For clarity, I would combine the size check conditional, and further, move it to it's own method to make the code a bit more clear. Working with objects (as Alex suggested), will make this much easier:

if(this.doubMatrix.length == doubMatrix.length)
        if(this.doubMatrix[0].length == doubMatrix[0].length) {
    // stuff
}

Change to something like:

if(this.isSameSize(doubMatrix))
{
    //stuff
}

// ...
// later
public boolean isSameSize(DoubleMatrix doub) {
    return // Comparison of column and rows of backing arrays
}

• Also in your addMatrix function, it looks like you are making some confusing/improper assignments:

  `this.doubMatrix = doubMatrix`
  

  ...is overwriting your current DoubleMatrix backing array with the one you are passing into the method (the this keyword refers to the class' scope, not the method's scope). I would suggest simply renaming the addMatrix parameter to something else so you don't have the confusion of working with this.doubMatrix and doubMatrix.

• Your addMatrix result array (tempArray) is also initialized to null and never properly initialized before you start placing values there. That's going to throw a NullPointerException.

For clarity, I would combine the size check conditional, and further, move it to it's own method to make the code a bit more clear. Working with objects (as @Alex suggested), will make this much easier:

if(this.doubMatrix.length == doubMatrix.length)
        if(this.doubMatrix[0].length == doubMatrix[0].length) {
    // stuff
}

Change to something like:

if(this.isSameSize(doubMatrix))
{
    //stuff
}

// ...
// later
public boolean isSameSize(DoubleMatrix doub) {
    return // Comparison of column and rows of backing arrays
}

• Also in your addMatrix() function, it looks like you are making some confusing/improper assignments:

   this.doubMatrix = doubMatrix
  

  ...is overwriting your current DoubleMatrix backing array with the one you are passing into the method (the this keyword refers to the class' scope, not the method's scope). I would suggest simply renaming the addMatrix() parameter to something else so you don't have the confusion of working with this.doubMatrix and doubMatrix.

• Your addMatrix() result array (tempArray) is also initialized to null and never properly initialized before you start placing values there. That's going to throw a NullPointerException.