Generic method for types

Question

I have the following interface:

public interface Caster{

    public boolean tryCast(Object value);

}

and its implementations:

public class IntegerCaster{

    public boolean tryCast(Object value){
        try{
            Integer.class.cast(value);
            return true;
        } catch (ClassCastException e){
            return false;
        }
   }
}
public class DateCaster{

        public boolean tryCast(Object value){
            try{
                Date.class.cast(value);
                return true;
            } catch (ClassCastException e){
                return false;
            }
       }
}

Is it possible to make such implementation generic? We can't quite take and declare Caster with type parameter, because we won't be able implement it as follows:

public interface Caster<T>{

    public boolean tryCast(Object value);

}

public class CasterImpl<T> implements Caster<T>{

    public boolean tryCast(Object value){
        try{
            T.class.cast(value); //fail
            return true;
        } catch (ClassCastException e){
            return false;
        }
   }
}

Why not using Integer.class.isInstance, Date.class.isInstance, etc instead of this interface? — Tagir Valeev
– Tagir Valeev, Commented May 14, 2015 at 10:47

Konstantin Yovkov · Accepted Answer · 2015-05-14 10:45:59Z

4

You have to inject the Class value, parameterized by T, within your Generic CasterImpl.

Something like this:

public class CasterImpl<T> implements Caster<T> {

    private Clazz<T> clazz;

    public CasterImpl(Class<T> clazz) {
        this.clazz = clazz;
    }

    public boolean tryCast(Object value){
        try{
            clazz.cast(value);
            return true;
        } catch (ClassCastException e){
            return false;
        }
   }
}

As a side note: I don't see a reason why the Caster interface is Generic, since you don't use the type-parameter within the interface.

answered May 14, 2015 at 10:45

Konstantin Yovkov

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4 Comments

prashant thakre Over a year ago

tryCast(Object value) must be written as tryCast(T value)

Konstantin Yovkov Over a year ago

Yes, if the method signature in the interface is changed.

St.Antario Over a year ago

Actually, pretty simple. I'll do it that way.

newacct Over a year ago

@prashantthakre: if the object to be casted is already T, then what's the point of casting it to T?

Tagir Valeev · Accepted Answer · 2015-05-14 10:50:07Z

4

This can be done without interface at all using standard Class.isInstance method. If you still want to implement this interface, use

public Caster getCaster(final Class<?> clazz) {
     return new Caster() {
         public boolean tryCast(Object value) {
             return clazz.isInstance(value);
         }
     };
}

Or simpler in Java 8:

public Caster getCaster(final Class<?> clazz) {
     return clazz::isInstance;
}

answered May 14, 2015 at 10:50

Tagir Valeev

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Comments

Has QUIT--Anony-Mousse · Accepted Answer · 2015-05-14 11:54:13Z

3

Generics in Java are implemented by erasure. Java produces a single bytecode for all T.

So this:

return T.class.cast(value);

would - if it were allowed - become essentially a

return Object.class.cast(value);

no matter which T you specify. If you want to check a specific class, you need a Class<T> object.

Use `someclass.isInstance(obj)` instead.

Right now, you are reinventing the Class<T> API.

edited May 14, 2015 at 11:54

answered May 14, 2015 at 10:50

Has QUIT--Anony-Mousse

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Comments

Gábor Bakos · Accepted Answer · 2015-05-14 10:47:18Z

1

Sure, just store the class of T to use later in the constructor:

public interface Caster<T>{

    public boolean tryCast(Object value);

}

public class CasterImpl<T> implements Caster<T>{
   private Class<? extends T> cls;
   public CasterImpl(Class<? extends T> cl) {
      this.cls = cl;
   }

    public boolean tryCast(Object value){
        try{
            cls.cast(value); //fail
            return true;
        } catch (ClassCastException e){
            return false;
        }
   }
}

answered May 14, 2015 at 10:47

Gábor Bakos

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Comments

aalku · Accepted Answer · 2015-05-14 11:04:09Z

Instances of a generic type do not store their generic type parameter.

You can store the class as kocko said or you can store the generic type parameter inside a new type this way:

public abstract class CasterImpl<T> implements Caster<T>{

    public boolean tryCast(Object value){
        try{
            getValueClass().cast(value);
            return true;
        } catch (ClassCastException e){
            return false;
        }
    }

    @SuppressWarnings("unchecked")
    public Class<T> getValueClass() {
        Class<? extends CasterImpl<T>> c = (Class<? extends CasterImpl<T>>) this.getClass();
        ParameterizedType x = (ParameterizedType) c.getGenericSuperclass();
        return (Class<T>) x.getActualTypeArguments()[0];
    }
}

That will only work with subclasses like this, that store the type in it's own type definition:

public class CasterIntegerImpl extends CasterImpl<Integer> {
    // No need to implement anything
}

Test:

    System.out.println(new CasterIntegerImpl().tryCast(1)); // true
    System.out.println(new CasterIntegerImpl().tryCast("")); // false

prashant thakre · Accepted Answer · 2015-05-14 10:51:15Z

0

Try this , use T instead of Object

public interface Caster<T>{

    public boolean tryCast(T value);

}


public class CasterImpl<T> implements Caster<T>{
   private Class<? extends T> cls;
   public CasterImpl(Class<? extends T> cl) {
      this.cls = cl;
   }

    public boolean tryCast(T value){
        try{
            cls.cast(value); //fail
            return true;
        } catch (ClassCastException e){
            return false;
        }
   }

answered May 14, 2015 at 10:51

prashant thakre

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Use `someclass.isInstance(obj)` instead.

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Use someclass.isInstance(obj) instead.

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Use `someclass.isInstance(obj)` instead.