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Barium Scoorge
Barium Scoorge

Reputation: 2008

Enum with a getter

Is an enum able to store references to a getter method, using a Supplier?

To be use like that :

String value = myEnum.getValue(object)

I can't figure how to write it without compiling errors.

Upvotes: 6

Views: 2365

Answers (2)

Tagir Valeev
Tagir Valeev

Reputation: 100169

It's not very difficult if the return type for all the getters is the same. Consider the following PoJo class:

public static class MyPoJo {
    final String foo, bar;

    public MyPoJo(String foo, String bar) {
        this.foo = foo;
        this.bar = bar;
    }

    public String getFoo() {
        return foo;
    }

    public String getBar() {
        return bar;
    }

    public int getBaz() {
        return 5;
    }
}

Then we may have such enum:

public static enum Getters {
    FOO(MyPoJo::getFoo), BAR(MyPoJo::getBar);

    private final Function<MyPoJo, String> fn;

    private Getters(Function<MyPoJo, String> fn) {
        this.fn = fn;
    }

    public String getValue(MyPoJo object) {
        return fn.apply(object);
    }
}

And use it like this:

System.out.println(Getters.FOO.getValue(new MyPoJo("fooValue", "barValue"))); // fooValue

However it would be problematic if you want to return different types. In this case I'd suggest to use normal class with predefined instances instead of enum:

public static final class Getters<T> {
    public static final Getters<String> FOO = new Getters<>(MyPoJo::getFoo);
    public static final Getters<String> BAR = new Getters<>(MyPoJo::getBar);
    public static final Getters<Integer> BAZ = new Getters<>(MyPoJo::getBaz);

    private final Function<MyPoJo, T> fn;

    private Getters(Function<MyPoJo, T> fn) {
        this.fn = fn;
    }

    public T getValue(MyPoJo object) {
        return fn.apply(object);
    }
}

Usage is the same:

System.out.println(Getters.FOO.getValue(new MyPoJo("fooValue", "barValue"))); // fooValue
System.out.println(Getters.BAZ.getValue(new MyPoJo("fooValue", "barValue"))); // 5

Upvotes: 5

Flown
Flown

Reputation: 11740

If I get you right then you want to do something like this:

import java.util.function.DoubleSupplier;

public class Test {

  enum MathConstants {

    PI(Test::getPi), E(Test::getE);

    private final DoubleSupplier supply;

    private MathConstants(DoubleSupplier supply) {
      this.supply = supply;
    }

    public double getValue() {
      return supply.getAsDouble();
    }
  }

  public static void main(String... args) {
    System.out.println(MathConstants.PI.getValue());
  }

  public static double getPi() {
    return Math.PI;
  }

  public static double getE() {
    return Math.E;
  }
}

Upvotes: 6

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