type witness and diamond in java

## type witness
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 with a type witness as follows:

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

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

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



## Diamond

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 substitute the parameterized type of the constructor with an empty set of type parameters (<>):

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

Note that to take advantage of type inference during generic class instantiation, you must use the diamond. 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