Question

I am trying to create a method that pretty much takes anything as a parameter, and returns a concatenated string representation of the value with some delimiter.

public static String getConcatenated(char delim, Object ...names) {
String[] stringArray = Arrays.copyOf(names, names.length, String[].class); //Exception here
return getConcatenated(delim, stringArray);
}

And the actual method

public static String getConcatenated(char delim, String ... names) {
if(names == null || names.length == 0)
    return "";

StringBuilder sb = new StringBuilder();

for(int i = 0; i < names.length; i++) {
    String n = names[i];
    if(n != null) {
        sb.append(n.trim());
        sb.append(delim);
    }
}

//Remove the last delim
return sb.substring(0, sb.length()-1).toString();
}

And I have the following JUnit test:

final String two = RedpillLinproUtils.getConcatenated(' ', "Shervin", "Asgari");
Assert.assertEquals("Result", "Shervin Asgari", two); //OK

final String three = RedpillLinproUtils.getConcatenated(';', "Shervin", "Asgari");
Assert.assertEquals("Result", "Shervin;Asgari", three); //OK

final String four = RedpillLinproUtils.getConcatenated(';', "Shervin", null, "Asgari", null);
Assert.assertEquals("Result", "Shervin;Asgari", four); //OK

final String five = RedpillLinproUtils.getConcatenated('/', 1, 2, null, 3, 4);
Assert.assertEquals("Result", "1/2/3/4", five); //FAIL

However, the test fails on the last part with the exception:

java.lang.ArrayStoreException
at java.lang.System.arraycopy(Native Method)
at java.util.Arrays.copyOf(Arrays.java:2763)

Can someone spot the error?

Answer 1

The cause

You can’t put an Integer in a String[]. That’s why you get ArrayStoreException. Arrays.copyOf does no conversion.

From the API:

T[] copyOf(U[] original, int newLength, Class< ? extends T[]> newType)

Throws: ArrayStoreException – if an element copied from original is not of a runtime type that can be stored in an array of class newType

The ArrayStoreException API has an example:

Thrown to indicate that an attempt has been made to store the wrong type of object into an array of objects. For example, the following code generates an ArrayStoreException:

    Object x[] = new String[3];
    x[0] = new Integer(0);

See also

• JLS 10.10 Array Store Exception

---

The fix

That said, you actually don’t need a String... and Arrays.copyOf to a String[]. Your getConcatenated can just take Object... and it’d work just fine.

public static String getConcatenated(char delim, Object... objs) {
    if(objs == null || objs.length == 0)
        return "";

    StringBuilder sb = new StringBuilder();
    for (Object o : objs) {
        if(o != null) {
            if (sb.length() > 0) sb.append(delim);
            sb.append(o.toString().trim());
        }
    }
    return sb.toString();
}

Now you can do the following:

    System.out.println(getConcatenated(';', "Shervin", null, "Asgari", null));
    // prints "Shervin;Asgari"

    System.out.println(getConcatenated('/', 1, 2, null, 3, 4));
    // prints "1/2/3/4"

    System.out.println(getConcatenated(':', "  ", null, "boo", "boo"));
    // prints "boo:boo"

Note that this follows the intended specification in the original code, i.e. trim() and skips null.