In java8, how to set the global value in the lambdas foreach block?

You could, of course, "make the outer value mutable" via a trick:

public void test() {
    String[] x = new String[1];
    List<String> list = Arrays.asList("a", "b", "c", "d");

    list.forEach(n -> {
        if (n.equals("d"))
            x[0] = "match the value";
    });
}

Get ready for a beating by the functional purist on the team, though. Much nicer, however, is to use a more functional approach (similar to Sleiman's approach):

public void test() {
    List<String> list = Arrays.asList("a", "b", "c", "d");
    String x = list.stream()
                   .filter("d"::equals)
                   .findAny()
                   .map(v -> "match the value")
                   .orElse(null);
}

1. No you can't do it. (Although you should have tried it yourself)
2. Because variables used within anonymous inner classes and lambda expression have to be effectively final.

3. you can achieve the same more concisely using filter and map.

   Optional<String> d = list.stream()
                            .filter(c -> c.equals("d"))
                            .findFirst()
                            .map(c -> "match the value");
   

In addition to already provided idiomatic examples, another hack would be to use AtomicReference, but I would only recommend it if you do need 'forEach' and prefer something more readable than true-functional variant:

public void test(){
    AtomicReference<String> x = new AtomicReference<>();
    List<String> list= Arrays.asList("a", "b", "c", "d");

    list.forEach(n->{
        if(n.equals("d"))
            x.set("match the value");
    });
}