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PODistinguish instances of objects implementing an interface, by an enum value or by its implementing interface
 

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  1. PODistinguish instances of objects implementing an interface, by an enum value or by its implementing interface
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    <p>The situation is that i have this interface:</p> <pre><code>interface ISymbol { } </code></pre> <p>and these classes:</p> <pre><code>class Letter implements ISymbol { } class Number implements ISymbol { } class LowerCaseLetter extends Letter { } class UpperCaseLetter extends Letter { } </code></pre> <p>Now lets say that i have this function, in a different class. The logic that this class performs DOES NOT belong in the implementation of the ISymbol classes:</p> <pre><code>class SymbolMonitor { void func(ISymbol[] symbols) { for (ISymbol iSymbol : symbols) { // If the symbol is a number // or a lower case letter do something. } } } </code></pre> <p>But i don't want to check for those two types specifically because i might have other implementing classes of ISymbol that will also require the same logic.</p> <p>My question is about the preferred way of action. I have two solutions to the problem. 1. Create an like so:</p> <pre><code>enum SymbolType { NORMAL, SPECIAL1, SPECIAL2, } </code></pre> <p>and add to the code:</p> <pre><code> interface ISymbol { SymbolType getType(); } class SymbolMonitor { void func(ISymbol[] symbols) { for (ISymbol iSymbol : symbols) { if(iSymbol.getType().equals(SymbolType.SPECIAL1)) { // Do special1 logic } if(iSymbol.getType().equals(SymbolType.SPECIAL2)) { // Do special2 logic } } } } </code></pre> <ol> <li><p>I can create empty interfaces and use the instanceof function. </p> <pre><code>interface ISpecialSymbol1 </code></pre> <p>{</p> <p>}</p> <p>interface ISpecialSymbol2 {</p> <p>}</p> <p>class LowerCaseLetter extends Letter implements ISpecialSymbol1 {</p> <p>}</p> <p>class Number implements ISymbol, ISpecialSymbol1 {</p> <p>}</p></li> </ol> <p>so that the function will be:</p> <pre><code>class SymbolMonitor { void func(ISymbol[] symbols) { for (ISymbol iSymbol : symbols) { if(iSymbol instanceof ISpecialSymbol1) { // Do special 1 logic. } if(iSymbol instanceof ISpecialSymbol2) { // Do special 2 logic. } } } } </code></pre> <p>I think that the second option is much more simpler, but I am new to java and I am not sure at all what is the correct way of action to use.</p>
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    1. COUse polymorphism to your advantage. Try to avoid `instanceof` and type checks.
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    2. COYou mean move the logic into the class? because i have an external function using the ISymbol, not logic the belongs inside the classes.
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    3. COThere are a number of patterns to achieve this indirection. `Visitor` might be one of them. Check it out.
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