Package edu.internet2.middleware.grouperInstaller.util

Class GcHashCodeBuilder

java.lang.Object
edu.internet2.middleware.grouperInstaller.util.GcHashCodeBuilder
public class GcHashCodeBuilder 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.

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 GcHashCodeBuilder(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 GcHashCodeBuilder.reflectionHashCode(this);
 }
Since:
1.0
Version:
$Id: GcHashCodeBuilder.java 161243 2005-04-14 04:30:28Z ggregory $

  • Constructor Details

    • GcHashCodeBuilder

      public GcHashCodeBuilder()

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

    • GcHashCodeBuilder

      public GcHashCodeBuilder(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 Details

    • 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

    • appendSuper

      public GcHashCodeBuilder appendSuper(int superHashCode)

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

      Parameters:
      superHashCode - the result of calling super.hashCode()
      Returns:
      this GcHashCodeBuilder, used to chain calls.
      Since:
      2.0

    • append

      public GcHashCodeBuilder append(Object object)

      Append a hashCode for an Object.

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

    • append

      public GcHashCodeBuilder append(long value)

      Append a hashCode for a long.

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

    • append

      public GcHashCodeBuilder append(int value)

      Append a hashCode for an int.

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

    • append

      public GcHashCodeBuilder append(short value)

      Append a hashCode for a short.

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

    • append

      public GcHashCodeBuilder append(char value)

      Append a hashCode for a char.

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

    • append

      public GcHashCodeBuilder append(byte value)

      Append a hashCode for a byte.

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

    • append

      public GcHashCodeBuilder append(double value)

      Append a hashCode for a double.

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

    • append

      public GcHashCodeBuilder append(float value)

      Append a hashCode for a float.

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

    • append

      public GcHashCodeBuilder 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 GcHashCodeBuilder append(Object[] array)

      Append a hashCode for an Object array.

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

    • append

      public GcHashCodeBuilder append(long[] array)

      Append a hashCode for a long array.

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

    • append

      public GcHashCodeBuilder append(int[] array)

      Append a hashCode for an int array.

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

    • append

      public GcHashCodeBuilder append(short[] array)

      Append a hashCode for a short array.

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

    • append

      public GcHashCodeBuilder append(char[] array)

      Append a hashCode for a char array.

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

    • append

      public GcHashCodeBuilder append(byte[] array)

      Append a hashCode for a byte array.

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

    • append

      public GcHashCodeBuilder append(double[] array)

      Append a hashCode for a double array.

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

    • append

      public GcHashCodeBuilder append(float[] array)

      Append a hashCode for a float array.

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

    • append

      public GcHashCodeBuilder append(boolean[] array)

      Append a hashCode for a boolean array.

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

    • toHashCode

      public int toHashCode()

      Return the computed hashCode.

      Returns:
      hashCode based on the fields appended