A Filter expects a stream of individual argument Tuples and returns a Boolean value for each one, stating whether it should be discarded. Like a Function, a Filter is used with an Each pipe, which may follow any pipe type.

To create a custom Filter, subclass the class cascading.operation.BaseOperation and implement the interface cascading.operation.Filter. Because BaseOperation has been subclassed, the isRemove method, as defined on the Filter interface, is the only method that must be implemented.

public class SomeFilter extends BaseOperation implements Filter
  {
  public boolean isRemove( FlowProcess flowProcess, FilterCall call )
    {
    // get the arguments TupleEntry
    TupleEntry arguments = call.getArguments();

    // initialize the return result
    boolean isRemove = false;

    // test the argument values and set isRemove accordingly

    return isRemove;
    }
  }

Filters must accept one or more values in a Tuple as arguments, and should declare the number of argument values they expect. If not specified, the default is to accept any number of values (Operation.ANY). Cascading verifies during planning that the number of arguments selected matches the number of arguments expected.

The number of arguments declaration must be done on the constructor, either by passing a default value to the super constructor, or by accepting the value from the user via a constructor implementation.

public class StringLengthFilter extends BaseOperation implements Filter
  {
  public StringLengthFilter()
    {
    // expects 2 arguments, fail otherwise
    super( 2 );
    }

  public boolean isRemove( FlowProcess flowProcess, FilterCall call )
    {
    // get the arguments TupleEntry
    TupleEntry arguments = call.getArguments();

    // filter out the current Tuple if the first argument length is greater
    // than the second argument integer value
    return arguments.getString( 0 ).length() > arguments.getInteger( 1 );
    }
  }

The example above implements a Filter that accepts two arguments and filters out the current Tuple if the first argument, String length, is greater than the integer value of the second argument.