str, int offset,
                                        int len)
    {
        if ((index < 0) || (index > length()))
            throw new StringIndexOutOfBoundsException(index);
        if ((offset < 0) || (len < 0) || (offset > str.length - len))
            throw new StringIndexOutOfBoundsException(
                "offset " + offset + ", len " + len + ", str.length "
                + str.length);
        ensureCapacityInternal(count + len);
        System.arraycopy(value, index, value, index + len, count - index);
        System.arraycopy(str, offset, value, index, len);
        count += len;
        return this;
    }

    public AbstractStringBuilder insert(int offset, Object obj) {
        return insert(offset, String.valueOf(obj));
    }
    
    public AbstractStringBuilder insert(int offset, String str) {
        if ((offset < 0) || (offset > length()))
            throw new StringIndexOutOfBoundsException(offset);
        if (str == null)
            str = "null";
        int len = str.length();
        ensureCapacityInternal(count + len);
        System.arraycopy(value, offset, value, offset + len, count - offset);
        str.getChars(value, offset);
        count += len;
        return this;
    }

    public AbstractStringBuilder insert(int offset, char[] str) {
        if ((offset < 0) || (offset > length()))
            throw new StringIndexOutOfBoundsException(offset);
        int len = str.length;
        ensureCapacityInternal(count + len);
        System.arraycopy(value, offset, value, offset + len, count - offset);
        System.arraycopy(str, 0, value, offset, len);
        count += len;
        return this;
    }

    public AbstractStringBuilder insert(int dstOffset, CharSequence s) {
        if (s == null)
            s = "null";
        if (s instanceof String)
            return this.insert(dstOffset, (String)s);
        return this.insert(dstOffset, s, 0, s.length());
    }

     public AbstractStringBuilder insert(int dstOffset, CharSequence s,
                                         int start, int end) {
        if (s == null)
            s = "null";
        if ((dstOffset < 0) || (dstOffset > this.length()))
            throw new IndexOutOfBoundsException("dstOffset "+dstOffset);
        if ((start < 0) || (end < 0) || (start > end) || (end > s.length()))
            throw new IndexOutOfBoundsException(
                "start " + start + ", end " + end + ", s.length() "
                + s.length());
        int len = end - start;
        ensureCapacityInternal(count + len);
        System.arraycopy(value, dstOffset, value, dstOffset + len,
                         count - dstOffset);
        for (int i=start; i<end; i++)
            value[dstOffset++] = s.charAt(i);
        count += len;
        return this;
    }

    public AbstractStringBuilder insert(int offset, boolean b) {
        return insert(offset, String.valueOf(b));
    }
    
    public AbstractStringBuilder insert(int offset, char c) {
        ensureCapacityInternal(count + 1);
        System.arraycopy(value, offset, value, offset + 1, count - offset);
        value[offset] = c;
        count += 1;
        return this;
    }

    public AbstractStringBuilder insert(int offset, int i) {
        return insert(offset, String.valueOf(i));
    }

    public AbstractStringBuilder insert(int offset, long l) {
        return insert(offset, String.valueOf(l));
    }

    public AbstractStringBuilder insert(int offset, float f) {
        return insert(offset, String.valueOf(f));
    }

    public AbstractStringBuilder insert(int offset, double d) {
        return insert(offset, String.valueOf(d));
    }

    public int indexOf(String str) {
        return indexOf(str, 0);
    }

    public int indexOf(String str, int fromIndex) {
        return String.indexOf(value, 0, count, str, fromIndex);
    }

    public int lastIndexOf(String str) {
        return lastIndexOf(str, count);
    }

    public int lastIndexOf(String str, int fromIndex) {
        return String.lastIndexOf(value, 0, count, str, fromIndex);
    }

    public AbstractStringBuilder reverse() {
        boolean hasSurrogates = false;
        int n = count - 1;
        for (int j = (n-1) >> 1; j >= 0; j--) {
            int k = n - j;
            char cj = value[j];
            char ck = value[k];
            value[j] = ck;
            value[k] = cj;
            if (Character.isSurrogate(cj) ||
                Character.isSurrogate(ck)) {
                hasSurrogates = true;
            }
        }
        if (hasSurrogates) {
            reverseAllValidSurrogatePairs();
        }
        return this;
    }

    private void reverseAllValidSurrogatePairs() {
        for (int i = 0; i < count - 1; i++) {
            char c2 = value[i];
            if (Character.isLowSurrogate(c2)) {
                char c1 = value[i + 1];
                if (Character.isHighSurrogate(c1)) {
                    value[i++] = c1;
                    value[i] = c2;
                }
            }
        }
    }

    @Override
    public abstract String toString();

    final char[] getValue() {
        return value;
    }

}

那么这个父类他是建造者吗?其实不然,因为我们发现他已实现了接口一些方法

public interface Appendable {

    Appendable append(CharSequence csq) throws IOException;

    Appendable append(CharSequence csq, int start, int end) throws IOException;

    Appendable append(char c) throws IOException;
}

刚刚在StringBuilder构造器中填入"hello,world",就调用了append

public final class StringBuilder
    extends AbstractStringBuilder
    implements java.io.Serializable, CharSequence
{

    
    static final long serialVersionUID = 4383685877147921099L;

    public StringBuilder() {
        super(16);
    }

    public StringBuilder(int capacity) {
        super(capacity);
    }

    public StringBuilder(String str) {
        super(str.length() + 16);
        append(str);
    }

    public StringBuilder(CharSequence seq) {
        this(seq.length() + 16);
        append(seq);
    }
    
}

背后调用的就是继承的父类方法,我们就能发现append其实已实现了

abstract class AbstractStringBuilder implements Appendable, CharSequence {

    //省略其他关键性代码.....
    public AbstractStringBuilder append(Object obj) {
        return append(String.valueOf(obj));
    }

    public AbstractStringBuilder append(String str) {
        if (str == null)
            return appendNull();
        int len = str.length();
        ensureCapacityInternal(count + len);
        str.getChars(0, len, value, count);
        count += len;
        return this;
    }

    public AbstractStringBuilder append(StringBuffer sb) {
        if (sb == null)
            return appendNull();
        int len = sb.length();
        ensureCapacityInternal(count + len);
        sb.getChars(0, len, value, count);
        count += len;
        return this;
    }

    AbstractStringBuilder append(AbstractStringBuilder asb) {
        if (asb == null)
            return appendNull();
        int len = asb.length();
        ensureCapacityInternal(count + len);
        asb.getChars(0, len, value, count);
        count += len;
        return this;
    }
}

那么怎么理解StringBuilder里的建造者模式,产品、指挥者等这些角色呢?

这里需要阐述一个观点:我们在去查看源码的时候,有可能发现在使用的时候设计模式思想很相似,但形式上又与我们画的类图保持完全的一致

所以我们在这里没有很准确的找到产品、建造者、指挥者、具体建造者

但是建造者模式是什么呢?

建造者模式是一步一步创建一个复杂的对象,它允许用户只通过指定复杂对象的类型和内容就可以构建它们, 用户不需要知道内部的具体构建细节

源码中建造者模式角色分析

所以我们来分析一下StringBuilder在建造者模式下的角色

  • Appendable 接口定义了多个 append 方法(抽象方法)

即 Appendable 为抽象建造者, 定义了抽象方法

  • AbstractStringBuilder 实现了Appendable接口方法

这里的 AbstractStringBuilder已经是建造者,只是不能实例化

  • StringBuilder 即充当了指挥者角色,同时充当了具体的建造者

建造方法的实现是由 AbstractStringBuilder 完成 , 而 StringBuilder 继承了 AbstractStringBuilder

public final class StringBuilder
    extends AbstractStringBuilder
    implements java.io.Serializable, CharSequence
{
    
    
    //省略其他关键性代码.....
    @Override
    public StringBuilder append(String str) {
        super.append(str);
        return this;
    }
    
}
//具体建造方法由父类AbstractStringBuilder的方法实现
//建造后返回自己的类型AbstractStringBuilder
abstract class AbstractStringBuilder implements Appendable, CharSequence {


    //省略其他关键性代码.....
    
    public AbstractStringBuilder append(String str) {
        if (str == null)
            return appendNull();
        int len = str.length();
        ensureCapacityInternal(count + len);
        str.getChars(0, len, value, count);
        count += len;
        return this;
    }
}

图片.png

七、建造者模式的注意事项和细节


客户端(使用程序)不必知道产品内部组成的细节,将产品本身与产品的创建过程解耦,使得相同的创建过程可以创建不同的产品对象

每一个具体建造者都相对独立,而与其他的具体建造者无关,因此可以很方便地替换具体建造者或增加新的具体建造者, 用户使用不同的具体建造者即可得到不同的产品对象

可以更加精细地控制产品的创建过程 。将复杂产品的创建步骤分解在不同的方法中,使得创建过程更加清晰, 也更方便使用程序来控制创建过程

增加新的具体建造者无须修改原有类库的代码,指挥者类针对抽象建造者类编程,系统扩展方便,符合“开闭原则

建造者模式所创建的产品一般具有较多的共同点,其组成部分相似,如果产品之间的差异性很大,则不适合使用建造者模式,因此其使用范围受到一定的限制。

如果产品的内部变化复杂,可能会导致需要定义很多具体建造者类来实现这种变化,导致系统变得很庞大,因此在这种情况下,要考虑是否选择建造者模式

参考资料


尚硅谷:设计模式(韩顺平老师):建造者模式

Refactoring.Guru:《深入设计模式》

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