The meaning of Java Generics. <SomeValue>

Question

I had an interview test and saw the following code:

EDIT:

public class TestValue { private Value<SomeValue> defaultValue; @Test public void Empty_Value_Has_No_Value() { Assert.assertFalse(Value.<SomeValue> createEmptyValue() .hasValue()); } @Test public void Default_Value_IsEmpty() { Assert.assertEquals(Value.<SomeValue> createEmptyValue(), defaultValue); } @Test public void Non_Empty_Value_Has_Value() { Assert.assertTrue(new Value<SomeValue>(true, new SomeValue()) .hasValue()); } } 

I've never seen java generic like

 Value.<SomeValue> 

The test is to implement the Value class with the unit test code above.

I tried to find below the signature of the Value method (implementation needed):

 public interface Value<T> { public boolean hasValue(); public Value<T> createEmptyValue(); } 

Does anyone know please help?

thanks

EDIT: should be like this, according to the answers below @marlon

 public class Value<T> { public boolean hasValue(){} public static <M> Value<M> createEmptyValue(){}; //need <M> } 

Key Syntax:

 Value.<SomeValue> //ClassName.<Type>method 

- a method of calling a static class method with a parameterized argument.

EDIT: according to @ snipes83, the syntax for calling a non-static class method with a parameterized argument.

 SomeObject.<Type>method 

Answer 1

Value.<SomeValue> is how generic methods are represented for methods.

Using Google Guava Optional as an example:

 Optional<String> email = Optional.<String>of(strEmail); 

See General Types - Calling General Methods

Since interfaces cannot declare static methods (shame on you java), just declare your method as static and forget about the interface, for example:

 class Value<T> { public static <T> Value<T> createEmptyValue(){ return null; } } 

Answer 2

Look at the Test class using the getEmptyList method below:

 public class Test { public <T> List<T> getEmptyList() { return new ArrayList<T>(); } } 

It returns an empty List object containing objects of type T

If you use Test like this

 Test t = new Test(); List<Integer> list = t.getEmptyList(); 

Then the type inference engine can infer the type parameter based on the type of the variable.

However, if you need to use the getEmptyList return value in a method invocation expression, as in the following example, where the printList method expects a single argument of type List<Integer> , then the type cannot be inferred from any type of variable.

 public void printList(List<Integer> list) { for (int i : list) { System.out.print(i); } } printList(t.getEmptyList()); // This will FAIL. 

In this case, you need to specify the type using the following:

 printList(t.<Integer>getEmptyList()); 

Answer 3

1) This is how generic methods are called. See → http://docs.oracle.com/javase/tutorial/java/generics/methods.html

2) <SomeValue> in Value.<SomeValue> optional . The compiler can infer the type. This is called TypeInference. See → http://docs.oracle.com/javase/tutorial/java/generics/genTypeInference.html

Answer Updated:

3) Value.<SomeValue> createEmptyValue() right and Value.<SomeValue>createEmptyValue() also right. Both methods are legal. Just tested it. Did not notice before.

Answer 4

Although Value itself is typed (based on the type of the instance variable Value<SomeValue> ), the static createEmptyValue() method is also introduced.

A reasonable assumption, if the naming conventions are followed, is that SomeValue extends (or implements) Value .

Although we do not have a single correct answer, a likely possibility for a Value signature is:

 public class Value<T extend Value> { public static <V extends Value> V createEmptyValue() { // some impl } }