第7条：在改写equals的时候请遵守通用约定

boolean java.lang.Object.equals(Object obj)

Indicates whether some other object is "equal to" this one.

The equals method implements an equivalence relation on non-null object references:

• It is reflexive: for any non-null reference value x, x.equals(x) should return true.
• It is symmetric: for any non-null reference values x and y, x.equals(y) should return true if and only if y.equals(x) returns true.
• It is transitive: for any non-null reference values x, y, and z, if x.equals(y) returns true and y.equals(z) returns true, then x.equals(z) should return true.
• It is consistent: for any non-null reference values x and y, multiple invocations of x.equals(y) consistently return true or consistently return false, provided no information used in equals comparisons on the objects is modified.
• For any non-null reference value x, x.equals(null) should return false.

The equals method for class Object implements the most discriminating possible equivalence relation on objects; that is, for any non-null reference values x and y, this method returns true if and only if x and y refer to the same object (x == y has the value true).

Note that it is generally necessary to override the hashCode method whenever this method is overridden, so as to maintain the general contract for the hashCode

obj the reference object with which to compare.

Returns: true if this object is the same as the obj argument;

false otherwise.

See Also: hashCode()

java.util.Hashtable

自反性：

对于任意的非null引用x，x.equals(x)返回true。

对称性：

对于任意的非null引用x和y，如果x.equals(y)返回true，那么y.equals(x)也返回true。

传递性：

对于任意的非null引用x、y和z，如果x.equals(y)返回true，y.equals(z)返回true，那么x.equals(z)也返回true。

一致性：

对于任意的非null引用x和y，如果没有修改equals，那么多次调用x.equals(y)，要么一致返回true，要么一致返回false。

非空性：

对于任意的非null引用x，x.equals(null)返回false。