实现通用解析器类

Question

I wanna to write a generic excel parser class. But there is a problem with passing functions to it.

A parser read excel rows and create an object from each rows. By a systemic view, it get an excel file, a class and a mapping between each column and an object field (its setter function) and generate a list of objects.

If we create a Parser for each pojos ( StudentParser for Student pojo and SchoolParser for School ), there is a lot of duplicates in these parsers. So, Implementation of a generic Parser class is required, but I have no idea for passing setter methods to the class. We need something like this:

private Map<Integer, java.util.function.Consumer> mapColumnIndexToSetter = new HashMap<>();
mapColumnIndexToSetter .put(3, Student::setName);

But Student::setName gets an error:

Error:(30, 41) java: incompatible types: invalid method reference
incompatible types: java.lang.Object cannot be converted to java.lang.String`

I know the above code does can not meet my requirements. But It gives some idea about what is the requirements.

I have two questions on implementation of this generic Parser class in java 8:

1. How can I pass class and setter methods to parser class?
2. How to new a generic class (call its constructor) and set its field using the mentioned setter?

Answer 1

The Function you are passing is not compatible with the Map's declaration.

At the Map declaration you missed to specify generic type arguments for Function , so those will be inferred as follows:

Function<Object, Object>

On the other hand you are passing:

// I guess getName() is returning a String
Function<Student, String> function = Student::getName;

Now generic type arguments in such references have to match exactly unless specified otherwise, which would look like:

Function<? extends Object, ? extends Object>

And as long as you are fine with getting Object from the Function call, extends Object is obviously pointless, so it can be reduced to:

Function<?, ?>

But you might as well add a generic restriction here, such as:

Function<?, ? extends Serializable>

Answer 2

A method like Student.setName(String) requires two arguments, the Student instance to invoke the method on and the String to be passed. Therefore, an appropriate functional interface would be BiConsumer<Student,String> .

By the way, the columns of an excel table are rarely that sparse that they justify the use of a HashMap from index to function. A List having a function for each column is straight-forward.

To instantiate the type, you may use a Supplier .

Putting it together, such a class may look like

public class Parser<T> {
    public static final BiConsumer<Object,Object> IGNORE = (x,y) -> {};

    private final Supplier<? extends T> instantiator;
    private final List<BiConsumer<? super T, ? super String>> setters;

    public Parser(Supplier<? extends T> instantiator,
                  List<BiConsumer<? super T, ? super String>> setters) {
        this.instantiator = Objects.requireNonNull(instantiator);
        this.setters = new ArrayList<>(setters);
        if(this.setters.contains(null)) throw new NullPointerException();
    }
    public Parser(Supplier<? extends T> instantiator,
                  BiConsumer<? super T, ? super String>... setters) {
        this(instantiator, Arrays.asList(setters));
    }

    // Replace the two-dimensional string array with actual xls parsing...
    public List<T> parse(String[][] data) {
        List<T> result = new ArrayList<>(data.length);
        for(String[] row: data) {
            T instance = instantiator.get();
            for (int ix = 0; ix < row.length; ix++)
                setters.get(ix).accept(instance, row[ix]);
            result.add(instance);
        }
        return result;
    }
}

Then, to instantiate Student s only using the 3rd column to initialize their names, you can use

Parser<Student> parser = new Parser<>(Student::new,
                                      Parser.IGNORE, Parser.IGNORE, Student::setName);
List<Student> list = parser.parse(new String[][] {
                                      {null,         null,          "Moe" },
                                      {null,         null,          "Larry" },
                                      {null,         null,          "Curly" },
});
list.forEach(System.out::println);