org.apache.commons.lang.builder

Class HashCodeBuilder

public class HashCodeBuilder extends Object

Assists in implementing {@link Object#hashCode()} methods.

This class enables a good hashCode method to be built for any class. It follows the rules laid out in the book Effective Java by Joshua Bloch. Writing a good hashCode method is actually quite difficult. This class aims to simplify the process.

All relevant fields from the object should be included in the hashCode method. Derived fields may be excluded. In general, any field used in the equals method must be used in the hashCode method.

To use this class write code as follows:

 public class Person {
   String name;
   int age;
   boolean isSmoker;
   ...

   public int hashCode() {
     // you pick a hard-coded, randomly chosen, non-zero, odd number
     // ideally different for each class
     return new HashCodeBuilder(17, 37).
       append(name).
       append(age).
       append(smoker).
       toHashCode();
   }
 }
 

If required, the superclass hashCode() can be added using {@link #appendSuper}.

Alternatively, there is a method that uses reflection to determine the fields to test. Because these fields are usually private, the method, reflectionHashCode, uses AccessibleObject.setAccessible to change the visibility of the fields. This will fail under a security manager, unless the appropriate permissions are set up correctly. It is also slower than testing explicitly.

A typical invocation for this method would look like:

 public int hashCode() {
   return HashCodeBuilder.reflectionHashCode(this);
 }
 

Since: 1.0

Version: $Id: HashCodeBuilder.java 161243 2005-04-14 04:30:28Z ggregory $

Author: Stephen Colebourne Gary Gregory Pete Gieser

Constructor Summary
HashCodeBuilder()

Uses two hard coded choices for the constants needed to build a hashCode.

HashCodeBuilder(int initialNonZeroOddNumber, int multiplierNonZeroOddNumber)

Two randomly chosen, non-zero, odd numbers must be passed in.

Method Summary
HashCodeBuilderappend(Object object)

Append a hashCode for an Object.

HashCodeBuilderappend(long value)

Append a hashCode for a long.

HashCodeBuilderappend(int value)

Append a hashCode for an int.

HashCodeBuilderappend(short value)

Append a hashCode for a short.

HashCodeBuilderappend(char value)

Append a hashCode for a char.

HashCodeBuilderappend(byte value)

Append a hashCode for a byte.

HashCodeBuilderappend(double value)

Append a hashCode for a double.

HashCodeBuilderappend(float value)

Append a hashCode for a float.

HashCodeBuilderappend(boolean value)

Append a hashCode for a boolean.

This adds iConstant * 1 to the hashCode and not a 1231 or 1237 as done in java.lang.Boolean.

HashCodeBuilderappend(Object[] array)

Append a hashCode for an Object array.

HashCodeBuilderappend(long[] array)

Append a hashCode for a long array.

HashCodeBuilderappend(int[] array)

Append a hashCode for an int array.

HashCodeBuilderappend(short[] array)

Append a hashCode for a short array.

HashCodeBuilderappend(char[] array)

Append a hashCode for a char array.

HashCodeBuilderappend(byte[] array)

Append a hashCode for a byte array.

HashCodeBuilderappend(double[] array)

Append a hashCode for a double array.

HashCodeBuilderappend(float[] array)

Append a hashCode for a float array.

HashCodeBuilderappend(boolean[] array)

Append a hashCode for a boolean array.

HashCodeBuilderappendSuper(int superHashCode)

Adds the result of super.hashCode() to this builder.

static intreflectionHashCode(Object object)

This method uses reflection to build a valid hash code.

This constructor uses two hard coded choices for the constants needed to build a hash code.

It uses AccessibleObject.setAccessible to gain access to private fields.

static intreflectionHashCode(Object object, boolean testTransients)

This method uses reflection to build a valid hash code.

This constructor uses two hard coded choices for the constants needed to build a hash code.

It uses AccessibleObject.setAccessible to gain access to private fields.

static intreflectionHashCode(int initialNonZeroOddNumber, int multiplierNonZeroOddNumber, Object object)

This method uses reflection to build a valid hash code.

It uses AccessibleObject.setAccessible to gain access to private fields.

static intreflectionHashCode(int initialNonZeroOddNumber, int multiplierNonZeroOddNumber, Object object, boolean testTransients)

This method uses reflection to build a valid hash code.

It uses AccessibleObject.setAccessible to gain access to private fields.

static intreflectionHashCode(int initialNonZeroOddNumber, int multiplierNonZeroOddNumber, Object object, boolean testTransients, Class reflectUpToClass)

This method uses reflection to build a valid hash code.

It uses AccessibleObject.setAccessible to gain access to private fields.

inttoHashCode()

Return the computed hashCode.

Constructor Detail

HashCodeBuilder

public HashCodeBuilder()

Uses two hard coded choices for the constants needed to build a hashCode.

HashCodeBuilder

public HashCodeBuilder(int initialNonZeroOddNumber, int multiplierNonZeroOddNumber)

Two randomly chosen, non-zero, odd numbers must be passed in. Ideally these should be different for each class, however this is not vital.

Prime numbers are preferred, especially for the multiplier.

Parameters: initialNonZeroOddNumber a non-zero, odd number used as the initial value multiplierNonZeroOddNumber a non-zero, odd number used as the multiplier

Throws: IllegalArgumentException if the number is zero or even

Method Detail

append

public HashCodeBuilder append(Object object)

Append a hashCode for an Object.

Parameters: object the Object to add to the hashCode

Returns: this

append

public HashCodeBuilder append(long value)

Append a hashCode for a long.

Parameters: value the long to add to the hashCode

Returns: this

append

public HashCodeBuilder append(int value)

Append a hashCode for an int.

Parameters: value the int to add to the hashCode

Returns: this

append

public HashCodeBuilder append(short value)

Append a hashCode for a short.

Parameters: value the short to add to the hashCode

Returns: this

append

public HashCodeBuilder append(char value)

Append a hashCode for a char.

Parameters: value the char to add to the hashCode

Returns: this

append

public HashCodeBuilder append(byte value)

Append a hashCode for a byte.

Parameters: value the byte to add to the hashCode

Returns: this

append

public HashCodeBuilder append(double value)

Append a hashCode for a double.

Parameters: value the double to add to the hashCode

Returns: this

append

public HashCodeBuilder append(float value)

Append a hashCode for a float.

Parameters: value the float to add to the hashCode

Returns: this

append

public HashCodeBuilder append(boolean value)

Append a hashCode for a boolean.

This adds iConstant * 1 to the hashCode and not a 1231 or 1237 as done in java.lang.Boolean. This is in accordance with the Effective Java design.

Parameters: value the boolean to add to the hashCode

Returns: this

append

public HashCodeBuilder append(Object[] array)

Append a hashCode for an Object array.

Parameters: array the array to add to the hashCode

Returns: this

append

public HashCodeBuilder append(long[] array)

Append a hashCode for a long array.

Parameters: array the array to add to the hashCode

Returns: this

append

public HashCodeBuilder append(int[] array)

Append a hashCode for an int array.

Parameters: array the array to add to the hashCode

Returns: this

append

public HashCodeBuilder append(short[] array)

Append a hashCode for a short array.

Parameters: array the array to add to the hashCode

Returns: this

append

public HashCodeBuilder append(char[] array)

Append a hashCode for a char array.

Parameters: array the array to add to the hashCode

Returns: this

append

public HashCodeBuilder append(byte[] array)

Append a hashCode for a byte array.

Parameters: array the array to add to the hashCode

Returns: this

append

public HashCodeBuilder append(double[] array)

Append a hashCode for a double array.

Parameters: array the array to add to the hashCode

Returns: this

append

public HashCodeBuilder append(float[] array)

Append a hashCode for a float array.

Parameters: array the array to add to the hashCode

Returns: this

append

public HashCodeBuilder append(boolean[] array)

Append a hashCode for a boolean array.

Parameters: array the array to add to the hashCode

Returns: this

appendSuper

public HashCodeBuilder appendSuper(int superHashCode)

Adds the result of super.hashCode() to this builder.

Parameters: superHashCode the result of calling super.hashCode()

Returns: this HashCodeBuilder, used to chain calls.

Since: 2.0

reflectionHashCode

public static int reflectionHashCode(Object object)

This method uses reflection to build a valid hash code.

This constructor uses two hard coded choices for the constants needed to build a hash code.

It uses AccessibleObject.setAccessible to gain access to private fields. This means that it will throw a security exception if run under a security manager, if the permissions are not set up correctly. It is also not as efficient as testing explicitly.

Transient members will be not be used, as they are likely derived fields, and not part of the value of the Object.

Static fields will not be tested. Superclass fields will be included.

Parameters: object the Object to create a hashCode for

Returns: int hash code

Throws: IllegalArgumentException if the object is null

reflectionHashCode

public static int reflectionHashCode(Object object, boolean testTransients)

This method uses reflection to build a valid hash code.

This constructor uses two hard coded choices for the constants needed to build a hash code.

It uses AccessibleObject.setAccessible to gain access to private fields. This means that it will throw a security exception if run under a security manager, if the permissions are not set up correctly. It is also not as efficient as testing explicitly.

If the TestTransients parameter is set to true, transient members will be tested, otherwise they are ignored, as they are likely derived fields, and not part of the value of the Object.

Static fields will not be tested. Superclass fields will be included.

Parameters: object the Object to create a hashCode for testTransients whether to include transient fields

Returns: int hash code

Throws: IllegalArgumentException if the object is null

reflectionHashCode

public static int reflectionHashCode(int initialNonZeroOddNumber, int multiplierNonZeroOddNumber, Object object)

This method uses reflection to build a valid hash code.

It uses AccessibleObject.setAccessible to gain access to private fields. This means that it will throw a security exception if run under a security manager, if the permissions are not set up correctly. It is also not as efficient as testing explicitly.

Transient members will be not be used, as they are likely derived fields, and not part of the value of the Object.

Static fields will not be tested. Superclass fields will be included.

Two randomly chosen, non-zero, odd numbers must be passed in. Ideally these should be different for each class, however this is not vital. Prime numbers are preferred, especially for the multiplier.

Parameters: initialNonZeroOddNumber a non-zero, odd number used as the initial value multiplierNonZeroOddNumber a non-zero, odd number used as the multiplier object the Object to create a hashCode for

Returns: int hash code

Throws: IllegalArgumentException if the Object is null IllegalArgumentException if the number is zero or even

reflectionHashCode

public static int reflectionHashCode(int initialNonZeroOddNumber, int multiplierNonZeroOddNumber, Object object, boolean testTransients)

This method uses reflection to build a valid hash code.

It uses AccessibleObject.setAccessible to gain access to private fields. This means that it will throw a security exception if run under a security manager, if the permissions are not set up correctly. It is also not as efficient as testing explicitly.

If the TestTransients parameter is set to true, transient members will be tested, otherwise they are ignored, as they are likely derived fields, and not part of the value of the Object.

Static fields will not be tested. Superclass fields will be included.

Two randomly chosen, non-zero, odd numbers must be passed in. Ideally these should be different for each class, however this is not vital. Prime numbers are preferred, especially for the multiplier.

Parameters: initialNonZeroOddNumber a non-zero, odd number used as the initial value multiplierNonZeroOddNumber a non-zero, odd number used as the multiplier object the Object to create a hashCode for testTransients whether to include transient fields

Returns: int hash code

Throws: IllegalArgumentException if the Object is null IllegalArgumentException if the number is zero or even

reflectionHashCode

public static int reflectionHashCode(int initialNonZeroOddNumber, int multiplierNonZeroOddNumber, Object object, boolean testTransients, Class reflectUpToClass)

This method uses reflection to build a valid hash code.

It uses AccessibleObject.setAccessible to gain access to private fields. This means that it will throw a security exception if run under a security manager, if the permissions are not set up correctly. It is also not as efficient as testing explicitly.

If the TestTransients parameter is set to true, transient members will be tested, otherwise they are ignored, as they are likely derived fields, and not part of the value of the Object.

Static fields will not be included. Superclass fields will be included up to and including the specified superclass. A null superclass is treated as java.lang.Object.

Two randomly chosen, non-zero, odd numbers must be passed in. Ideally these should be different for each class, however this is not vital. Prime numbers are preferred, especially for the multiplier.

Parameters: initialNonZeroOddNumber a non-zero, odd number used as the initial value multiplierNonZeroOddNumber a non-zero, odd number used as the multiplier object the Object to create a hashCode for testTransients whether to include transient fields reflectUpToClass the superclass to reflect up to (inclusive), may be null

Returns: int hash code

Throws: IllegalArgumentException if the Object is null IllegalArgumentException if the number is zero or even

Since: 2.0

toHashCode

public int toHashCode()

Return the computed hashCode.

Returns: hashCode based on the fields appended

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