java中如何使用"自定义类"作为HashMap中的key呢?

实现思路:
    我们只需重写类的
	  hashCode()和equals()方法

重写HashCode的功能

重写hashCode():
    由于需要计算存储数据的存储位置,
	而且使用hashCode()用于确定数据的存储位置
	编写好的hashCode()可减少hash碰撞

重写equals的功能

由于需遵守自反性、对称性、传递性、一致性以及
 对任何非null的引用值x
  x.equals(null)必须返回false的这几个特性
 其目的是为了保证key在哈希表中的唯一性

String类源码

    /**
     * Compares this string to the specified object.  The result is {@code
     * true} if and only if the argument is not {@code null} and is a {@code
     * String} object that represents the same sequence of characters as this
     * object.
     *
     * <p>For finer-grained String comparison, refer to
     * {@link java.text.Collator}.
     *
     * @param  anObject
     *         The object to compare this {@code String} against
     *
     * @return  {@code true} if the given object represents a {@code String}
     *          equivalent to this string, {@code false} otherwise
     *
     * @see  #compareTo(String)
     * @see  #equalsIgnoreCase(String)
     */
    public boolean equals(Object anObject) {
        if (this == anObject) {
            return true;
        }
        return (anObject instanceof String aString)
                && (!COMPACT_STRINGS || this.coder == aString.coder)
                && StringLatin1.equals(value, aString.value);
    }

 public int hashCode() {
        int h = hash;
        if (h == 0 && !hashIsZero) {
            h = isLatin1() ? StringLatin1.hashCode(value)
                           : StringUTF16.hashCode(value);
            if (h == 0) {
                hashIsZero = true;
            } else {
                hash = h;
            }
        }
        return h;
    }