方法中有多个返回类型

Question

Is there a way to get different return type from a method like int or String or double?

what i can think of is using generics wildcard,not sure

like:

public x m1(){
//implementation
}

x can be int,string,long etc

Answer 1

No, because Java is statically typed. You can use generics or polymorphic types, but you cannot just return arbitrary type (well, you can return Object ...) How are you suppose to work with return value?

??? value = m1()

The type of value is unknown and you cannot do anything about it.

You can however think of some generic, strongly typed container, similar to Either . Apache Commons Lang 3 has Pair<L,R> , you can take it as an example.

Answer 2

No, a method can have only one return type. If you want to return objects of different types you can return Object but you'd still have to cast in that case. You also can't return unboxed primitives in that case (ie int or double ).

Generics could help somewhat in that they add some of those casts implicitly. However, depending on what you want to achieve it might not be worth it.

We're using something like this, for example:

class BasicBuilder<T extends BasicBuilder<T>> {
  private int value;

  public T setValue( int v ) {
    value = v;
     return (T)this;
  }
}

class ConcreteBuilder extends BasicBuilder<ConcreteBuilder> {
  //more here
}

This way, when I have a ConcreteBuilder instance and call setValue(...) on it, I'll get a ConreteBuilder reference back. Note that this only works if the generics are used properly and the casts are valid.

Answer 3

You could do this in HotSpot Java 6 with generics, but this was considered a bug and Java 5.0 nor Java 7 supports this.

http://vanillajava.blogspot.com/2011/02/with-generics-return-type-is-part-of.html

Java in general is not context aware, but this is changing with implied type support in Java 7 and not implied types with Java 8.

The common workaround is to specify the type in the name. eg ByteBuffer or DataInput with getInt() , getLong() , getDouble()

Answer 4

Use Object class as return type and then use Integer or String (Wrapper) classes. So the point where you are returning you can use Object's getClass() method to identify its class.