Package org.apache.commons.lang3.builder

Class HashCodeBuilder

java.lang.Object
org.apache.commons.lang3.builder.HashCodeBuilder
All Implemented Interfaces:
Builder<Integer>
public class HashCodeBuilder extends Object implements Builder<Integer>
Assists in implementing 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.

The following is the approach taken. When appending a data field, the current total is multiplied by the multiplier then a relevant value for that data type is added. For example, if the current hashCode is 17, and the multiplier is 37, then appending the integer 45 will create a hash code of 674, namely 17 * 37 + 45.

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 smoker;
   ...

   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 appendSuper(int).

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);
 }

The HashCodeExclude annotation can be used to exclude fields from being used by the reflectionHashCode methods.

Since:
1.0

  • Constructor Details

    • HashCodeBuilder

      public HashCodeBuilder()
      Uses two hard coded choices for the constants needed to build a hashCode.

    • HashCodeBuilder

      public HashCodeBuilder(int initialOddNumber, int multiplierOddNumber)
      Two randomly chosen, 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:
      initialOddNumber - an odd number used as the initial value
      multiplierOddNumber - an odd number used as the multiplier
      Throws:
      IllegalArgumentException - if the number is even

  • Method Details

    • reflectionHashCode

      public static int reflectionHashCode(int initialNonZeroOddNumber, int multiplierNonZeroOddNumber, Object object)
      Uses reflection to build a valid hash code from the fields of object.

      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. This will be the returned value if no fields are found to include in the hash code
      multiplierNonZeroOddNumber - a non-zero, odd number used as the multiplier
      object - the Object to create a hashCode for
      Returns:
      int hash code
      Throws:
      NullPointerException - if the Object is null
      IllegalArgumentException - if the number is zero or even
      See Also:

    • reflectionHashCode

      public static int reflectionHashCode(int initialNonZeroOddNumber, int multiplierNonZeroOddNumber, Object object, boolean testTransients)
      Uses reflection to build a valid hash code from the fields of object.

      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. This will be the returned value if no fields are found to include in the hash code
      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:
      NullPointerException - if the Object is null
      IllegalArgumentException - if the number is zero or even
      See Also:

    • reflectionHashCode

      public static <T> int reflectionHashCode(int initialNonZeroOddNumber, int multiplierNonZeroOddNumber, T object, boolean testTransients, Class<? super T> reflectUpToClass, String... excludeFields)
      Uses reflection to build a valid hash code from the fields of object.

      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.

      Type Parameters:
      T - the type of the object involved
      Parameters:
      initialNonZeroOddNumber - a non-zero, odd number used as the initial value. This will be the returned value if no fields are found to include in the hash code
      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
      excludeFields - array of field names to exclude from use in calculation of hash code
      Returns:
      int hash code
      Throws:
      NullPointerException - if the Object is null
      IllegalArgumentException - if the number is zero or even
      Since:
      2.0
      See Also:

    • reflectionHashCode

      public static int reflectionHashCode(Object object, boolean testTransients)
      Uses reflection to build a valid hash code from the fields of object.

      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. If no fields are found to include in the hash code, the result of this method will be constant.

      Parameters:
      object - the Object to create a hashCode for
      testTransients - whether to include transient fields
      Returns:
      int hash code
      Throws:
      NullPointerException - if the object is null
      See Also:

    • reflectionHashCode

      public static int reflectionHashCode(Object object, Collection<String> excludeFields)
      Uses reflection to build a valid hash code from the fields of object.

      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. If no fields are found to include in the hash code, the result of this method will be constant.

      Parameters:
      object - the Object to create a hashCode for
      excludeFields - Collection of String field names to exclude from use in calculation of hash code
      Returns:
      int hash code
      Throws:
      NullPointerException - if the object is null
      See Also:

    • reflectionHashCode

      public static int reflectionHashCode(Object object, String... excludeFields)
      Uses reflection to build a valid hash code from the fields of object.

      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. If no fields are found to include in the hash code, the result of this method will be constant.

      Parameters:
      object - the Object to create a hashCode for
      excludeFields - array of field names to exclude from use in calculation of hash code
      Returns:
      int hash code
      Throws:
      NullPointerException - if the object is null
      See Also:

    • append

      public HashCodeBuilder append(boolean value)
      Append a hashCode for a boolean.

      This adds 1 when true, and 0 when false to the hashCode.

      This is in contrast to the standard Boolean.hashCode() handling, which computes a hashCode value of 1231 for Boolean instances that represent true or 1237 for Boolean instances that represent false.

      This is in accordance with the Effective Java design.

      Parameters:
      value - the boolean to add to the hashCode
      Returns:
      this instance.

    • append

      public HashCodeBuilder append(boolean[] array)
      Append a hashCode for a boolean array.
      Parameters:
      array - the array to add to the hashCode
      Returns:
      this instance.

    • append

      public HashCodeBuilder append(byte value)
      Append a hashCode for a byte.
      Parameters:
      value - the byte to add to the hashCode
      Returns:
      this instance.

    • append

      public HashCodeBuilder append(byte[] array)
      Append a hashCode for a byte array.
      Parameters:
      array - the array to add to the hashCode
      Returns:
      this instance.

    • append

      public HashCodeBuilder append(char value)
      Append a hashCode for a char.
      Parameters:
      value - the char to add to the hashCode
      Returns:
      this instance.

    • append

      public HashCodeBuilder append(char[] array)
      Append a hashCode for a char array.
      Parameters:
      array - the array to add to the hashCode
      Returns:
      this instance.

    • append

      public HashCodeBuilder append(double value)
      Append a hashCode for a double.
      Parameters:
      value - the double to add to the hashCode
      Returns:
      this instance.

    • append

      public HashCodeBuilder append(double[] array)
      Append a hashCode for a double array.
      Parameters:
      array - the array to add to the hashCode
      Returns:
      this instance.

    • append

      public HashCodeBuilder append(float value)
      Append a hashCode for a float.
      Parameters:
      value - the float to add to the hashCode
      Returns:
      this instance.

    • append

      public HashCodeBuilder append(float[] array)
      Append a hashCode for a float array.
      Parameters:
      array - the array to add to the hashCode
      Returns:
      this instance.

    • append

      public HashCodeBuilder append(int value)
      Append a hashCode for an int.
      Parameters:
      value - the int to add to the hashCode
      Returns:
      this instance.

    • append

      public HashCodeBuilder append(int[] array)
      Append a hashCode for an int array.
      Parameters:
      array - the array to add to the hashCode
      Returns:
      this instance.

    • append

      public HashCodeBuilder append(long value)
      Append a hashCode for a long.
      Parameters:
      value - the long to add to the hashCode
      Returns:
      this instance.

    • append

      public HashCodeBuilder append(long[] array)
      Append a hashCode for a long array.
      Parameters:
      array - the array to add to the hashCode
      Returns:
      this instance.

    • append

      public HashCodeBuilder append(Object object)
      Append a hashCode for an Object.
      Parameters:
      object - the Object to add to the hashCode
      Returns:
      this instance.

    • append

      public HashCodeBuilder append(Object[] array)
      Append a hashCode for an Object array.
      Parameters:
      array - the array to add to the hashCode
      Returns:
      this instance.

    • append

      public HashCodeBuilder append(short value)
      Append a hashCode for a short.
      Parameters:
      value - the short to add to the hashCode
      Returns:
      this instance.

    • append

      public HashCodeBuilder append(short[] array)
      Append a hashCode for a short array.
      Parameters:
      array - the array to add to the hashCode
      Returns:
      this instance.

    • 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 instance.
      Since:
      2.0

    • build

      public Integer build()
      Returns the computed hashCode.
      Specified by:
      build in interface Builder<Integer>
      Returns:
      hashCode based on the fields appended
      Since:
      3.0

    • equals

      public boolean equals(Object obj)
      Implements equals using the hash code.
      Overrides:
      equals in class Object
      Since:
      3.13.0

    • hashCode

      public int hashCode()
      The computed hashCode from toHashCode() is returned due to the likelihood of bugs in mis-calling toHashCode() and the unlikeliness of it mattering what the hashCode for HashCodeBuilder itself is.
      Overrides:
      hashCode in class Object
      Returns:
      hashCode based on the fields appended
      Since:
      2.5

    • toHashCode

      public int toHashCode()
      Returns the computed hashCode.
      Returns:
      hashCode based on the fields appended