Comparison of usage methods of java.lang.Void and void

This article mainly introduces the comparison and usage of java.lang.Void and void. The editor thinks it is quite good. I will share it with you here. Friends who need it can refer to it.

void is not a function, it is a modifier of a method. Void means that the method has no return value, which means that the method will only run the statements in the method, but will not return anything.

java.lang.Void is a type. For example, assign null to a Void reference.

Void nil = null;

As you can see from the code of the Void class, the Void type cannot be inherited and instantiated.

public final
class Void {
  /**
   * The {@code Class} object representing the pseudo-type corresponding to
   * the keyword {@code void}.
   */
  @SuppressWarnings("unchecked")
  public static final Class<Void> TYPE = (Class<Void>) Class.getPrimitiveClass("void");
  /*
   * The Void class cannot be instantiated.
   */
  private Void() {}
}

Void as the return result of the function means that the function returns null (other types cannot be returned except null).

Void function(int a, int b) {
  //do something
  return null;
 }

Before the emergence of generics, Void was generally used in reflection. For example, the following code prints the name of a method with a return type of void.

public class Test {
  public void print(String v) {}
  public static void main(String args[]){
    for(Method method : Test.class.getMethods()) {
      if(method.getReturnType().equals(Void.TYPE)) {
        System.out.println(method.getName());
      }
    }
  }
}

After the emergence of generics, the Void type will be used in certain scenarios. For example, Future is used to save the results. Future's get method will return the result (type T).

But what if the operation does not return a value? In this case, it can be represented by Future. When get is called and the result is calculated, null will be returned.
In addition, Void is also used in valueless Maps, such as Map, so the map will have the same function as Set.

So when you use generics, the function does not need to return a result or when an object does not need a value, it can be represented by the java.lang.Void type.

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