generate hashcode based on effective java

HashCodeGenerator .java

package baiyanhuang;

import java.util.Arrays;

/**
 * 
 * A utility class to help generate hash code: This is based on the theory from Effective Java, Item 9
 * 
 */
public class HashCodeGenerator {

    private static final int INIT_VALUE = 17;

    private static final int MULTI_COEF = 31;

    private int hashCode = INIT_VALUE;

    private void updateHashCode(final int value) {
        hashCode = hashCode * MULTI_COEF + value;
    }

    // 1. boolean
    public void addField(final boolean field) {
        int value = field ? 1 : 0;
        updateHashCode(value);
    }

    // 2. byte, char, short, int
    public void addField(final byte field) {
        int value = (int) field;
        updateHashCode(value);
    }

    public void addField(final char field) {
        int value = (int) field;
        updateHashCode(value);
    }

    public void addField(final short field) {
        int value = (int) field;
        updateHashCode(value);
    }

    public void addField(final int field) {
        int value = (int) field;
        updateHashCode(value);
    }

    // 3. long - what is the rationale? (exclusive OR of the two halves of the primitive long value )
    public void addField(final long field) {
        int value = (int) (field ^ (field >>> 32));
        updateHashCode(value);
    }

    // 4. float
    public void addField(final float field) {
        int value = Float.floatToIntBits(field);
        updateHashCode(value);
    }

    // 5. double
    public void addField(final double field) {
        long value = Double.doubleToLongBits(field);
        addField(value);
    }

    // 6. object
    public void addField(final Object field) {
        int value = field == null ? 0 : field.hashCode();
        updateHashCode(value);
    }

    // 7. arrays
    public <T> void addField(final T[] field) {
        int value = Arrays.hashCode(field);
        updateHashCode(value);
    }

    /**
     * return the computed hash code
     */
    public int hashCode() {
        return hashCode;
    }
}