Java Type Conversion (Done Well)

This lightweight library (with no dependencies) provides a versatile, extensible, and robust mechanism for converting a Java object to a different type. For example, you can convert a String to an Integer using the TypeConverter like this:

import com.toddfast.util.convert.TypeConverter;

// Convert
Integer i = TypeConverter.convert(Integer.class, "123");

// Or more simply
int i = TypeConverter.asInt("123");

Converting from a string to an integer is hardly noteworthy, but you can use the TypeConverter class to convert from any type to any other type just as easily, in exactly the same way. For example:

// Register your custom conversion class
TypeConverter.registerTypeConversion(new FooConversion());

Bar bar = new Bar();
Foo foo = TypeConverter.convert(Foo.class, bar);
// -- or e.g. --
String s = "bar";
Foo foo = TypeConverter.convert(Foo.class, s);

Why?

Type conversion is one of those tedious things that frequently comes up, but supporting robust, general type conversion, especially in an extensible way, is often not a priority for a project. And so developers limp by with half-baked solutions.

Unfortunately, other libraries that provide type conversion in some form or another either don't do it extensibly, don't make it available as an independent API, or bring along significant bloat.

Instead, this tiny library aims to solve this problem once, cleanly (eyes on you, JDK) so you can get on with the more important aspects of your project.

Usage

The primary class, TypeConverter, comes ready to convert all the primitive Java types, plus a few more like BigDecimal and java.sql.Date. You can also add custom type conversions; see the information in the next section.

TypeConverter allows specification of an arbitrary type key in the registerTypeConversion(Object,TypeConversion) and convert(Object,Object) methods. This allows a conversion object to simultaneously be registered under various keys, such as a Class instance, a class name, and one or more logical type names. For example, the following are valid ways of converting a string to an int:

Integer i = TypeConverter.convert(Integer.class, "123");
Integer i = (Integer) TypeConverter.convert("java.lang.Integer", "123");
Integer i = (Integer) TypeConverter.convert(TypeConverter.TYPE_INT, "123");
Integer i = (Integer) TypeConverter.convert(TypeConverter.TYPE_INTEGER, "123");
Integer i = (Integer) TypeConverter.convert("int", "123");
Integer i = (Integer) TypeConverter.convert("integer", "123");
int i = TypeConverter.asInt("123");

Default type conversions have been registered under the following keys:

Classes

• java.lang.Object
• java.lang.String
• java.lang.Integer
• java.lang.Integer.TYPE (int)
• java.lang.Double
• java.lang.Double.TYPE (double)
• java.lang.Boolean
• java.lang.Boolean.TYPE (boolean)
• java.lang.Long
• java.lang.Long.TYPE (long)
• java.lang.Float
• java.lang.Float.TYPE (float)
• java.lang.Short
• java.lang.Short.TYPE (short)
• java.lang.Byte
• java.lang.Byte.TYPE (byte)
• java.lang.Character
• java.lang.Character.TYPE (char)
• java.math.BigDecimal
• java.sql.Date
• java.sql.Time
• java.sql.Timestamp

Class name strings

• "java.lang.Object"
• "java.lang.String"
• "java.lang.Integer"
• "java.lang.Double"
• "java.lang.Boolean"
• "java.lang.Long"
• "java.lang.Float"
• "java.lang.Short"
• "java.lang.Byte"
• "java.lang.Character"
• "java.math.BigDecimal"
• "java.sql.Date"
• "java.sql.Time"
• "java.sql.Timestamp"

Logical type name string constants

• TypeConverter.TYPE_UNKNOWN ("null")
• TypeConverter.TYPE_OBJECT ("object")
• TypeConverter.TYPE_STRING ("string")
• TypeConverter.TYPE_INT ("int")
• TypeConverter.TYPE_INTEGER ("integer")
• TypeConverter.TYPE_DOUBLE ("double")
• TypeConverter.TYPE_BOOLEAN ("boolean")
• TypeConverter.TYPE_LONG ("long")
• TypeConverter.TYPE_FLOAT ("float")
• TypeConverter.TYPE_SHORT ("short")
• TypeConverter.TYPE_BYTE ("byte")
• TypeConverter.TYPE_CHAR ("char")
• TypeConverter.TYPE_CHARACTER ("character")
• TypeConverter.TYPE_BIG_DECIMAL ("bigdecimal")
• TypeConverter.TYPE_SQL_DATE ("sqldate")
• TypeConverter.TYPE_SQL_TIME ("sqltime")
• TypeConverter.TYPE_SQL_TIMESTAMP ("sqltimestamp")

The TypeConverter treats type keys of type Class slightly differently than other keys. If the provided value is already of the type specified by the type key class, it is returned without a conversion taking place. For example, a value of type MySub that extends the class MySuper would not be converted in the following situation because it is already of the necessary type:

MySub o = TypeConverter.convert(MySuper.class, mySub);

Extensibility

TypeConverter is extensible by registering classes that implement the TypeConverter.Conversion interface for conversion to a custom type. For example, you can define a class to convert arbitrary objects to type Foo, and register it for use throughout your application:

// Register your custom conversion class
TypeConverter.registerTypeConversion(new FooConversion());

Foo foo;

Bar bar = new Bar();
foo = TypeConverter.convert(Foo.class, bar);

String s = "bar";
foo = TypeConverter.convert(Foo.class, s);

Discovery

TypeConverter.Conversion classes are also discovered using the JDK's standard java.util.ServiceLoader mechanism. To make a conversion discoverable, place a file named

META-INF/services/com.toddfast.util.typeconverter.TypeConverter$Conversion

in your project, the contents of which are fully qualified TypeConverter.Conversion class names, one per line. See the ServiceLoader documentation for more details on how to use the META-INF/services mechanism, or take a look at how registration of the default conversions is done.

Customizing conversion

Finally, a class can optionally implement the TypeConverter.Listener and/or TypeConverter.Convertible interfaces to receive conversion events or provide its own TypeConverter.Conversion instance, respectively. This capability allows a class to implement very rich custom type conversion logic.

Getting started

This library is in Maven Central and can be used with the following dependency:

<dependency>
  <groupId>com.toddfast.typeconverter</groupId>
  <artifactId>typeconverter</artifactId>
  <version>1.0</version>
</dependency>

Or, you can download the artifacts directly.