Java: how to initialize String[]?

You need to initialize errorSoon, as indicated by the error message, you have only declared it.

String[] errorSoon;                   // <--declared statement
String[] errorSoon = new String[100]; // <--initialized statement

You need to initialize the array so it can allocate the correct memory storage for the String elements before you can start setting the index.

If you only declare the array (as you did) there is no memory allocated for the String elements, but only a reference handle to errorSoon, and will throw an error when you try to initialize a variable at any index.

As a side note, you could also initialize the String array inside braces, { } as so,

String[] errorSoon = {"Hello", "World"};

which is equivalent to

String[] errorSoon = new String[2];
errorSoon[0] = "Hello";
errorSoon[1] = "World";