Package fr.becpg.repo.repository.impl

Class BeCPGHashCodeBuilder

java.lang.Object
fr.becpg.repo.repository.impl.BeCPGHashCodeBuilder
public class BeCPGHashCodeBuilder extends Object

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 hashcode 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 BeCPGHashCodeBuilder(17, 37).
       append(name).
       append(age).
       append(smoker).
       toHashCode();
   }
 }

If required, the superclass hashCode() can be added using 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 BeCPGHashCodeBuilder.reflectionHashCode(this);
 }
Since:
1.0
Version:
$Id: BeCPGHashCodeBuilder.java 1057009 2011-01-09 19:48:06Z niallp $ beCPG FIX when Integer, Float or Double is null give same hashCode as when egal 0
Author:
Apache Software Foundation, Gary Gregory, Pete Gieser, Matthieu L

  • Field Details

    • iConstant

      private static final int iConstant
      Constant to use in building the hashCode.
      See Also:

    • NULL_MULTIPLIER

      private static final int NULL_MULTIPLIER
      See Also:

    • annotatedMethodsCache

      private static final Map<Class<?>,List<Method>> annotatedMethodsCache

    • logger

      private static org.apache.commons.logging.Log logger

  • Constructor Details

    • BeCPGHashCodeBuilder

      public BeCPGHashCodeBuilder()

  • Method Details

    • reflectionAppend

      private long reflectionAppend(RepositoryEntity object, Set<RepositoryEntity> visited)

    • getCachedMethods

      private List<Method> getCachedMethods(Class<?> clazz)
      Extract method caching logic for better readability

    • isAnnotatedMethod

      private boolean isAnnotatedMethod(Method method)
      Check if method has relevant annotations

    • appendAspects

      private long appendAspects(long total, AspectAwareDataItem aspectAwareDataItem, Set<RepositoryEntity> visited)
      Optimized aspect handling

    • printDiff

      public static String printDiff(RepositoryEntity obj1, RepositoryEntity obj2)

      printDiff.

      Parameters:
      obj1 - a RepositoryEntity object.
      obj2 - a RepositoryEntity object.
      Returns:
      a String object.

    • appendDiffDetails

      private static void appendDiffDetails(StringBuilder ret, Object fieldValue, Object fieldValue2, RepositoryEntity obj1, RepositoryEntity obj2)
      Extract diff details logic for better readability

    • appendListDiff

      private static void appendListDiff(StringBuilder ret, List<?> list1, List<?> list2)
      Extract list diff logic

    • appendNodeRefDiff

      private static void appendNodeRefDiff(StringBuilder ret, RepositoryEntity obj1, RepositoryEntity obj2)
      Extract nodeRef diff logic

    • appendAspectsDiff

      private static void appendAspectsDiff(StringBuilder ret, RepositoryEntity obj1, RepositoryEntity obj2)
      Extract aspects diff logic

    • calculateAspectsHash

      private static int calculateAspectsHash(RepositoryEntity obj)
      Calculate aspects hash for diff comparison

    • reflectionHashCode

      public static long reflectionHashCode(RepositoryEntity 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

    • append

      private long append(long total, 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 java.lang.Boolean.hashCode handling, which computes a hashCode value of 1231 for java.lang.Boolean instances that represent true or 1237 for java.lang.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

    • append

      private long append(long total, boolean[] array)

      Append a hashCode for a boolean array.

      Parameters:
      array - the array to add to the hashCode
      Returns:
      this

    • append

      private long append(long total, byte value)

      Append a hashCode for a byte.

      Parameters:
      value - the byte to add to the hashCode
      Returns:
      this

    • append

      private long append(long total, byte[] array)

      Append a hashCode for a byte array.

      Parameters:
      array - the array to add to the hashCode
      Returns:
      this

    • append

      private long append(long total, char value)

      Append a hashCode for a char.

      Parameters:
      value - the char to add to the hashCode
      Returns:
      this

    • append

      private long append(long total, char[] array)

      Append a hashCode for a char array.

      Parameters:
      array - the array to add to the hashCode
      Returns:
      this

    • append

      private long append(long total, double value)

      Append a hashCode for a double.

      Parameters:
      value - the double to add to the hashCode
      Returns:
      this

    • append

      private long append(long total, double[] array)

      Append a hashCode for a double array.

      Parameters:
      array - the array to add to the hashCode
      Returns:
      this

    • append

      private long append(long total, float value)

      Append a hashCode for a float.

      Parameters:
      value - the float to add to the hashCode
      Returns:
      this

    • append

      private long append(long total, float[] array)

      Append a hashCode for a float array.

      Parameters:
      array - the array to add to the hashCode
      Returns:
      this

    • append

      private long append(long total, int value)

      Append a hashCode for an int.

      Parameters:
      value - the int to add to the hashCode
      Returns:
      this

    • append

      private long append(long total, int[] array)

      Append a hashCode for an int array.

      Parameters:
      array - the array to add to the hashCode
      Returns:
      this

    • append

      private long append(long total, long value)

      Append a hashCode for a long.

      Parameters:
      value - the long to add to the hashCode
      Returns:
      this

    • append

      private long append(long total, long[] array)

      Append a hashCode for a long array.

      Parameters:
      array - the array to add to the hashCode
      Returns:
      this

    • append

      private long append(long total, Object object, Set<RepositoryEntity> visited)

      Append a hashCode for an Object.

      Parameters:
      object - the Object to add to the hashCode
      Returns:
      this

    • appendArray

      private long appendArray(long total, Object object, Set<RepositoryEntity> visited)
      Optimized array handling

    • append

      public long append(long total, Object[] array, Set<RepositoryEntity> visited)

      Append a hashCode for an Object array.

      Parameters:
      array - the array to add to the hashCode
      total - a long.
      visited - a Set object.
      Returns:
      this

    • append

      private long append(long total, short value)

      Append a hashCode for a short.

      Parameters:
      value - the short to add to the hashCode
      Returns:
      this

    • append

      private long append(long total, short[] array)

      Append a hashCode for a short array.

      Parameters:
      array - the array to add to the hashCode
      Returns:
      this