Interpolatd

Interpolatd is a string interpolation library for Java. It aspires to be easy to use and yet flexible and powerful. It is string interpolation done right.

Getting Started

Download the latest .jar. Or, add the following dependency to your pom.xml:

<dependency>
    <groupId>org.bigtesting</groupId>
    <artifactId>interpolatd</artifactId>
    <version>1.0.0</version>
</dependency>

Usage

There is a single class of interest in the library: Interpolator. It exposes a fluent interface for configuring interpolations. Normally, you'll create a single instance of this class in your application. It is thread-safe.

First, you create an instance of an Interpolator:

Interpolator<String> interpolator = new Interpolator<String>();

The Interpolator class is generic. The type parameter is the type of argument (in this case a String) expected in the Interpolator's interpolate() method, as you'll see next.

Next, you configure the Interpolator:

interpolator.when().enclosedBy("#{").and("}")
    .handleWith(new Substitutor<String>() {
        public String substitute(String captured, String arg) {
            return arg;
        }
    });

In the snippet above, we are configuring the Interpolator such that it will substitute anything enclosed by #{ and }. The handleWith() method accepts a Substitutor. The Substitutor is also generic, and its type parameter must match the Interpolator's type parameter. In the Subsitutor's subsitute() method, the captured argument is the value captured by the pattern. In the example above, this will be the value between the #{ and }.

Next, you call the interpolate() method of the Interpolator:

interpolator.interpolate("Hello #{name}!", "World");
//returns "Hello World!"

You can also substitute values prefixed by a specific String:

interpolator.when().prefixedBy(":")
    .handleWith(new Substitutor<String>() {
        public String substitute(String captured, String arg) {
            return arg;
        }
    });

The Interpolator will now interpolate a String like "Hello :name!".

There's no reason we need to be limited to one pattern or another. With the Interpolator configured as above, we can interpolate Strings containing both patterns:

interpolator.interpolate("Hello :name #{name}!", "foo");
//returns "Hello foo foo!"

In practice, you will not be passing along a String as an argument to the interpolate() method. You will likely be using a Map, or some other more application-specific type, to return more meaningful values, based on the value of the captured argument.

Character Classes

If you want to place restrictions on what can be prefixed or enclosed, you can specify your own character classes while configuring the Interpolator:

Interpolator<String[]> interpolator = new Interpolator<String[]>();
        
interpolator.when("[0-9]").enclosedBy("#{").and("}")
    .handleWith(new Substitutor<String[]>() {
        public String substitute(String captured, String[] arg) {
            return arg[Integer.valueOf(captured)];
        }
    });

In the configuration above, we are saying that only single digits from 0-9 that are enclosed by #{ and } should be substituted. We can then use the Interpolator as follows:

interpolator.interpolate("Hello #{0}, #{1}, but not #{you}.", 
    new String[]{"John", "Jane"});
//returns "Hello John, Jane, but not #{you}."

We could also have specified that any number of digits from 0-9 be matched, by using "[0-9]+" as an argument, etc.

Escaping Substitution Patterns

At times, you may want to escape substitution patterns. The Interpolator will not substitute patterns if there is no value to substitute with. For example, if the Substitutor returns null when asked to handle "Hello #{name}", the interpolation result will be "Hello #{name}". However, if you want to explicitly escape a substitution pattern because the Substitutor may return a value, you can specify an escape character or sequence:

Interpolator<String> interpolator = new Interpolator<String>();
        
interpolator.when().enclosedBy("#{").and("}")
    .handleWith(new Substitutor<String>() {
        public String substitute(String captured, String arg) {
            return arg;
        }
    });

interpolator.escapeWith("^");

In the snippet above, we are specifying ^ as an escape character. There can only be one. On their own, escape characters are treated like any other character. However, if they are placed next to a substitution pattern, they act as genuine escape characters:

interpolator.interpolate("Hello #{name}, but not ^#{you} or ^.", "World");
//returns "Hello World, but not #{you} or ^."

If you want to produce an escape character next to a substitution pattern, you can escape the escape. For example:

interpolator.interpolate("Hello ^^ ^^#{name}", "World");
//returns "Hello ^^ ^World"

Notice that the escape characters on their own--that are not next to a substitution pattern--are not treated as escape characters.