Java 中的equals方法

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

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

Note that it is generally necessary to override the hashCode method whenever this method is overridden, so as to maintain the general contract for thehashCode method, which states that equal objects must have equal hash codes.

Parameters:

obj the reference object with which to compare.

Returns:

true if this object is the same as the obj argument; false otherwise.

适合单个引用性对象之间的相等,所以使用了单三人称equals而不是equal 或者same.

从java说明文档上看,这个方法主要是用来判断某个对象和当前对象是否相等,注意这个地方说的是相等而不是相同.

equals实现了两个非空对象的相等关系判断,注意对象非空.

这个方法具有自反性(任何非空对象x equals x),对称性(任何非空对象x equals y,y也一定equals x,) ,传递性(任何非空对象 x equals y, y equals z, z equals y)和持续性(多次调用 x equals y返回结果应该一样)

非空性(任何非空对象 x equals null 返回false).

参数:需要被比较的引用性对象

返回值:boolean类型,本对象和参数对象相等返回true 否则返回false

备注:null对象没有非静态方法和属性,使用对象前没有判断null会出现java.lang.NullPointerException.