【java学习】object中的equals方法

在java.lang.Object类中可以看到对equals方法的说明
/**
* Indicates whether some other object is “equal to” this one.
*

* The {@code equals} method implements an equivalence relation
* on non-null object references:
*

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

*

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

* Note that it is generally necessary to override the {@code hashCode}
* method whenever this method is overridden, so as to maintain the
* general contract for the {@code hashCode} method, which states
* that equal objects must have equal hash codes.
*
* @param obj the reference object with which to compare.
* @return {@code true} if this object is the same as the obj
* argument; {@code false} otherwise.
* @see #hashCode()
* @see java.util.HashMap
*/
如果英文不行，就看中文jdk文档：

总结一下：
①性质：自反 对称 传递 一致 (也就是离散数学那些东西)
②euqals比较的是地址(引用)，因此true的条件比==更加严格
③重写时要重写hashCode方法

实际操作一波：

e1和e2的地址不同，所以false，s1和s2的内容相同但是地址不同，本来按照Object的equals方法应当输出false，但是String类重写了equals方法，当内容相同时会输出true。