You need to ensure that all values added to a Map satisfy a set of arbitrary conditions.

Decorate an existing Map with PredicatedMap from Commons Collections. Predicates add an inbound validation to a Map, validating keys or values any time an entry is added to a PredicatedMap. If a Predicate assigned to a key or value returns false, put() throws an IllegalArgumentException. The following example decorates a HashMap with PredicatedMap—two Predicates are created to validate the keys and the values:

import java.util.*;
import org.apache.commons.collections.map.PredicatedMap;
import org.apache.commons.collections.Predicate;
import org.apache.commons.collections.functors.EqualPredicate;
import org.apache.commons.collections.functors.InstanceofPredicate;
import org.apache.commons.collections.functors.OrPredicate;

// Create a Predicate that only accepts Strings
               Predicate onlyStrings = new InstanceofPredicate( String.class );

               // Create a Predicate that only allows "green" or "red"
Predicate onlyGreen = new EqualPredicate( "green" );
Predicate onlyRed = new EqualPredicate( "red" );
Predicate greenOrRed = new OrPredicate( onlyGreen, onlyRed );

               // Created a Decorated Map - accepting String keys and "green" or "red" values
               Map map = PredicatedMap.decorate( new HashMap( ), onlyStrings, greenOrRed );

// All of these puts should work
map.put( "tony" , "green" );
map.put( "alice" , "red" );
map.put( "mike" , "red" );
map.put( "bobby" , "green" );

// All of these puts should throw an IllegalArgumentException
map.put( new Double(4.0) , "green" );
map.put( "alice" , "purple" );
map.put( new Long(32) , new Long(43) );

In the previous example, keys are validated by a simple InstanceofPredicate, which ensures that each key is a String. The values are validated with an OrPredicate, which combines two EqualPredicates; values are accepted if they are equal to the strings “green” or “red.”

A PredicatedMap can work with any Predicate no matter how simple or complex. Passing in a null value for either the key or value Predicate instructs the PredicatedMap not to validate keys or values. In the following example, a PredicatedMap decorates a HashMap to ensure that a Map contains valid Team values:

import org.apache.commons.collections.functors.AndPredicate;
import org.apache.commons.collections.map.PredicatedMap;

// Create the Predicates
ValidTeamPredicate validTeam = new ValidTeamPredicate( );
ValidCoachPredicate validCoach = new ValidCoachPredicate( );

// Tie two Predicates together into an AndPredicate
AndPredicate valuePredicate = new AndPredicate( validTeam, validCoach );
        
// Decorate a HashMap with a predicate on the value
Map teamMap = PredicatedMap.decorate( new HashMap( ), null, valuePredicate);
        
// Manufacture some teams
Team redSox = new Team( "Red Sox", new Coach( "Patrick", "Moloney") );
Team yankees= new Team( "Yankees", new Coach( "David", "McGarry") );
Team dodgers = new Team( "Dodgers", new Coach( "Nick", "Taylor") );
Team twins = new Team( null, new Coach( "Patrick", "Moloney") );
Team braves = new Team( "Braves", null );
        
// The following put calls should work fine
teamMap.put( "RedSox", redSox );
teamMap.put( "Yankees", yankees );
teamMap.put( "Dodgers", dodgers );
        
// This put should fail because the team name is null
try {
    teamMap.put( "Twins", twins);
} catch( IllegalArgumentException iae ) {
    System.out.println( "Twins put failed, as expected" );
}

// This put should fail because the coach is null
try {
    teamMap.put( "Braves", braves);
} catch( IllegalArgumentException iae ) {
    System.out.println( "Braves put failed, as expected" );
}

An application can assume that every team in teamMap has met the following requirements:

The previous example uses two Predicate objects to validate both the team and the coach. These two custom predicates (ValidTeamPredicate and ValidCoachPredicate) are defined in Examples Example 5-15 and Example 5-16. These Predicate objects are passed to the PredicatedMap.decorate( ) method to decorate a Map with these conditions. The first Predicate, ValidTeamPredicate, validates a Team object to see if it has a non-null name property.

import org.apache.commons.collections.Predicate;

public class ValidTeamPredicate implements Predicate {

    public boolean evaluate(Object object) {
        Team team = (Team) object;
        return team.getName( ) != null;        
    }

}

import org.apache.commons.collections.Predicate;

public class ValidCoachPredicate implements Predicate {

    public boolean evaluate(Object object) {
        boolean validCoach = false;
        Team team = (Team) object;
        if( team.getCoach( ) != null && 
            team.getCoach( ).getFirstName( ) != null &&
            team.getCoach( ).getLastName( ) != null ) {
            validCoach = true;
        }
        return validCoach;
    }

}

The second Predicate, ValidCoachPredicate, validates a Team object, checking to see if the coach property is non-null, and that the coach has a first and last name.

In Example 5-16, a Predicate is created by combining the ValidTeamPredicate and ValidCoachPredicate in an AndPredicate. A HashMap is then decorated with PredicatedMap.decorate( ), passing the AndPredicate in as the predicate to validate Map values. A few Team objects are created, all of which are valid with the exception of the Twins and the Braves, and, as expected, the Twins and Braves cause put( ) to throw an IllegalArgumentException.