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copied!<p>Strategies are higher-level things that are used to change how things are computed. With dependency injection, you can change not just how things are computed but also change what is there.</p> <p>For me, it becomes clear when using unit tests. For production code execution, you have all the data hidden (i.e. private or protected); whereas, with unit tests, most of the data is public so I can look at it with the Asserts.</p> <hr> <p>Example of strategy:</p> <pre><code>public class Cosine { private CalcStrategy strat; // Constructor - strategy passed in as a type of DI public Cosine(CalcStrategy s) { strat = s; } } public abstract class CalcStrategy { public double goFigure(double angle); } public class RadianStrategy extends CalcStrategy { public double goFigure(double angle) { return (...); } } public class DegreeStrategy extends CalcStrategy { public double goFigure(double angle) { return (...); } } </code></pre> <p>Notice that there is no public data that is different between the strategies. Nor is there any different methods. Both strategies share all the same functions and signatures.</p> <hr> <p>Now for the dependency injection:</p> <pre><code>public class Cosine { private Calc strat; // Constructor - Dependency Injection. public Cosine(Calc s) { strat = s; } } public class Calc { private int numPasses = 0; private double total = 0; private double intermediate = 0; public double goFigure(double angle) { return(...); } public class CalcTestDouble extends Calc { // NOTICE THE PUBLIC DATA. public int numPasses = 0; public double total = 0; public double intermediate = 0; public double goFigure(double angle) { return (...); } } </code></pre> <p>Use:</p> <pre><code>public CosineTest { @Test public void testGoFigure() { // Setup CalcTestDouble calc = new CalcTestDouble(); Cosine instance = new Cosine(calc); // Exercise double actualAnswer = instance.goFigure(0.0); // Verify double tolerance = ...; double expectedAnswer = ...; assertEquals("GoFigure didn't work!", expectedAnswer, actualAnswer, tolerance); int expectedNumPasses = ...; assertEquals("GoFigure had wrong number passes!", expectedNumPasses, calc.numPasses); double expectedIntermediate = ...; assertEquals("GoFigure had wrong intermediate values!", expectedIntermediate, calc.intermediate, tolerance); } } </code></pre> <p>Notice the last 2 checks. They used the public data in the test double that was injected into the class under test. I couldn't do this with production code because of the data hiding principle. I didn't want to have special purpose testing code inserted in the production code. The public data had to be in a different class.</p> <p>The test double was injected. That is different than just a strategy since it affected <strong>data</strong> and not just functions.</p>

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