Java Array concatenation

Question

I am trying to concatenate two arrays to add labels to the columns of a numeric array and also to add a total after the last row of the array. I have found some code on another Stack Overflow thread How can I concatenate two arrays in Java?. But I get an error

Exception in thread "AWT-EventQueue-0" java.lang.ClassCastException: [Ljava.lang.Object; cannot be cast to [[Ljava.lang.Object;

Here is my code

public void getpdf(double[][] pricelist, ArrayList<Piece> in) { PDDocument doc = new PDDocument(); PDPage page = new PDPage(); doc.addPage( page ); try { PDPageContentStream contentStream = new PDPageContentStream(doc, page); JTable table=gettable(pricelist,in); Object[] headercol={"Type","Asc","Ref","Commandes","Prix unitaire","Prix total"}; Object[][] content=getTableData(table); Object[][] global=(Object[][]) concatenate (headercol,content); //drawTable(page, contentStream, 700, 75, headercol); drawTable(page, contentStream, 700, 75, content); contentStream.close(); doc.save("bill.pdf" ); } catch (IOException ex) { ex.printStackTrace(); } } public <T> T[] concatenate (T[] a, T[][] b) { int aLen = a.length; int bLen = b.length; @SuppressWarnings("unchecked") T[] c = (T[]) Array.newInstance(a.getClass().getComponentType(), aLen+bLen); System.arraycopy(a, 0, c, 0, aLen); System.arraycopy(b, 0, c, aLen, bLen); return c; }

You are trying to put in a T[] c a T[][] b, which are two incompatible types — DamCx
– DamCx, Commented Feb 22, 2017 at 9:59

Haris Osmanagić · Accepted Answer · 2017-02-22 09:59:44Z

3

public <T> T[] concatenate (T[] a, T[][] b)

You cannot concatenate two array of different types. a is an array of Ts, b is an array of T[]s, i.e. it is an array of arrays of Ts.

answered Feb 22, 2017 at 9:59

Haris Osmanagić

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1 Comment

m_h Over a year ago

Thanks. That is true. However, the two arrays I am trying to concatenate have the same number of columns. Declaring Object[][] headercol throws Type mismatch: cannot convert String to Object[]

tobias_k · Accepted Answer · 2017-02-22 15:51:10Z

It seems like you want to concatenate the T[] a header and the T[][] b content, but your concatenate method returns a T[]. You have to return a T[][] instead, i.e. create an array of a.getClass() instead of a.getClass().getComponentType(). Also, in that new T[][] c, the T[] a will only take a single position, so the total length is just b.length + 1. Try this:

public <T> T[][] concatenate (T[] a, T[][] b) { T[][] c = (T[][]) Array.newInstance(a.getClass(), b.length + 1); c[0] = a; System.arraycopy(b, 0, c, 1, b.length); return c; }

Since the above can only concatenate a 1D array to a 2D array, but not the other way around (as you'd need for adding a 'totals' line below the table, a more general approach would be to write a generic "join 2 arrays with same dimensions" method, and another method to wrap an element into an array.

public <T> T[] wrap(T x) { T[] a = (T[]) Array.newInstance(x.getClass(), 1); a[0] = x; return a; } public <T> T[] concatenate (T[] a, T[] b) { T[] c = (T[]) Array.newInstance(a.getClass().getComponentType(), a.length + b.length); System.arraycopy(a, 0, c, 0, a.length); System.arraycopy(b, 0, c, a.length, b.length); return c; }

Example:

String[] headercol = {"Type","Asc","Ref","Commandes","Prix unitaire","Prix total"}; String[][] content = {{"A","B","C","D","E","F","G","H"}, {"1","2","3","4","5","6","7","8"}}; String[] footer = {"a","b","c","d","e","f","g","h"}; String[][] global = (String[][]) concatenate (wrap(headercol),content); global = (String[][]) concatenate (global, wrap(footer)); for (String[] row : global) { System.out.println(Arrays.toString(row)); }

Output:

[Type, Asc, Ref, Commandes, Prix unitaire, Prix total] [A, B, C, D, E, F, G, H] [1, 2, 3, 4, 5, 6, 7, 8] [a, b, c, d, e, f, g, h]

However, when trying to add the last line to the table, the one with the total price, I get an ArrayStoreException. It seems to me, I am doing exactly the same thing in the concatenate2 function as in the concatenate function. Here is the code I am using
@m_h The difference is that this function is adding the header as the first element in the array, whereas the footer would need to be the last line.
I added the concatenate2 to your suggestion above since it is very similar to your concatenate function. Yet this throws the ArrayStoreException as I stated above.
@m_h See my update for a more general solution. BTW, the exception you got was because if a is the 2D array you have to use a.getClass().getComponentType() (or b.getClass())

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