You need a quick way to implement compareTo( ) methods.

Commons Lang CompareToBuilder provides a builder for compareTo( ) methods. CompareToBuilder can perform a comparison via reflection, or a comparison can be customized by passing parameters to an instance of CompareToBuilder. The following example demonstrates the use of the reflection builder to implement a compareTo( ) method. This implementation compares all nonstatic and nontransient member variables in both classes.

import org.apache.commons.lang.builder.CompareToBuilder;
// Build a compareTo function from reflection 
public int compareTo(Object o) {
    return CompareToBuilder.reflectionCompare(this, obj);
}

CompareToBuilder.reflectionCompare( ) takes two objects and compares the non-static, nontransient member variables of these objects. In Example 1-2, the comparison involves the name properties of two PoliticalCandidate objects from Example 1-3, which are considered equal if both firstName and lastName are equal. reflectionCompare( ) ignores static fields and transients; therefore, in Example 1-3, averageAge and fullName do not contribute to the automated comparison.

public class PoliticalCandidate implements Comparable {
  private String lastName;
  private String firstName;
  private transient String fullName;
  
  // Static Variable
  private static String averageAge;
  
  
    // Constructors
    // get/set methods
    
    // Build a compareTo function from reflection 
    public int compareTo(Object o) {
        return CompareToBuilder.reflectionCompare(this, o);
    }    
  
}

In addition to a comparison by reflection, the CompareToBuilder can be configured to compare two objects by a set of variables in a particular order. The order of comparisons plays a key role when a comparison involves multiple member variables; this order is not specified when using the reflectionCompare() method. Assume that the default sorting behavior for PoliticalCandidate objects should be lastName and then firstName; if two objects have the same lastName, then sort by the firstName. The following example demonstrates a customization of the compareTo( ) method.

Calling append( ) specifies what variables will be compared and in what order they will be compared. The order of the calls to append( ) are backward—similar to pushing an object onto the top of a stack. The last property "pushed" onto the CompareToBuilder is the first property to be compared. Objects are compared by last name, and first name is used as a "tiebreaker." Example 1-4 will compare two PoliticalCandidate objects by lastName, falling back to firstName only if the lastName values were equal.

// A compare to that mimics the behavior of equals( )
public int compareTo(Object o) {
    int compare = -1; // By default return less-than
    if( o != null && 
        PoliticalCandidate.class.isAssignableFrom( o.getClass( ) ) ) {
            PoliticalCandidate pc = (PoliticalCandidate) o;
            compare = (new CompareToBuilder( )
                          .append(firstName, pc.firstName)
                          .append(lastName, pc.lastName)).toComparison( );
    }
    return compare;
}

Remember to keep the behavior of equals( ) and compareTo( ) consistent to avoid problems when sorting collections. Automating the compareTo( ) method via reflection may not compare objects in a way that is consistent with equals( ).

compareTo( ) methods provide the natural sorting order for a set of objects, and they are frequently used when sorting a collection of JavaBeans©. If you are trying to sort a collection of beans, you are better off using the BeanComparator, which is described in Recipe 3.10.

Instead of capturing comparison logic in a compareTo() method, consider using a Comparator object. The Commons Collections project contains a number of supplements to the Comparator interface, such as utilities to reverse and chain comparators. Comparator utilities are discussed in Chapter 4.