Java value object comparison methods

Question

I'm new to Java and trying to learn.

Java doesn't have operator overloading, so when coding a value object I understand that you need to compare objects with an overridden 'equals' method instead of the == operator, but I have yet to read about the other methods you need to override. What about the equivalent methods for the other common operators: >, <, >=, and <=. Do these methods need to be overridden, or do I have to create them if I need them. If I have to create these methods, there must be some standard naming convention for them ('gt', 'lt', 'gte', 'lte'). What is it?

Answer 1

查看Comparable接口。

Answer 2

You need to override hashCode() (used by all hash collections) and equals() . It's a rule that if a.equals(b) , a.hashCode() should equal b.hashCode() .

For comparisons you may implement the interface Comparable . For this interface you'll need to implement a compareTo method which is an equivalent of <, =, > (note that a.compareTo(b) == 0 does NOT imply or require that a.equals(b) !). Implementing Comparable is optional and only useful if you're interested in sorting or certain order-based collections such as TreeSet .

Answer 3

In java, the Comparable interface is meant exactly for that. For a "canonical example", have a look into the the java.lang.Integer class and see the implementation of equals() , hashCode() and compareTo() . Also, have a look at the JavaDoc info available for these methods in java.lang.Object and java.lang.Comparable .