Java 8：反射Field.get（）尝试捕获错误

Question

Issue

I am receiving an error in Eclipse stating that I have an unhandled exception with my call to Field.get(Object) . When I wrap the code in the recommended try-catch, the error persists. I assume that I have to wrap the parameter, in the get() method, with a try-catch. Is this possible?

Error

Unhandled exception type IllegalAccessException

Line in question:

return list.stream().map(e -> fieldType.cast(f.get(e))).collect(Collectors.toList());

Code

Main.java

import java.lang.reflect.Field;
import java.util.ArrayList;
import java.util.Arrays;
import java.util.List;
import java.util.stream.Collectors;

public class Main {
    public static void main(String[] args) {
        List<Entity> entities = Arrays.asList(
                new Item(0, "Apple"),
                new Item(1, "Banana"),
                new Item(2, "Grape"),
                new Item(3, "Orange")
                );

        List<Long> ids = null;
        try {
            ids = pluck("id", Long.class, entities, Entity.class, true);
        } catch (Exception e) {
            e.printStackTrace();
        }
        System.out.println(ids);
    }

    public static <E extends Entity> List<Long> pluckIds(List<E> list) {
        return list.stream().map(e -> e.getId()).collect(Collectors.toList());
    }

    public static <T,F> List<F> pluck(String fieldName, Class<F> fieldType, 
            List<T> list, Class<T> listType, boolean useLambda)
            throws NoSuchFieldException, IllegalAccessException,
            IllegalArgumentException {
        Field f = listType.getDeclaredField(fieldName);
        f.setAccessible(true);

        if (useLambda) {
            return list.stream().map(e -> fieldType.cast(f.get(e))).collect(Collectors.toList());
        } else {
            List<F> result = new ArrayList<F>();
            for (T element : list) {
                result.add(fieldType.cast(f.get(element)));
            }
            return result;
        }
    }
}

Item.java

public class Item extends Entity {
    private String name;

    public Item(long id, String name) {
        super(id);
        this.setName(name);
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }
}

Entity.java

public abstract class Entity {
    private long id;

    public Entity(long id) {
        this.id = id;
    }

    public long getId() {
        return id;
    }

    public void setId(long id) {
        this.id = id;
    }
}

Answer 1

Eclipse is kind of stupid right now (there's probably a bug open).

You have a lambda expression in here

return list.stream().map(e -> fieldType.cast(f.get(e))).collect(Collectors.toList());

Field#get(Object) is declared as throwing a checked exception. But Function#apply(Object) , which is the functional interface method you're implementing, does not. Eclipse is stupid in that it recommends that you wrap the statement in a try block, but it adds the try block around the whole thing

try {
    return list.stream().map(e -> fieldType.cast(f.get(e))).collect(Collectors.toList());
} catch (Exception e) {
}

when in reality it should be adding it to the body of the lambda

return list.stream().map(e -> {
    try {
        return fieldType.cast(f.get(e));
    } catch (Exception e) {
        return /* something else */ null; // or throw an unchecked exception
    }
}).collect(Collectors.toList());

This seems to happen when your lambda body is a simple expression rather than a block.

Answer 2

You need to handle IllegalAccessException which can be thrown as a result of fieldType.cast(f.get(e)) . You can surround the code within try/catch

if (useLambda) {
            return list.stream().map(e -> {
                try {
                    return fieldType.cast(f.get(e));
                } catch (IllegalAccessException e1) {
                    e1.printStackTrace();
                    return null;
                }
            }).collect(Collectors.toList());
        }