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Builder Pattern in java

  • Difficulty Level : Medium
  • Last Updated : 08 Aug, 2018

Method Chaining: In java, Method Chaining is used to invoke multiple methods on the same object which occurs as a single statement. Method-chaining is implemented by a series of methods that return the this reference for a class instance.

Implementation: As return values of methods in a chain is this reference, this implementation allows us to invoke methods in chain by having the next method invocation on the return value of the previous method in the chain.




// Java code to demonstrate method chaining
final class Student {
  
    // instance fields
    private int id;
    private String name;
    private String address;
  
    // Setter Methods
    // Note that all setters method
    // return this reference
    public Student setId(int id)
    {
        this.id = id;
        return this;
    }
  
    public Student setName(String name)
    {
        this.name = name;
        return this;
    }
  
    public Student setAddress(String address)
    {
        this.address = address;
        return this;
    }
  
    @Override
    public String toString()
    {
        return "id = " + this.id + ", name = " + this.name + 
                               ", address = " + this.address;
    }
}
  
// Driver class
public class MethodChainingDemo {
    public static void main(String args[])
    {
        Student student1 = new Student();
        Student student2 = new Student();
  
        student1.setId(1).setName("Ram").setAddress("Noida");
        student2.setId(2).setName("Shyam").setAddress("Delhi");
  
        System.out.println(student1);
        System.out.println(student2);
    }
}

Output:

id = 1, name = Ram, address = Noida
id = 2, name = Shyam, address = Delhi

Need of Builder Pattern : Method chaining is a useful design pattern but however if accessed concurrently, a thread may observe some fields to contain inconsistent values. Although all setter methods in above example are atomic, but calls in the method chaining can lead to inconsistent object state when the object is modified concurrently. The below example can lead us to a Student instance in an inconsistent state, for example, a student with name Ram and address Delhi.




// Java code to demonstrate need of Builder Pattern
  
// Server Side Code
final class Student {
  
    // instance fields
    private int id;
    private String name;
    private String address;
  
    // Setter Methods
    // Note that all setters method
    // return this reference
    public Student setId(int id)
    {
        this.id = id;
        return this;
    }
  
    public Student setName(String name)
    {
        this.name = name;
        return this;
    }
  
    public Student setAddress(String address)
    {
        this.address = address;
        return this;
    }
  
    @Override
    public String toString()
    {
        return "id = " + this.id + ", name = " + this.name + 
                               ", address = " + this.address;
    }
}
  
// Client Side Code
class StudentReceiver {
  
    private final Student student = new Student();
  
    public StudentReceiver()
    {
  
        Thread t1 = new Thread(new Runnable() {
            @Override
            public void run()
            {
                student.setId(1).setName("Ram").setAddress("Noida");
            }
        });
  
        Thread t2 = new Thread(new Runnable() {
            @Override
            public void run()
            {
                student.setId(2).setName("Shyam").setAddress("Delhi");
            }
        });
  
        t1.start();
        t2.start();
    }
  
    public Student getStudent()
    {
        return student;
    }
}
  
// Driver class
public class BuilderNeedDemo {
    public static void main(String args[])
    {
        StudentReceiver sr = new StudentReceiver();
        System.out.println(sr.getStudent());
    }
}

Output may be:



id = 2, name = Shyam, address = Noida

Another inconsistent output may be

id = 0, name = null, address = null

Note : Try running main method statements in loop(i.e. multiple requests to server simultaneously).

To solve this problem, there is Builder pattern to ensure the thread-safety and atomicity of object creation.

Implementation : In Builder pattern, we have a inner static class named Builder inside our Server class with instance fields for that class and also have a factory method to return an new instance of Builder class on every invocation. The setter methods will now return Builder class reference. We will also have a build method to return instances of Server side class, i.e. outer class.




// Java code to demonstrate Builder Pattern
  
// Server Side Code
final class Student {
  
    // final instance fields
    private final int id;
    private final String name;
    private final String address;
  
    public Student(Builder builder)
    {
        this.id = builder.id;
        this.name = builder.name;
        this.address = builder.address;
    }
  
    // Static class Builder
    public static class Builder {
  
        /// instance fields
        private int id;
        private String name;
        private String address;
  
        public static Builder newInstance()
        {
            return new Builder();
        }
  
        private Builder() {}
  
        // Setter methods
        public Builder setId(int id)
        {
            this.id = id;
            return this;
        }
        public Builder setName(String name)
        {
            this.name = name;
            return this;
        }
        public Builder setAddress(String address)
        {
            this.address = address;
            return this;
        }
  
        // build method to deal with outer class
        // to return outer instance
        public Student build()
        {
            return new Student(this);
        }
    }
  
    @Override
    public String toString()
    {
        return "id = " + this.id + ", name = " + this.name + 
                               ", address = " + this.address;
    }
}
  
// Client Side Code
class StudentReceiver {
  
    // volatile student instance to ensure visibility
    // of shared reference to immutable objects
    private volatile Student student;
  
    public StudentReceiver()
    {
  
        Thread t1 = new Thread(new Runnable() {
            @Override
            public void run()
            {
                student = Student.Builder.newInstance()
                              .setId(1)
                              .setName("Ram")
                              .setAddress("Noida")
                              .build();
            }
        });
  
        Thread t2 = new Thread(new Runnable() {
            @Override
            public void run()
            {
                student = Student.Builder.newInstance()
                              .setId(2)
                              .setName("Shyam")
                              .setAddress("Delhi")
                              .build();
            }
        });
  
        t1.start();
        t2.start();
    }
  
    public Student getStudent()
    {
        return student;
    }
}
  
// Driver class
public class BuilderDemo {
    public static void main(String args[])
    {
        StudentReceiver sr = new StudentReceiver();
        System.out.println(sr.getStudent());
    }
}

Output is guaranteed to be one of below:

id = 1, name = Ram, address = Noida

OR

id = 2, name = Shyam, address = Delhi

The Builder.newInstance() factory method can also be called with any required arguments to obtain a Builder instance by overloading it. The object of Student class is constructed with the invocation of the build() method.
The above implementation of Builder pattern makes the Student class immutable and consequently thread-safe.

Also note that the student field in client side code cannot be declared final because it is assigned a new immutable object. But it be declared volatile to ensure visibility of shared reference to immutable objects. Also private members of Builder class maintain encapsulation.

Please have a look at append method of StringBuilder class in java.lang package to understand implementations of Builder pattern more.

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