Trail: Learning the Java Language
Lesson: Generics (Updated)
Type Inference
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Type Inference

Type inference is a Java compiler's ability to look at each method invocation and corresponding declaration to determine the type argument (or arguments) that make the invocation applicable. The inference algorithm determines the types of the arguments and, if available, the type that the result is being assigned, or returned. Finally, the inference algorithm tries to find the most specific type that works with all of the arguments.

To illustrate this last point, in the following example, inference determines that the second argument being passed to the pick method is of type String:

static <T> T pick(T a1, T a2) { return a2; }
Serializable s = pick("d", new ArrayList<String>());

Type Inference and Generic Methods

Generic Methods introduced you to type inference, which enables you to invoke a generic method as you would an ordinary method, without specifying a type between angle brackets. Consider the following example, BoxDemo, which requires the Box class:

public class BoxDemo {

  public static <U> void addBox(U u, 
      java.util.List<Box<U>> boxes) {
    Box<U> box = new Box<>();
    box.set(u);
    boxes.set(box);
  }

  public static <U> void outputBoxes(java.util.List<Box<U>> boxes) {
    int counter = 0;
    for (Box<U> box: boxes) {
      U boxContents = box.get();
      System.out.println("Box #" + counter + " contains [" +
             boxContents.toString() + "]");
      counter++;
    }
  }

  public static void main(String[] args) {
    java.util.ArrayList<Box<Integer>> listOfIntegerBoxes =
      new java.util.ArrayList<>();
    BoxDemo.<Integer>addBox(Integer.valueOf(10), listOfIntegerBoxes);
    BoxDemo.addBox(Integer.valueOf(20), listOfIntegerBoxes);
    BoxDemo.addBox(Integer.valueOf(30), listOfIntegerBoxes);
    BoxDemo.outputBoxes(listOfIntegerBoxes);
  }
}

The following is the output from this example:

Box #0 contains [10]
Box #1 contains [20]
Box #2 contains [30]

The generic method addBox defines one type parameter named U. Generally, a Java compiler can infer the type parameters of a generic method call. Consequently, in most cases, you do not have to specify them. For example, to invoke the generic method addBox, you can specify the type parameter as follows:

BoxDemo.<Integer>addBox(Integer.valueOf(10), listOfIntegerBoxes);

Alternatively, if you omit the type parameters, a Java compiler automatically infers (from the method's arguments) that the type parameter is Integer:

BoxDemo.addBox(Integer.valueOf(20), listOfIntegerBoxes);

Type Inference and Instantiation of Generic Classes

You can replace the type arguments required to invoke the constructor of a generic class with an empty set of type parameters (<>) as long as the compiler can infer the type arguments from the context. This pair of angle brackets is informally called the diamond.

For example, consider the following variable declaration:

Map<String, List<String>> myMap = new HashMap<String, List<String>>();

You can subsitute the parameteried type of the constructor with an empty set of type parameters (<>):

Map<String, List<String>> my Map = newHashMap<>();

Note that to take advantage of type inference during generic class instantiation, you must diamond notation. In the following example, the compiler generates an unchecked conversion warning because the HashMap() constructor refers to the HashMap raw type, not the Map<String, List<String>> type:

Map<String, List<String>> myMap = new HashMap(); // unchecked conversion warning

Java supports limited type inference for generic instance creation; you can only use type inference if the parameterized type of the constructor is obvious from the context. For example, the following code does not compile:

List<String> list = new ArrayList<>();
list.add("A");

// The following statement should fail since addAll expects
// Collection<? extends String>
list.addAll(new ArrayList<>());

Note that the diamond often works in method calls; however, for greater clarity, it is suggested that you use the diamond primarily to initialize a variable where it is declared.

In comparison, the following example compiles:

List<? extends String> list2 = new ArrayList<>();
list.addAll(list2);

Type Inference and Generic Constructors of Generic and Non-Generic Classes

Note that constructors can be generic (in other words, declare their own formal type parameters) in both generic and non-generic classes. Consider the following example:

class MyClass<X> {
  <T> MyClass(T t) {
    // ...
  }
}

Consider the following instantiation of the class MyClass:

new MyClass<Integer>("")

This statement creates an instance of the parameterized type MyClass<Integer>; the statement explicitly specifies the type Integer for the formal type parameter, X, of the generic class MyClass<X>. Note that the constructor for this generic class contains a formal type parameter, T. The compiler infers the type String for the formal type parameter, T, of the constructor of this generic class (because the actual parameter of this constructor is a String object).

Compilers from releases prior to Java SE 7 are able to infer the actual type parameters of generic constructors, similar to generic methods. However, compilers in Java SE 7 and later can infer the actual type parameters of the generic class being instantiated if you use the diamond (<>). Consider the following example:

MyClass<Integer> myObject = new MyClass<>("");

In this example, the compiler infers the type Integer for the formal type parameter, X, of the generic class MyClass<X>. It infers the type String for the formal type parameter, T, of the constructor of this generic class.


Note: It is important to note that the inference algorithm uses only invocation arguments and, possibly, an obvious expected return type to infer types. The inference algorithm does not use results from later in the program.

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