What Is Type Argument Inference?

Java Tutorials - Herong's Tutorial Examples - Version 7.03, by Dr. Herong Yang

Java Tutorials - Herong's Tutorial Examples

∟What Is Type Argument Inference?

This section describes what is type argument inference - a process performed by the compiler to determine the actual type argument for a type parameter when invoking in a generic method.

What Is Type Argument Inference? Type argument inference is a process performed by the compiler to determine the actual type argument for a type parameter when invoking in a generic method.

Type argument inference also happens when the compiler tries to determine the actual type argument for a type parameter when instantiating an object from a generic class. See other parts of the book for more details.

Why Type Argument Inference Is Needed? Type argument inference is needed only if you do not specify type arguments explicitly when invoking a generic method.

When type arguments are specified as shown below in the type argument list, the compiler knows what the type argument is. It will not perform the type argument inference process:

   // No, type argument inference is not needed 
   Collections.<String>sort(myArrayList);

When type arguments are not specified as shown below, the compiler does not know what the type argument is. It will perform the type argument inference process to determine type arguments:

   // Yes, type argument inference is needed
   Collections.sort(myArrayList);

Personally, I will always specify type arguments explicitly to avoid type argument inference, because compiler may infer to incorrect type arguments and lead to type incompatibility issues. But if you are a lazy programmer, you can take the risk and let the compiler to do the work for you using rules described below.

How Type Argument Inference Is Done? If you are not specifying type arguments when invoking generic methods, the compiler will perform the type argument inference process using these rules:

If a type parameter is used in one method parameter, the compiler will look at the type of the corresponding argument to deduce the type argument.
If a type parameter is used in multiple method parameters, the compiler will look at types of all corresponding arguments to deduce a single type argument. If this not possible, a compilation error is raised.
If a type parameter is not used in any method parameter, it will be used in the return value type. The compiler will look at the expression context where this method is invoked to deduce the type argument.
If the compiler failed to deduce the type argument from the context, it will use the default type argument, Object.

Next sections will show you some examples on how compiler does the type argument inference.

Last update: 2014.

Table of Contents

About This Book

Installing JDK 1.8 on Windows

Execution Process, Entry Point, Input and Output

Primitive Data Types and Literals

Bits, Bytes, Bitwise and Shift Operations

Managing Bit Strings in Byte Arrays

Reference Data Types and Variables

StringBuffer - The String Buffer Class

System Properties and Runtime Object Methods