Can you explain odd behaviour of Boxing and Generics in Java

Question

According to the Java docs the following code ought to cause a compile error:

import java.util.*;

public class GenericTest1 {

    // Add T-array of objects to collection<T>
    static <T> void fromArrayToCollection(T[] a, Collection<T> c) {
        for (T o : a) {
            c.add(o);
        }
    }

   public static void main( String[] args ) {
       Number[] na = new Number[100];
       Collection<Number> cn = new ArrayList<Number>();


       // This should work and does
       fromArrayToCollection( na, cn );


       Collection<String> cs = new ArrayList<String>();

       // This should fail to copile and does
       fromArrayToCollection( na, cs );
   }
}

and it does:

GenericTest1.java:25: error: method fromArrayToCollection in class GenericTest1 cannot be applied to given types;
       fromArrayToCollection( na, cs );
       ^
  required: T[],Collection<T>
  found: Number[],Collection<String>
  reason: inference variable T has incompatible bounds
    equality constraints: String
    lower bounds: Number
  where T is a type-variable:
    T extends Object declared in method <T>fromArrayToCollection(T[],Collection<T>)

However, this compiles and runs perfectly.

public class GenericTest2 {

    // Test for equality of two objects of type T
    static <T> boolean testEquality(T first, T second ) {
        return first.equals( second );
    }


   public static void main( String[] args ) {
       // Should work
       System.out.println( testEquality( "One", "One" ) );

       // Shouldn't this refuse to compile ?
       System.out.println( testEquality( "One", 1 ) );

       // Shouldn't this refuse to compile ?
       Number one = new Integer( 1 );
       System.out.println( testEquality( "One", one ) );

   }
}

Output is :

true
false
false

Can anyone explain why ?

score 2 · Accepted Answer · edited May 23 '17 at 11:45

2

It works because one(Number) and "One"(String) are both Objects, as are 1(Integer due to autoboxing) and "One"(String). So T is evaluated to Object, equals gets called and returns false. It does not work with the Collection(and other generics for that matter) because a Collection<String> can not be cast to a Collection<Object>

edited May 23 '17 at 11:45

Community

1
1

answered Jun 02 '15 at 07:37

LionC

3,106
1
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31

Thanks! And for the link. – Dave Durbin Jun 02 '15 at 07:51

score 1 · Answer 2 · answered Jun 02 '15 at 07:42

1

testEquality( "One", 1 )

As a result of autoboxing, 1 here will be converted to Integer(1), which is an object. Both String("One") and Integer(1) inherited .equals function from Object, thus it can compile without error.

answered Jun 02 '15 at 07:42

tning

1,231
10
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score 0 · Answer 3 · answered Jun 02 '15 at 07:38

0

In your second test, the 1 will simply be boxed into an Integer instance, which is also an instance of Object.

answered Jun 02 '15 at 07:38

Mirko Stocker

2,232
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Can you explain odd behaviour of Boxing and Generics in Java

3 Answers3