Implementing Comparable allows:

• calling Collections.sort and Collections.binarySearch
• calling Arrays.sort and Arrays.binarySearch
• using objects as keys in a TreeMap
• using objects as elements in a TreeSet

• anticommutation :  x.compareTo(y) is the opposite sign of y.compareTo(x)
• exception symmetry : x.compareTo(y) throws exactly the same exceptions as y.compareTo(x)
• transitivity :  if x.compareTo(y)>0 and y.compareTo(z)>0, then x.compareTo(z)>0  (and same for less than)
•  if x.compareTo(y)==0, then x.compareTo(z) has the same sign as y.compareTo(z)
• consistency with equals is highly recommended, but not required : x.compareTo(y)==0, if and only if x.equals(y) ; consistency with equals is required for ensuring sorted collections (such as TreeSet) are well-behaved.

When a class extends a concrete Comparable class and adds a significant field, a correct implementation of compareTo cannot be constructed. The only alternative is to use composition instead of inheritance. (See Effective Java for more information.)

Compare the various types of fields as follows:

• numeric primitive : use < and >. There is an exception to this rule: float and double primitives should be compared using Float.compare(float, float) and Double.compare(double, double). This avoids problems associated with special border values. (Thanks to Roger Orr in the UK for pointing this out.)
• boolean primitive :  use tests of the form (x && !y)
• Object : use compareTo. (Note that possibly-null fields present a problem : while x.equals(null) returns false, x.compareTo(null) will always throw a NullPointerException)
• type-safe enumeration : use compareTo, like any Object
• collection or array : Comparable does not seem to be intended for these kinds of fields. For example, List, Map and Set do not implement Comparable. As well, some collections have no definite order of iteration, so doing an element-by-element comparison cannot be meaningful in those cases.

An alternative to implementing Comparable is passing Comparator objects as parameters. Be aware that if a Comparator compares only one of several significant fields, then the Comparator is very likely not synchronized with equals.

All primitive wrapper classes implement Comparable. Note that Boolean did not implement Comparable until version 1.5, however.

Example

import java.util.*;

/** @since Java 7 */
public final class Account implements Comparable<Account> {
  
  enum AccountType {CASH, MARGIN, RRSP};

   public Account (
      String firstName,
      String lastName,
      int accountNumber,
      int balance,
      boolean isNewAccount,
      AccountType accountType
  ) {
      //..detailed parameter validations elided
      this.firstName = Objects.requireNonNull(firstName);
      this.lastName = Objects.requireNonNull(lastName);
      this.accountNumber = accountNumber;
      this.balance = balance;
      this.isNewAccount = isNewAccount;
      this.accountType = accountType;
   }

  /**
  * @param that is a non-null Account.
  */
  @Override public int compareTo(Account that) {
    final int BEFORE = -1;
    final int EQUAL = 0;
    final int AFTER = 1;

    //this optimization is usually worthwhile, and can
    //always be added
    if (this == that) return EQUAL;

    //primitive numbers follow this form
    if (this.accountNumber < that.accountNumber) return BEFORE;
    if (this.accountNumber > that.accountNumber) return AFTER;

    //booleans follow this form
    if (!this.isNewAccount && that.isNewAccount) return BEFORE;
    if (this.isNewAccount && !that.isNewAccount) return AFTER;

    //objects, including type-safe enums, follow this form
    //note that null objects will throw an exception here
    int comparison = this.accountType.compareTo(that.accountType);
    if (comparison != EQUAL) return comparison;

    comparison = this.lastName.compareTo(that.lastName);
    if (comparison != EQUAL) return comparison;

    comparison = this.firstName.compareTo(that.firstName);
    if (comparison != EQUAL) return comparison;

    if (this.balance < that.balance) return BEFORE;
    if (this.balance > that.balance) return AFTER;

    //all comparisons have yielded equality
    //verify that compareTo is consistent with equals (optional)
    assert this.equals(that) : "compareTo inconsistent with equals.";

    return EQUAL;
  }

   /**
   * Define equality of state.
   */
   @Override public boolean equals(Object aThat) {
     if (this == aThat) return true;
     if (!(aThat instanceof Account)) return false;
     Account that = (Account)aThat;
     for(int i = 0; i < this.getSigFields().length; ++i){
       if (!Objects.equals(this.getSigFields()[i], that.getSigFields()[i])){
         return false;
       }
     }
     return true;     
   }

   /**
   * A class that overrides equals must also override hashCode.
   */
   @Override public int hashCode() {
     return Objects.hash(getSigFields());     
   }

   //PRIVATE

   private String firstName; //non-null
   private String lastName;  //non-null
   /** These are not Integer, Boolean here, in order to exercise different cases:*/
   private int accountNumber;
   private int balance;
   private boolean isNewAccount;

   /**
   * Type of the account, expressed as a type-safe enumeration (non-null).
   */
   private AccountType accountType;
   
   private Object[] getSigFields() {
     Object[] result = {
      accountNumber, firstName, lastName, balance, isNewAccount, accountType
     };
     return result;     
   }

   /** Exercise compareTo.  */
   public static void main (String[] aArguments) {
     //Note the difference in behaviour in equals and compareTo, for nulls:
     String text = "blah";
     Integer number = 10;
     //x.equals(null) always returns false:
     log("false: " + text.equals(null));
     log("false: " + number.equals(null) );
     //x.compareTo(null) always throws NullPointerException:
     //System.out.println( text.compareTo(null) );
     //System.out.println( number.compareTo(null) );

     Account flaubert = new Account(
      "Gustave", "Flaubert", 1003, 0, true, AccountType.MARGIN
     );

     //all of these other versions of "flaubert" differ from the
     //original in only one field
     Account flaubert2 = new Account(
       "Guy", "Flaubert", 1003, 0, true, AccountType.MARGIN
     );
     Account flaubert3 = new Account(
       "Gustave", "de Maupassant", 1003, 0, true, AccountType.MARGIN
     );
     Account flaubert4 = new Account(
       "Gustave", "Flaubert", 2004, 0, true, AccountType.MARGIN
     );
     Account flaubert5 = new Account(
       "Gustave", "Flaubert", 1003, 1, true, AccountType.MARGIN
     );
     Account flaubert6 = new Account(
       "Gustave", "Flaubert", 1003, 0, false, AccountType.MARGIN
     );
     Account flaubert7 = new Account(
       "Gustave", "Flaubert", 1003, 0, true, AccountType.CASH
     );

     log( "0: " +  flaubert.compareTo(flaubert) );
     log( "first name +: " +  flaubert2.compareTo(flaubert) );
     //Note capital letters precede small letters
     log( "last name +: " +  flaubert3.compareTo(flaubert) );
     log( "acct number +: " +  flaubert4.compareTo(flaubert) );
     log( "balance +: " +  flaubert5.compareTo(flaubert) );
     log( "is new -: " +  flaubert6.compareTo(flaubert) );
     log( "account type -: " +  flaubert7.compareTo(flaubert) );
   }
   
   private static void log(String text) {
     System.out.println(text);
   }
   
} 

In the older JDK 1.4, there are two differences :

• the type-safe version of the Comparable  interface cannot be used. Instead, Object appears, along with a related cast operation
• Boolean objects must be treated differently from other wrapper classes, since Boolean did not implement Comparable until JDK 1.5.

import java.util.*;
import java.io.*;

public final class AccountOld implements Comparable {

   public AccountOld (
      String aFirstName,
      String aLastName,
      int aAccountNumber,
      int aBalance,
      boolean aIsNewAccount,
      AccountType aAccountType
   ) {
      //..parameter validations elided
      fFirstName = aFirstName;
      fLastName = aLastName;
      fAccountNumber = aAccountNumber;
      fBalance = aBalance;
      fIsNewAccount = aIsNewAccount;
      fAccountType = aAccountType;
   }

  /**
  * @param aThat is a non-null AccountOld.
  *
  * @throws NullPointerException if aThat is null.
  * @throws ClassCastException if aThat is not an AccountOld object.
  */
  public int compareTo(Object aThat) {
    final int BEFORE = -1;
    final int EQUAL = 0;
    final int AFTER = 1;

    //this optimization is usually worthwhile, and can
    //always be added
    if ( this == aThat ) return EQUAL;

    final AccountOld that = (AccountOld)aThat;

    //primitive numbers follow this form
    if (this.fAccountNumber < that.fAccountNumber) return BEFORE;
    if (this.fAccountNumber > that.fAccountNumber) return AFTER;

    //booleans follow this form
    if (!this.fIsNewAccount && that.fIsNewAccount) return BEFORE;
    if (this.fIsNewAccount && !that.fIsNewAccount) return AFTER;

    //Objects, including type-safe enums, follow this form.
    //Exception : Boolean implements Comparable in JDK 1.5, but not in 1.4
    //Note that null objects will throw an exception here.
    int comparison = this.fAccountType.compareTo(that.fAccountType);
    if ( comparison != EQUAL ) return comparison;

    comparison = this.fLastName.compareTo(that.fLastName);
    if ( comparison != EQUAL ) return comparison;

    comparison = this.fFirstName.compareTo(that.fFirstName);
    if ( comparison != EQUAL ) return comparison;

    if (this.fBalance < that.fBalance) return BEFORE;
    if (this.fBalance > that.fBalance) return AFTER;

    //all comparisons have yielded equality
    //verify that compareTo is consistent with equals (optional)
    assert this.equals(that) : "compareTo inconsistent with equals.";

    return EQUAL;
  }

   /**
   * Define equality of state.
   */
   public boolean equals(Object aThat) {
     if ( this == aThat ) return true;
     if ( !(aThat instanceof Account) ) return false;

     AccountOld that = (AccountOld)aThat;
     return
       ( this.fAccountNumber == that.fAccountNumber ) &&
       ( this.fAccountType == that.fAccountType ) &&
       ( this.fBalance == that.fBalance ) &&
       ( this.fIsNewAccount == that.fIsNewAccount ) &&
       ( this.fFirstName.equals(that.fFirstName) ) &&
       ( this.fLastName.equals(that.fLastName) );
   }

   /**
   * A class that overrides equals must also override hashCode.
   */
   public int hashCode() {
     int result = HashCodeUtil.SEED;
     result = HashCodeUtil.hash( result, fAccountNumber );
     result = HashCodeUtil.hash( result, fAccountType );
     result = HashCodeUtil.hash( result, fBalance );
     result = HashCodeUtil.hash( result, fIsNewAccount );
     result = HashCodeUtil.hash( result, fFirstName );
     result = HashCodeUtil.hash( result, fLastName );
     return result;
   }

   //PRIVATE

   private String fFirstName; //non-null
   private String fLastName;  //non-null
   private int fAccountNumber;
   private int fBalance;
   private boolean fIsNewAccount;

   /**
   * Type of the account, expressed as a type-safe enumeration (non-null).
   */
   private AccountType fAccountType;

   /**
   * Exercise compareTo.
   */
   public static void main (String[] aArguments) {
     //Note the difference in behaviour in equals and compareTo, for nulls:
     String text = "blah";
     Integer number = new Integer(10);
     //x.equals(null) always returns false:
     System.out.println("false: " + text.equals(null));
     System.out.println("false: " + number.equals(null) );
     //x.compareTo(null) always throws NullPointerException:
     //System.out.println( text.compareTo(null) );
     //System.out.println( number.compareTo(null) );

     AccountOld flaubert = new AccountOld(
       "Gustave", "Flaubert", 1003, 0, true, AccountType.MARGIN
     );

     //all of these other versions of "flaubert" differ from the
     //original in only one field
     AccountOld flaubert2 = new AccountOld(
       "Guy", "Flaubert", 1003, 0, true, AccountType.MARGIN
     );
     AccountOld flaubert3 = new AccountOld(
       "Gustave", "de Maupassant", 1003, 0, true, AccountType.MARGIN
     );
     AccountOld flaubert4 = new AccountOld(
       "Gustave", "Flaubert", 2004, 0, true, AccountType.MARGIN
     );
     AccountOld flaubert5 = new AccountOld(
       "Gustave", "Flaubert", 1003, 1, true, AccountType.MARGIN
     );
     AccountOld flaubert6 = new AccountOld(
       "Gustave", "Flaubert", 1003, 0, false, AccountType.MARGIN
     );
     AccountOld flaubert7 = new AccountOld(
       "Gustave", "Flaubert", 1003, 0, true, AccountType.CASH
     );

     System.out.println( "0: " +  flaubert.compareTo(flaubert) );
     System.out.println( "first name +: " +  flaubert2.compareTo(flaubert) );
     //Note capital letters precede small letters
     System.out.println( "last name +: " +  flaubert3.compareTo(flaubert) );
     System.out.println( "acct number +: " +  flaubert4.compareTo(flaubert) );
     System.out.println( "balance +: " +  flaubert5.compareTo(flaubert) );
     System.out.println( "is new -: " +  flaubert6.compareTo(flaubert) );
     System.out.println( "account type -: " +  flaubert7.compareTo(flaubert) );
   }
}