I have List<City> cities. I need to convert cities into a map Map<String, Integer>, where the value (Integer) has to be auto-generated.

I tried this, but it seems not allowed to use counter like that because of atomic error. Ho to solve this task?

public Map<String, Integer> convertListToMap(List<City> cities) {
    Integer counter=0;
    return cities.stream().forEach(elem->tollFreeVehicles.put(elem.getName(), counter  ));
}

CodePudding user response：

Local variables that are allowed to be used in the lambda expressions needs to be final or effectively final.

Have a look at the Oracle's tutorial on lambdas. A short excerpt:

a lambda expression can only access local variables and parameters of the enclosing block that are final or effectively final. In this example, the variable z is effectively final; its value is never changed after it's initialized.

You can construct a map from a list using indices of items in the list as values by utilizing IntStream.range().

Also note that forEach() doesn't return a value. In order to generate a map as a result of the execution of the stream pipeline that will be returned from the method, you need to use collect() as terminal operation.

Map<String, Integer> result = 
    IntStream.range(0, cities.size())
             .boxed()
             .collect(Collectors.toMap(i -> cities.get(i).getName(),
                                       Function.identity()));