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 a convenience mechanism that allows multiple method invocations on the same object to occur in a single statement. Classes that support method chaining provide several setter methods that return the {{this}} reference. 

Method chaining should not be used in a multi-threaded environment because chained invocations of a set of methods is non-atomic and consequently, noncompliant with [CON07-J. Do not assume that a group of calls to independently atomic methods is atomic].
 

h2. Noncompliant Code Example

Method chaining is useful for building an object and setting its optional fields. However, in a multi-threaded environment, a thread may observe a shared field to contain inconsistent values. This noncompliant code example shows the Javabeans pattern which is not safe for multithreaded use.

{code:bgColor=#FFcccc}
final class USCurrency {
  // Change requested, denomination (optional fields)
  private int quarters = 0;
  private int dimes = 0;
  private int nickels = 0;
  private int pennies = 0;

  public USCurrency() {}

  // Setter methods 
  public USCurrency setQuarters(int quantity) { 
    quarters = quantity; 
    return this;
  } 
  public USCurrency setDimes(int quantity) { 
    dimes = quantity; 
    return this;
  }
  public USCurrency setNickels(int quantity) { 
    nickels = quantity;
    return this;
  }
  public USCurrency setPennies(int quantity) { 
    pennies = quantity;
    return this;
  }
}

// ...

// Client code:
final USCurrency currency = new USCurrency();
new Thread(new Runnable() {
  public void run() {    
    USCurrency currency = new USCurrency().currency.setQuarters(1).setDimes(1);
  }
}).start();
new Thread(new Runnable() {
  public void run() {
    USCurrency currency = new USCurrency().currency.setQuarters(2).setDimes(2);
  }
}).start();
{code}

The Javabeans pattern uses a no-argument constructor along with a series of parallel setter methods to build an object. This pattern is not thread-safe and can lead to inconsistent object state. In this example

{code}

The {{Builder}} constructor is called with the _required_ arguments (if any) to obtain a _builder_ object. The _optional_ parameters are set using the setter methods on the builder. The object construction concludes with the invocation of the {{build()}} method. This also makes the class {{Currency}} immutable, and consequently, thread-safe.

If input needs to be validated, make sure that the values are copied from the builder class to the containing outer class's fields prior to the validation. The builder class does not violate [SCP03-J. Do not expose sensitive private members of the outer class from within a nested class] because it maintains a copy of the variables defined in the scope of the containing class. These take precedence and as a result, do not break encapsulation.

If the number of fields is small, it is better to synchronize the setter methods instead of using this design pattern. But care must be taken to ensure that the setter methods provide the required degree of atomicity. (see [CON07-J. Do not assume that a group of calls to independently atomic methods is atomic] for more information.)


h2. Exceptions

*EX1*: A class may use method chaining in a multithreaded environment if it sufficiently documents this fact. Client code must externally use some locking to ensure that the method calls are thread-safe.

{mc}
I don't think this class is necessary. (to avoid bloat; most readers would know what we are talking about by now)

{code:bgColor=#ccccff}
// This class is not thread-safe!. A client must provide locking on any
// USCurrency object in a multi-threaded environment!.
final class USCurrency {
  // Change requested, denomination (optional fields)
  private int quarters = 0;
  private int dimes = 0;
  private int nickels = 0;
  private int pennies = 0;

  public USCurrency() {}

  // Setter methods 
  public USCurrency setQuarters(int quantity) { 
    quarters = quantity; 
    return this;
  } 
  public USCurrency setDimes(int quantity) { 
    dimes = quantity; 
    return this;
  }
  public USCurrency setNickels(int quantity) { 
    nickels = quantity;
    return this;
  }
  public USCurrency setPennies(int quantity) { 
    pennies = quantity;
    return this;
  }
}

// ...

// Client code:
final USCurrency currency = new USCurrency();
final Object lock = new Object();
new Thread(new Runnable() {
  public void run() {
    synchronized (lock) {
      currency.setQuarters(1).setDimes(1);
    }
  }
}).start();
new Thread(new Runnable() {
  public void run() {
    synchronized (lock) {
      currency.setQuarters(2).setDimes(2);
    }
  }
}).start();
{code}

This code achieves thread-safety by having the client code perform all modification of the {{USCurrency}} object only while a {{lock}} is held