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A variant type for Java, with customisable automatic type conversion
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The aim of Variance is to provide a general-purpose, custom-wirable junction box for conversion between Java types.

You put a value - any value - into a Variant, like so:

Variant myVariant = Variant.of(myValue);

You get it out again like so:

MyType myInstance =;

The value you put in doesn't have to belong to the type you pull out, but it must be convertible to that type.

By default, Variants can convert:

  • Anything to String
  • Any type of Number to any other type of Number (and implement the Number interface themselves)
  • String to any kind of Number, and
  • Arrays/Iterables of things it knows how to convert into arrays/Iterables of converted things
assertThat(Variant.of("12").doubleValue(), is(12.0));
assertThat(Variant.of(12.0).toString(), is("12"));
assertThat(Variant.of(1, 2, 3, 4).asIterableOf(String.class), Contains.inOrder("1", "2", "3", "4"));
assertThat(Variant.of("1,2,3,4").asIterableOf(Integer.class), Contains.inOrder(1,2,3,4));

You can wire in other conversions by adding them to the context the Variant uses to resolve requests for different types. A vanilla Variant will pull its context out of a ThreadLocal, such that you can write:

\\ Enter a context in which numbers are formatted to 4d.p. on conversion to string

assertThat(Variant.of(12).as(String.class), is("12.0000"));

\\ Leave the custom context

assertThat(Variant.of(12).as(String.class), is("12"));

Alternatively, you can bind a Variant explicitly to a context:

assertThat(Variant.of(12).in(myCustomContext).as(String.class), is("12.0000"));

In most cases, you will want to extend an existing context rather than create a new context from scratch:

TypeConversionContext extended = ImplicitTypeConversions.getCurrent().extendedWith(extensions);
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