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POHow to define common operations between different objects in java?
 

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  1. POHow to define common operations between different objects in java?
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    <p>I have a number of operands of different types and want to define how these operands can operate on each other. This seems like a generic problem but I fail to find a good approach for implementing it.</p> <p>Say for a start that I have scalars (i.e. numbers), matrix, and sets of matrix or scalars and that I want to define the plus operation on these:</p> <ul> <li><p>scalar1.plus(scalar2) returns a scalar equal to scalar1+scalar2,</p></li> <li><p>matrix.plus(scalar1) or scalar.plus(matrix) both return a matrix where 'scalar1' each element has been added to each element of the matrix,</p></li> <li><p>matrixSet.plus(matrix1) returns a set where each matrix has been added to matrix1</p></li> </ul> <p>And so on... Note that some operations may throw exceptions, such as adding matrix of different sizes or adding sets that do not have the same number of elements.</p> <p>My goals are the following:</p> <p>1- reuse as much code as possible (important because I will have many more operations and data types),</p> <p>2- do as much validation at compile time as possible</p> <p>3- make it easy to add new data types or operations in the future (for example, I may want to add a vector data type later on)</p> <p>I initially thought I would define an Operand interface, which my scalar, matrix, and set of these elements would implement, and which would include methods like:</p> <pre><code>public Operand plus(Operand op); </code></pre> <p>This would allow be to define the plus operation from the bottom up, starting with the simplest element (scalar):</p> <pre><code>public Operand plus(Operand operand) { if (Scalar.class.equals(operand.getClass())) { return new Scalar(this.value + ((Scalar) operand).getValue()); } else { // assume the other operand will define the addition for me // since add is commutative return operand.plus(this); } } </code></pre> <p>And then for matrix:</p> <pre><code>public Operand plus(Operand operand) { if (Scalar.class.equals(operand.getClass())) { // return a matrix where we add scalar value to all matrix elements } else if (Matrix.class.equals(operand.getClass())) { // return a matrix where we perform a scalar addition between each element } else { // assume the other operand will define the addition for me // since add is commutative return operand.plus(this); } } </code></pre> <p>The idea being that whenever I introduce a new class implementing the Operand type (say a vector), I will only have to define the plus operation betwen that new operand and all the other existing operands. I.e. I wouldn't have to rewrite the plus method of the Scalar and Matrix classes.</p> <p>This approach meets goals 1 and 3 but not 2: all operations return Operand objects, and forces me to do a lot of casting and class checking at run time, which doesn't seem like a good practice.</p> <p>Any better way of approach my problem?</p>
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    1. COFor starters, use the `instanceof` operator instead of `getClass()`
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    2. COThanks. That makes sense.
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