Builder

Intent

Separate the construction of a complex object from its representation so that the same construction process can create different representations.

Class diagram of the original GoF pattern

Improvement in MixJuice

Solved Problem(s)

Extensibility problem

Adding new methods to Builder and ConcreteBuilder classes is impossible without modifying the source code. (This problem is mentioned at p.100,line.32-p.101,line.4 in the GoF book.)

Used programming technique(s)

Method addition to Builder and ConcreteBuilder classes.

Consequences

• (+) New methods can be added to Builder and ConcreteBuilder classes without modifying the source code.
• (-) Addition of both a new method and a new ConcreteBuilder requires a complementary module; however, if default behavior of the method can be implemented in Builder, e.g. default behavior is to do nothing, no complementary module is required.

Structure and pseudo code

module original {
 define abstract class Builder {
   define void buildPartA(){}
   define void buildPartB(){}
 }
 define class ConcreteBuilder extends Builder {
   void buildPartA(){...}
   void buildPartB(){...}
 }
 define class Director {
   Builder builder;
   define void construct(){
     builder.buildPartA();
     builder.buildPartB();
   }
 }
}
module extension extends original {
 class Builder {
   define void buildPartC(){}
 }
 define class ConcreteBuilder2 extends Builder {
   void buildPartA(){...}
   void buildPartB(){...}
   void buildPartC(){...}
 }
 class Director {
   void construct(){
     original();
     builder.buildPartC();
   }
 }
}

Another solution in MixJuice

Solved Problem(s)

Complexity problem

The class structure of the original GoF pattern is complicated.

Used programming technique(s)

Implementation selection for Builder.

Consequences

• (+) Class structure is not complicated.
• (-) This solution is applicable only if just one implementation of Builder is used in a program.

Structure and pseudo code

module builder {
  define class Builder {
    // This class has Director's function.
    define void construct() { ... }

    define void buildPartA() {}
    define void buildPartB() {}
    define void buildPartC() {}
  }
}

module builder.implementation1 extends builder {
  class Builder {
    void buildPartA() { ... }
    void buildPartB() { ... }
    void buildPartC() { ... }
  }
}

module builder.implementation2 extends builder {
  class Builder {
    void buildPartA() { ... }
    void buildPartB() { ... }
    void buildPartC() { ... }
  }
}

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