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    <p>A java subtype is any class that extends another class (or even implements an interface). A true subtype is not something language specific:</p> <p>A true subtype can always be substituted for the supertype. "Any property guaranteed by supertype must be guaranteed by subtype (true subtyping)"</p> <p><a href="http://www.cs.washington.edu/education/courses/cse331/10au/lectures/subtypingexamples.pdf">http://www.cs.washington.edu/education/courses/cse331/10au/lectures/subtypingexamples.pdf</a></p> <p>The link contains a very enlightning example. Let's say you have a class Point2D that stores values x and y. You could now create a subtype Point3D and add a value z. If you don't override any methods and take care with your equals and hashcode methods you could substitute a Point3D instance at any time for a Point2D instance. </p> <p>This is a simple example of course. One could argue why not have only Point3D. Maybe the classes both offer some methods that can - by dividing up into to classes - be better recognized as belonging to the 2D or 3D realm. In this case it would probably be purely a design decision.</p> <pre><code>class Point2D { int x; int y; } //true subtype class Point3D extends Point2D { int z; } </code></pre> <p>A more complex example might arise if you take a Person class and then have two subtypes: Employee and Customer. Both Employee and Customer offer all the same fields and methods as a Person.</p> <pre><code>class Person { String name; Date birthday; @Override public boolean equals(Object o){ //simplified equals implementation, this does not fulfill equals contract! return name.equals(((Person)o).name); } } //true subtype, same behaviour class Employee extends Person { long departmentId; } //not a true subtype, different behaviour -&gt; equals class Customer extends Person { long customerId; Date lastContact; String city; public boolean equals(Object o){ //simplified equals implementation, this does not fulfill equals contract! return customerId.equals(((Customer)o).customerId); } } </code></pre> <p>In this example Employee would be a true subtype of Person. However Customer is not a true subtype because the equals implementation differs (and presumably hashCode() as well) and it would not behave the same and could probably not be substituted for a Person object at all times. </p>
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    1. POWhat is the difference between "Java subtype" and "true subtype"
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    1. COIf you have two types which can always be substituted for each other, its unclear to me why you have two types in the first place.
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    2. CODoes it mean that a true subtype will return the same output for the same input as the super class returns, but that Java subtype will return a different result for a method of the super class with same input?
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    3. COin the link i posted is a nice example. true subtyping is a rather theoretical construct. i'll add to the answer, to clarify
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