HashCode generation routines.
+ * + * This class enables a good hashcode to be built for any class. It follows + * the rules laid out in the book Effective Java, by Joshua Bloch. Writing a + * good hashCode is actually quite difficult. This class aims to simplify the + * process. + *
+ * All relevant fields from the object should be included in the hashCode. 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 HashCodeBuilder(17, 37).
+ * append(name).
+ * append(age).
+ * append(smoker).
+ * toHashCode();
+ * }
+ * }
+ *
+ *
+ * Alternatively, there is a method that uses reflection to determine
+ * the fields to test. Because these fields are usually private, the method,
+ * reflectionTest, uses Field.setAccessible to change
+ * the visibility of the fields. This will fail under a security manager,
+ * unless the appropriate permissions are set. It is also slower than testing
+ * explicitly.
+ *
+ * A typical invocation for this method would look like:
+ *
+ * public boolean hashCode(Object o) {
+ * return HashCodeBuilder.reflectionHashCode(this);
+ * }
+ *
+ *
+ * @author Stephen Colebourne
+ * @version $Id: HashCodeBuilder.java,v 1.1 2002/09/12 21:59:01 scolebourne Exp $
+ */
+public class HashCodeBuilder {
+
+ /**
+ * Constant to use in building the hashCode
+ */
+ private final int iConstant;
+ /**
+ * Running total of the hashCode
+ */
+ private int iTotal = 0;
+
+ /**
+ * Constructor for HashCodeBuilder.
+ * This constructor uses two hard coded choices for the constants needed
+ * to build a hashCode.
+ */
+ public HashCodeBuilder() {
+ super();
+ iConstant = 37;
+ iTotal = 17;
+ }
+
+ /**
+ * Constructor for HashCodeBuilder.
+ * 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.
+ *
+ * @param initialNonZeroOddNumber
+ * @param multiplierNonZeroOddNumber
+ * @throws IllegalArgumentException if the number is zero or even
+ */
+ public HashCodeBuilder(int initialNonZeroOddNumber, int multiplierNonZeroOddNumber) {
+ super();
+ if (initialNonZeroOddNumber == 0) {
+ throw new IllegalArgumentException("HashCodeBuilder requires a non zero initial value");
+ }
+ if (initialNonZeroOddNumber % 2 == 0) {
+ throw new IllegalArgumentException("HashCodeBuilder requires an odd initial value");
+ }
+ if (multiplierNonZeroOddNumber == 0) {
+ throw new IllegalArgumentException("HashCodeBuilder requires a non zero multiplier");
+ }
+ if (multiplierNonZeroOddNumber % 2 == 0) {
+ throw new IllegalArgumentException("HashCodeBuilder requires an odd multiplier");
+ }
+ iConstant = multiplierNonZeroOddNumber;
+ iTotal = initialNonZeroOddNumber;
+ }
+
+ //-------------------------------------------------------------------------
+
+ /**
+ * This method uses reflection to build a valid hash code.
+ *
+ * It uses Field.setAccessible to gain access to private fields. This means + * that it will throw a security exception if run under a security manger, if + * the permissions are not set up. + * 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. + * This constructor uses two hard coded choices for the constants needed + * to build a hash code. + * + * @param object the object to create a hash code for + * @return int hash code + * @throws IllegalArgumentException if the object is null + */ + public static int reflectionHashCode(Object object) { + return reflectionHashCode(object, false); + } + + /** + * This method uses reflection to build a valid hash code. + *
+ * It uses Field.setAccessible to gain access to private fields. This means + * that it will throw a security exception if run under a security manger, if + * the permissions are not set up. + * 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. + *
+ * 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. + * + * @param initialNonZeroOddNumber + * @param multiplierNonZeroOddNumber + * @param object the object to create a hash code for + * @return int hash code + * @throws IllegalArgumentException if the object is null + * @throws IllegalArgumentException if the number is zero or even + */ + public static int reflectionHashCode( + int initialNonZeroOddNumber, int multiplierNonZeroOddNumber, + Object object) { + return reflectionHashCode(initialNonZeroOddNumber, multiplierNonZeroOddNumber, object); + } + + /** + * This method uses reflection to build a valid hash code. + *
+ * It uses Field.setAccessible to gain access to private fields. This means + * that it will throw a security exception if run under a security manger, if + * the permissions are not set up. + * 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. + * This constructor uses two hard coded choices for the constants needed + * to build a hash code. + * + * @param object the object to create a hash code for + * @param testTransients whether to include transient fields + * @return int hash code + * @throws IllegalArgumentException if the object is null + */ + public static int reflectionHashCode(Object object, boolean testTransients) { + return reflectionHashCode(17, 37, object, testTransients); + } + + /** + * This method uses reflection to build a valid hash code. + *
+ * It uses Field.setAccessible to gain access to private fields. This means + * that it will throw a security exception if run under a security manger, if + * the permissions are not set up. + * 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. + *
+ * 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. + * + * @param initialNonZeroOddNumber + * @param multiplierNonZeroOddNumber + * @param object the object to create a hash code for + * @param testTransients whether to include transient fields + * @return int hash code + * @throws IllegalArgumentException if the object is null + * @throws IllegalArgumentException if the number is zero or even + */ + public static int reflectionHashCode( + int initialNonZeroOddNumber, int multiplierNonZeroOddNumber, + Object object, boolean testTransients) { + + if (object == null) { + throw new IllegalArgumentException("The object to build a hash code for must not be null"); + } + HashCodeBuilder hashCodeBuilder = new HashCodeBuilder(initialNonZeroOddNumber, multiplierNonZeroOddNumber); + Field[] fields = object.getClass().getDeclaredFields(); + Field.setAccessible(fields, true); + for (int i = 0; i < fields.length; ++i) { + Field f = fields[i]; + if (testTransients || !Modifier.isTransient(f.getModifiers())) { + try { + hashCodeBuilder.append(f.get(object)); + } catch (IllegalAccessException e) { + //this can't happen. Would get a Security exception instead + //throw a runtime exception in case the impossible happens. + throw new InternalError("Unexpected IllegalAccessException"); + } + } + } + return hashCodeBuilder.toHashCode(); + } + + //------------------------------------------------------------------------- + + /** + * Append a hashCode for an Object. + * + * @param object the object to add to the hashCode + * @return this + */ + public HashCodeBuilder append(Object object) { + if (object == null) { + iTotal = iTotal * iConstant; + + } else { + if (object.getClass().isArray() == false) { + //the simple case, not an array, just the element + iTotal = iTotal * iConstant + object.hashCode(); + + } else { + //'Switch' on type of array, to dispatch to the correct handler + // This handles multi dimensional arrays + if (object instanceof long[]) { + append((long[]) object); + } else if (object instanceof int[]) { + append((int[]) object); + } else if (object instanceof short[]) { + append((short[]) object); + } else if (object instanceof char[]) { + append((char[]) object); + } else if (object instanceof byte[]) { + append((byte[]) object); + } else if (object instanceof double[]) { + append((double[]) object); + } else if (object instanceof float[]) { + append((float[]) object); + } else if (object instanceof boolean[]) { + append((boolean[]) object); + } else { + // Not an array of primitives + append((Object[]) object); + } + } + } + return this; + } + + /** + * Append a hashCode for a long. + * + * @param value the long to add to the hashCode + * @return this + */ + public HashCodeBuilder append(long value) { + iTotal = iTotal * iConstant + ((int) (value ^ (value >> 32))); + return this; + } + + /** + * Append a hashCode for an int. + * + * @param value the int to add to the hashCode + * @return this + */ + public HashCodeBuilder append(int value) { + iTotal = iTotal * iConstant + value; + return this; + } + + /** + * Append a hashCode for a short. + * + * @param value the short to add to the hashCode + * @return this + */ + public HashCodeBuilder append(short value) { + iTotal = iTotal * iConstant + (int) value; + return this; + } + + /** + * Append a hashCode for a char. + * + * @param value the char to add to the hashCode + * @return this + */ + public HashCodeBuilder append(char value) { + iTotal = iTotal * iConstant + (int) value; + return this; + } + + /** + * Append a hashCode for a byte. + * + * @param value the byte to add to the hashCode + * @return this + */ + public HashCodeBuilder append(byte value) { + iTotal = iTotal * iConstant + (int) value; + return this; + } + + /** + * Append a hashCode for a double. + * + * @param value the double to add to the hashCode + * @return this + */ + public HashCodeBuilder append(double value) { + return append(Double.doubleToLongBits(value)); + } + + /** + * Append a hashCode for a float. + * + * @param value the float to add to the hashCode + * @return this + */ + public HashCodeBuilder append(float value) { + iTotal = iTotal * iConstant + Float.floatToIntBits(value); + return this; + } + + /** + * Append a hashCode for a long. + * + * @param value the long to add to the hashCode + * @return this + */ + public HashCodeBuilder append(boolean value) { + iTotal = iTotal * iConstant + (value ? 0 : 1); + return this; + } + + /** + * Append a hashCode for an Object array. + * + * @param array the array to add to the hashCode + * @return this + */ + public HashCodeBuilder append(Object[] array) { + if (array == null) { + iTotal = iTotal * iConstant; + } else { + for (int i = 0; i < array.length; i++) { + append(array[i]); + } + } + return this; + } + + /** + * Append a hashCode for a long array. + * + * @param array the array to add to the hashCode + * @return this + */ + public HashCodeBuilder append(long[] array) { + if (array == null) { + iTotal = iTotal * iConstant; + } else { + for (int i = 0; i < array.length; i++) { + append(array[i]); + } + } + return this; + } + + /** + * Append a hashCode for an int array. + * + * @param array the array to add to the hashCode + * @return this + */ + public HashCodeBuilder append(int[] array) { + if (array == null) { + iTotal = iTotal * iConstant; + } else { + for (int i = 0; i < array.length; i++) { + append(array[i]); + } + } + return this; + } + + /** + * Append a hashCode for a short array. + * + * @param array the array to add to the hashCode + * @return this + */ + public HashCodeBuilder append(short[] array) { + if (array == null) { + iTotal = iTotal * iConstant; + } else { + for (int i = 0; i < array.length; i++) { + append(array[i]); + } + } + return this; + } + + /** + * Append a hashCode for a char array. + * + * @param array the array to add to the hashCode + * @return this + */ + public HashCodeBuilder append(char[] array) { + if (array == null) { + iTotal = iTotal * iConstant; + } else { + for (int i = 0; i < array.length; i++) { + append(array[i]); + } + } + return this; + } + + /** + * Append a hashCode for a byte array. + * + * @param array the array to add to the hashCode + * @return this + */ + public HashCodeBuilder append(byte[] array) { + if (array == null) { + iTotal = iTotal * iConstant; + } else { + for (int i = 0; i < array.length; i++) { + append(array[i]); + } + } + return this; + } + + /** + * Append a hashCode for a double array. + * + * @param array the array to add to the hashCode + * @return this + */ + public HashCodeBuilder append(double[] array) { + if (array == null) { + iTotal = iTotal * iConstant; + } else { + for (int i = 0; i < array.length; i++) { + append(array[i]); + } + } + return this; + } + + /** + * Append a hashCode for a float array. + * + * @param array the array to add to the hashCode + * @return this + */ + public HashCodeBuilder append(float[] array) { + if (array == null) { + iTotal = iTotal * iConstant; + } else { + for (int i = 0; i < array.length; i++) { + append(array[i]); + } + } + return this; + } + + /** + * Append a hashCode for a boolean array. + * + * @param array the array to add to the hashCode + * @return this + */ + public HashCodeBuilder append(boolean[] array) { + if (array == null) { + iTotal = iTotal * iConstant; + } else { + for (int i = 0; i < array.length; i++) { + append(array[i]); + } + } + return this; + } + + /** + * Return the computed hashCode + * + * @return int hashCode based on the fields appended + */ + public int toHashCode() { + return iTotal; + } + +}