By design the for each variable 'x' (in this case) is not meant to be assigned to. I'm surprised that it even compiles fine.

String[] currentState = new String[answer.length()]; 
for (String x : currentState) { 
    x = "_"; // x is not a reference to some element of currentState 
}

The following code maybe shows what you're in effect are doing. Note that this is not how enumerations work but it exemplifies why you can't assign 'x'. It's a copy of the element at location 'i'. (Edit: note that the element is a reference type, as such it's a copy of that reference, assignment to that copy does not update the same memory location i.e. the element at location 'i')

String[] currentState = new String[answer.length()]; 
for (int i = 0; i < answer.length(); i++) { 
    String x = currentState[i];
    x = "_";
}

The better practice is to use for-each. Besides violating the Keep It Simple, Stupid principle, the new-fangled forEach() has at least the following deficiencies:

• Can't use non-final variables. So, code like the following can't be turned into a forEach lambda:

Object prev = null;
for(Object curr : list)
{
    if( prev != null )
        foo(prev, curr);
    prev = curr;
}

• Can't handle checked exceptions. Lambdas aren't actually forbidden from throwing checked exceptions, but common functional interfaces like Consumer don't declare any. Therefore, any code that throws checked exceptions must wrap them in try-catch or Throwables.propagate(). But even if you do that, it's not always clear what happens to the thrown exception. It could get swallowed somewhere in the guts of forEach()

• Limited flow-control. A return in a lambda equals a continue in a for-each, but there is no equivalent to a break. It's also difficult to do things like return values, short circuit, or set flags (which would have alleviated things a bit, if it wasn't a violation of the no non-final variables rule). "This is not just an optimization, but critical when you consider that some sequences (like reading the lines in a file) may have side-effects, or you may have an infinite sequence."

• Might execute in parallel, which is a horrible, horrible thing for all but the 0.1% of your code that needs to be optimized. Any parallel code has to be thought through (even if it doesn't use locks, volatiles, and other particularly nasty aspects of traditional multi-threaded execution). Any bug will be tough to find.

• Might hurt performance, because the JIT can't optimize forEach()+lambda to the same extent as plain loops, especially now that lambdas are new. By "optimization" I do not mean the overhead of calling lambdas (which is small), but to the sophisticated analysis and transformation that the modern JIT compiler performs on running code.

• If you do need parallelism, it is probably much faster and not much more difficult to use an ExecutorService. Streams are both automagical (read: don't know much about your problem) and use a specialized (read: inefficient for the general case) parallelization strategy (fork-join recursive decomposition).

• Makes debugging more confusing, because of the nested call hierarchy and, god forbid, parallel execution. The debugger may have issues displaying variables from the surrounding code, and things like step-through may not work as expected.

• Streams in general are more difficult to code, read, and debug. Actually, this is true of complex "fluent" APIs in general. The combination of complex single statements, heavy use of generics, and lack of intermediate variables conspire to produce confusing error messages and frustrate debugging. Instead of "this method doesn't have an overload for type X" you get an error message closer to "somewhere you messed up the types, but we don't know where or how." Similarly, you can't step through and examine things in a debugger as easily as when the code is broken into multiple statements, and intermediate values are saved to variables. Finally, reading the code and understanding the types and behavior at each stage of execution may be non-trivial.

• Sticks out like a sore thumb. The Java language already has the for-each statement. Why replace it with a function call? Why encourage hiding side-effects somewhere in expressions? Why encourage unwieldy one-liners? Mixing regular for-each and new forEach willy-nilly is bad style. Code should speak in idioms (patterns that are quick to comprehend due to their repetition), and the fewer idioms are used the clearer the code is and less time is spent deciding which idiom to use (a big time-drain for perfectionists like myself!).

As you can see, I'm not a big fan of the forEach() except in cases when it makes sense.

Particularly offensive to me is the fact that Stream does not implement Iterable (despite actually having method iterator) and cannot be used in a for-each, only with a forEach(). I recommend casting Streams into Iterables with (Iterable<T>)stream::iterator. A better alternative is to use StreamEx which fixes a number of Stream API problems, including implementing Iterable.

That said, forEach() is useful for the following:

• Atomically iterating over a synchronized list. Prior to this, a list generated with Collections.synchronizedList() was atomic with respect to things like get or set, but was not thread-safe when iterating.

• Parallel execution (using an appropriate parallel stream). This saves you a few lines of code vs using an ExecutorService, if your problem matches the performance assumptions built into Streams and Spliterators.

• Specific containers which, like the synchronized list, benefit from being in control of iteration (although this is largely theoretical unless people can bring up more examples)

• Calling a single function more cleanly by using forEach() and a method reference argument (ie, list.forEach (obj::someMethod)). However, keep in mind the points on checked exceptions, more difficult debugging, and reducing the number of idioms you use when writing code.

Articles I used for reference:

• Everything about Java 8
• Iteration Inside and Out (as pointed out by another poster)

EDIT: Looks like some of the original proposals for lambdas (such as http://www.javac.info/closures-v06a.html Google Cache) solved some of the issues I mentioned (while adding their own complications, of course).