Creating Mock Objects in Java

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Creating Mock Objects
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This test uses two mock turtles, which we ask the mockery to create The rst is a eld in the test class:
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private final Turtle turtle = contextmock(Turtleclass);
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The second is local to the test, so it s held in a variable:
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final Turtle turtle2 = contextmock(Turtleclass, "turtle2");
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The variable has to be nal so that the anonymous expectations block has access to it we ll return to this soon This second mock turtle has a speci ed name, turtle2 Any mock can be given a name which will be used in the report if the test fails; the default name is the type of the object If there s more than one mock object of the same type, jMock enforces that only one uses the default name; the others must be given names when declared This is so that failure reports can make clear which mock instance is which when describing the state of the test
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1 At the time of writing, JUnit was introducing the concept of Rule We expect to extend the jMock API to adopt this technique
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Tests with Expectations
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Tests with Expectations
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A test sets up its expectations in one or more expectation blocks, for example:
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contextchecking(new Expectations() {{ oneOf (turtle)turn(45); }});
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An expectation block can contain any number of expectations A test can contain multiple expectation blocks; expectations in later blocks are appended to those in earlier blocks Expectation blocks can be interleaved with calls to the code under test
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What s with the Double Braces The most disconcerting syntax element in jMock is its use of double braces in an expectations block It s a hack, but with a purpose If we reformat an expectations block, we get this:
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contextchecking(new Expectations() { { oneOf (turtle)turn(45); } });
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We re passing to the checking() method an anonymous subclass of Expectations ( rst set of braces) Within that subclass, we have an instance initialization block (second set of braces) that Java will call after the constructor Within the initialization block, we can reference the enclosing Expectations object, so oneOf() is actually an instance method as are all of the expectation structure clauses we describe in the next section The purpose of this baroque structure is to provide a scope for building up expectations All the code in the expectation block is de ned within an anonymous instance of Expectations, which collects the expectation components that the code generates The scoping to an instance allows us to make this collection implicit, which requires less code It also improves our experience in the IDE, since code completion will be more focused, as in Figure A1 Referring back to the discussion in Building Up to Higher-Level Programming (page 65), Expectations is an example of the Builder pattern
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Appendix A
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Figure A1
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Narrowed scope gives better code completion
Expectations
Expectations have the following structure:
invocation-count(mock-object)method(argument-constraints); inSequence(sequence-name); when(state-machineis(state-name)); will(action); then(state-machineis(new-state-name));
The invocation-count and mock-object are required, all the other clauses are optional You can give an expectation any number of inSequence, when, will, and then clauses Here are some common examples:
oneOf (turtle)turn(45); // The turtle must be told exactly once to turn 45 degrees atLeast(1)of (turtle)stop(); // The turtle must be told at least once to stop allowing (turtle)flashLEDs(); // The turtle may be told any number of times, // including none, to flash its LEDs allowing (turtle)queryPen(); will(returnValue(PEN_DOWN)); // The turtle may be asked about its pen any // number of times and will always return PEN_DOWN ignoring (turtle2); // turtle2 may be told to do anything This test ignores it
Invocation Count
The invocation count is required to describe how often we expect a call to be made during the run of the test It starts the de nition of an expectation