HashCodeBuilder (Javadocs: proscene)

java.lang.Object
- remixlab.util.HashCodeBuilder

```
public class HashCodeBuilder
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 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);
 }
 
```
Since:

1.0

Author:

Apache Software Foundation, Gary Gregory, Pete Gieser, Daniel Bell

Constructor Summary

Constructors
Constructor and Description
`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

All Methods Instance Methods Concrete Methods
Modifier and Type	Method and Description
`HashCodeBuilder`	`append(boolean value)` Append a `hashCode` for a `boolean`.
`HashCodeBuilder`	`append(boolean[] array)` Append a `hashCode` for a `boolean` array.
`HashCodeBuilder`	`append(byte value)` Append a `hashCode` for a `byte`.
`HashCodeBuilder`	`append(byte[] array)` Append a `hashCode` for a `byte` array.
`HashCodeBuilder`	`append(char value)` Append a `hashCode` for a `char`.
`HashCodeBuilder`	`append(char[] array)` Append a `hashCode` for a `char` array.
`HashCodeBuilder`	`append(double value)` Append a `hashCode` for a `double`.
`HashCodeBuilder`	`append(double[] array)` Append a `hashCode` for a `double` array.
`HashCodeBuilder`	`append(float value)` Append a `hashCode` for a `float`.
`HashCodeBuilder`	`append(float[] array)` Append a `hashCode` for a `float` array.
`HashCodeBuilder`	`append(int value)` Append a `hashCode` for an `int`.
`HashCodeBuilder`	`append(int[] array)` Append a `hashCode` for an `int` array.
`HashCodeBuilder`	`append(long value)` Append a `hashCode` for a `long`.
`HashCodeBuilder`	`append(long[] array)` Append a `hashCode` for a `long` array.
`HashCodeBuilder`	`append(Object object)` Append a `hashCode` for an `Object`.
`HashCodeBuilder`	`append(Object[] array)` Append a `hashCode` for an `Object` array.
`HashCodeBuilder`	`append(short value)` Append a `hashCode` for a `short`.
`HashCodeBuilder`	`append(short[] array)` Append a `hashCode` for a `short` array.
`HashCodeBuilder`	`appendSuper(int superHashCode)` Adds the result of super.hashCode() to this builder.
`Integer`	`build()` Returns the computed `hashCode`.
`int`	`hashCode()` The computed `hashCode` from toHashCode() is returned due to the likelyhood of bugs in mis-calling toHashCode() and the unlikelyness of it mattering what the hashCode for HashCodeBuilder itself is.
`int`	`toHashCode()` Return the computed `hashCode`.

Methods inherited from class java.lang.Object
equals, getClass, notify, notifyAll, toString, wait, wait, wait

- 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(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.
    
    Parameters:
    
    value - the boolean 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
  - 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(byte[] array)
```
    Append a hashCode for a byte array.
    
    Parameters:
    
    array - the array 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(char[] array)
```
    Append a hashCode for a char array.
    
    Parameters:
    
    array - the array 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(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 value)
```
    Append a hashCode for a float.
    
    Parameters:
    
    value - the float 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(int value)
```
    Append a hashCode for an int.
    
    Parameters:
    
    value - the int 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(long value)
```
    Append a hashCode for a long.
    
    Parameters:
    
    value - the long 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(Object object)
```
    Append a hashCode for an Object.
    
    Parameters:
    
    object - the Object 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(short value)
```
    Append a hashCode for a short.
    
    Parameters:
    
    value - the short 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
  - 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
  - toHashCode
```
public int toHashCode()
```
    Return the computed hashCode.
    
    Returns:
    
    hashCode based on the fields appended
  - build
```
public Integer build()
```
    Returns the computed hashCode.
    
    Returns:
    
    hashCode based on the fields appended
    
    Since:
    
    3.0
  - hashCode
```
public int hashCode()
```
    The computed hashCode from toHashCode() is returned due to the likelyhood of bugs in mis-calling toHashCode() and the unlikelyness 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

Class HashCodeBuilder

Constructor Summary

Method Summary

Methods inherited from class java.lang.Object

Constructor Detail

HashCodeBuilder

HashCodeBuilder

Method Detail

append

append

append

append

append

append

append

append

append

append

append

append

append

append

append

append

append

append

appendSuper

toHashCode

build

hashCode