1.java

ConstraintMapping mapping = config.createConstraintMapping();
mapping.type( GreetingService.class )
    .method( "greet", User.class )
		.returnValue()
			.valid()
				.convertGroup( Default.class ).to( User.class )
				.convertGroup( Foo.class ).to( Bar.class );

//more "fluent" name?
ConstraintMapping mapping = config.createConstraintMapping();
mapping.type( GreetingService.class )
    .method( "greet", User.class )
    	.returnValue()
			.valid()
				.convertingFrom( Default.class ).to( User.class )
				.convertingFrom( Foo.class ).to( Bar.class );

// Emmanuel's variant
ConstraintMapping mapping = config.createConstraintMapping();
mapping.type( GreetingService.class )
    .method( "greet", User.class )
		.returnValue()
			.valid()
			.convertGroup( Default.class ).to( User.class )
			.convertGroup( Foo.class ).to( Bar.class );

2.java

//using an object for representing conversions
ConstraintMapping mapping = config.createConstraintMapping();
mapping.type( GreetingService.class )
	.method( "greet", User.class )
		.returnValue()
			.valid()
				.convertGroup( new GroupConversion( Default.class, User.class ) )
				.convertGroup( new GroupConversion( Foo.class, Bar.class ) );

3.java

//using an object within valid(GroupConversion... conversions)
ConstraintMapping mapping = config.createConstraintMapping();
mapping.type( GreetingService.class )
    .method( "greet", User.class )
		.returnValue()
			.valid(
				new GroupConversion( Default.class, User.class ),
				new GroupConversion( Foo.class, Bar.class )
			);

4.java

ConstraintMapping mapping = config.createConstraintMapping();
mapping.type( GreetingService.class )
    .method( "greet", User.class )
        .returnValue()
			.valid()
				.withConversion( Default.class, User.class )
				.withConversion( Foo.class, Bar.class );