Java Doc for HashCodeBuilder.java in  » Library » Apache-common-lang » org » apache » commons » lang » builder » Java Source Code / Java DocumentationJava Source Code and Java Documentation

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 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 HashCodeBuilder.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);
 }
 

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

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

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.

Append a hashCode for a boolean array.

Append a hashCode for a byte.

Append a hashCode for a byte array.

Append a hashCode for a char.

Append a hashCode for a char array.

Append a hashCode for a double.

Append a hashCode for a double array.

Append a hashCode for a float.

Append a hashCode for a float array.

Append a hashCode for an int.

Append a hashCode for an int array.

Append a hashCode for a long.

Append a hashCode for a long array.

Append a hashCode for an Object.

Append a hashCode for an Object array.

Append a hashCode for a short.

Append a hashCode for a short array.

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

Returns the registry of objects being traversed by the reflection methods in the current thread.

Returns true if the registry contains the given object.

This method uses reflection to build a valid hash code.

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

This method uses reflection to build a valid hash code.

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

This method uses reflection to build a valid hash code.

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

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 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 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 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.

Registers the given object.

Return the computed hashCode.

Unregisters the given object.

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

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.

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.

Append a hashCode for a boolean array.

Append a hashCode for a byte.

Append a hashCode for a byte array.

Append a hashCode for a char.

Append a hashCode for a char array.

Append a hashCode for a double.

Append a hashCode for a double array.

Append a hashCode for a float.

Append a hashCode for a float array.

Append a hashCode for an int.

Append a hashCode for an int array.

Append a hashCode for a long.

Append a hashCode for a long array.

Append a hashCode for an Object.

Append a hashCode for an Object array.

Append a hashCode for a short.

Append a hashCode for a short array.

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

Returns the registry of objects being traversed by the reflection methods in the current thread.

Returns true if the registry contains the given object. Used by the reflection methods to avoid infinite loops.

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.

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.

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.

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.

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.

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.

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.

Registers the given object. Used by the reflection methods to avoid infinite loops.

Return the computed hashCode.

Unregisters the given object.

Used by the reflection methods to avoid infinite loops.
Parameters:
  value - The object to unregister.
since:
   2.3