谈谈java的hashcode使用场景

hashcode是在Object就已经定义了一个方法，名叫散列码，来看看Object是怎么描述它的

    /**
     * Returns a hash code value for the object. This method is
     * supported for the benefit of hash tables such as those provided by
     * {@link java.util.HashMap}.
     * <p>
     * The general contract of {@code hashCode} is:
     * <ul>
     * <li>Whenever it is invoked on the same object more than once during
     *     an execution of a Java application, the {@code hashCode} method
     *     must consistently return the same integer, provided no information
     *     used in {@code equals} comparisons on the object is modified.
     *     This integer need not remain consistent from one execution of an
     *     application to another execution of the same application.
     * <li>If two objects are equal according to the {@code equals(Object)}
     *     method, then calling the {@code hashCode} method on each of
     *     the two objects must produce the same integer result.
     * <li>It is <em>not</em> required that if two objects are unequal
     *     according to the {@link java.lang.Object#equals(java.lang.Object)}
     *     method, then calling the {@code hashCode} method on each of the
     *     two objects must produce distinct integer results.  However, the
     *     programmer should be aware that producing distinct integer results
     *     for unequal objects may improve the performance of hash tables.
     * </ul>
     * <p>
     * As much as is reasonably practical, the hashCode method defined by
     * class {@code Object} does return distinct integers for distinct
     * objects. (This is typically implemented by converting the internal
     * address of the object into an integer, but this implementation
     * technique is not required by the
     * Java&trade; programming language.)
     *
     * @return  a hash code value for this object.
     * @see     java.lang.Object#equals(java.lang.Object)
     * @see     java.lang.System#identityHashCode
     */
    public native int hashCode();

hashcode如果没有覆盖重写，那么默认是由Object导出的对象存储地址。主要应用场景是HashMap和HashSet等等的Hash集合类里面

实例1??：String的hashCode()方法

public static void main(String[] args) {
    String a1=new String("张");
    String a2=new String("张");
    String a3=new String("吴");
    HashSet<String> h1=new HashSet<>();
    h1.add(a1);
    h1.add(a2);
    h1.add(a3);
    System.out.println(h1);
}

输出结果：

我明明add了三个对象进去，为什么HashSet里面只有两个元素，这是因为String覆盖了Object的hashcode()方法，我们来看看String的hashcode()

    /**
     * Returns a hash code for this string. The hash code for a
     * {@code String} object is computed as
     * <blockquote><pre>
     * s[0]*31^(n-1) + s[1]*31^(n-2) + ... + s[n-1]
     * </pre></blockquote>
     * using {@code int} arithmetic, where {@code s[i]} is the
     * <i>i</i>th character of the string, {@code n} is the length of
     * the string, and {@code ^} indicates exponentiation.
     * (The hash value of the empty string is zero.)
     *
     * @return  a hash code value for this object.
     */
    public int hashCode() {
        int h = hash;
        if (h == 0 && value.length > 0) {
            char val[] = value;

            for (int i = 0; i < value.length; i++) {
                h = 31 * h + val[i];
            }
            hash = h;
        }
        return h;
    }

String的hashcode值为每个字符chat值相加。HashSet虽然添加了三个对象，但是其中两个对象的值是一样的，所以HashSet只保存两个元素。

实例2??：Object的hashCode()方法

我们将实例1??的中的String类型换成Object类型，代码如下：

public static void main(String[] args) {
    Object a1=new Object();
    Object a2=new Object();
    Object a3=new Object();
    HashSet<Object> h1=new HashSet<>();
    h1.add(a1);
    h1.add(a2);
    h1.add(a3);
    System.out.println(h1);
}

输出结果：

OK，HashSet保存了三个对象，没问题，因为Object的hashcode默认就是对象的存储地址

实例3??自定义类覆盖hashCode()方法

我在这里定义了一个类，代码如下：

public class HashCodeObject {
    private int a;

    public int getA() {
        return a;
    }
    public void setA(int a) {
        this.a = a;
    }
    @Override
    public boolean equals(Object obj) {
        if(obj instanceof HashCodeObject){
            return this.a==((HashCodeObject)obj).getA();
        }
        return false;
    }
    @Override
    public int hashCode() {
        return this.a;
    }
}

然后，再将实例2??的Object换成我们自己定义的类，如下

public static void main(String[] args) {
    HashCodeObject a1=new HashCodeObject();
    HashCodeObject a2=new HashCodeObject();
    HashCodeObject a3=new HashCodeObject();
    HashSet<HashCodeObject> h1=new HashSet<>();
    h1.add(a1);
    h1.add(a2);
    h1.add(a3);
    System.out.println(h1);
}

输出结果：

注意，覆盖hashcode()方法的同时也要覆盖equal()方法，不然达不到效果，并且equal()和hashcode()定义必须要一致，如果equal()返回true，那么hashcode应该

具有相同的值。