What does “contract” of a class mean

Question

I am reading the book The Java Programming Language. In the chapter which explains overriding method, it says:

Making an override met

Answer 1

Contract of type (class, interface, enum) is the, well, the contract this type promises to comply to. It states:

• what are acceptable parameters to constructor and/or methods of this type;
• what you should and/or should not expect this type to be doing. For example, it may state that you shall expect this class to be thread-safe or not thread-safe;
• what are invariants supported by this type. For example, type method addData(float) of MathAverage class which calculates average of it's input may state that every time your call to add(float) is return, you shall expect call to MathAverage.getAverage() to return correct average of current input.
• generally speaking, your type can specify any constraint all it's subtypes must follow. For example, it may say "no method of this type must take longer than 1 second to execute".

Contract is specified in free-form in javadoc of type. There are some tools/practices to enforce execution of contracts, but they are limited, exactly because contract can be arbitrary, or, even, self-contradictory, in case of programmer's error.

Since subtyping(subclassing) can extend/modify behavior of supertype methods in arbitrary way, it may, as well, violate some parts of supertype's contract. Example of this would be extending HashMap, which accepts null values and keys, with some implementation which prohibits null values in calls to it's methods.

Other important aspect about type contract is that subtype can have stronger contract (covering subset of constraints in type's contract), but can't have weaker contract (covering superset of constraints in type's contract). For example, if your type's method 'doX(n)' promises to take O(n) (linear) time, 'doX(n)' in subtype can take O(1) (constant) time, but can not take O(n^2) time.