java type inference in builder pattern

问题

i want to create a parameterized (generic) class MyType<T> and its builder. builder will have some methods that ignore the type T and some methods that use the type. but it seems like using this pattern i have to repeat the T declaration:

                     how to omit this declaration?
                                   |
                                   V
MyType<String> myType = Builder.<String>of()
                               .method1(argument_can_be_of_any_type)
                               .method2(argument_must_be_of_type_T = String)
                               .build();

is it possible to use some java syntax tricks or other design patterns to omit the second type declaration and make the API more user friendly? like for example:

List<String> aList = Lists.emptyList();

List<String> aList = new LinkedList<>();

i don't want to enforce any order of methods in the builder

回答1:

I've reworked things a bit since my original answer didn't work as advertised. (Thanks to shmosel for spotting the problem, and to Daniel Pryden for the suggested solution.)

/* contents of Box.java */
public class Box<T>
{
    private T contents;
    private Object data;

    protected Box(T contents, Object data) {
        this.contents = contents;
        this.data = data;
    }

    public static BoxBuilder builder() {
        return new BoxBuilder();
    }

    public T getContents() {
        return contents;
    }
}

/* contents of BoxBuilder.java */
public class BoxBuilder
{
    private Object data;

    public BoxBuilder withAnything(Object o) {
        this.data = o;
        return this;
    }

    // Infers new type from argument
    public <T> TypedBoxBuilder<T> withBoxContent(T contents) {
        TypedBoxBuilder<T> builder = new TypedBoxBuilder<T>();
        builder.setData(data);
        builder.setContents(contents);

        return builder;
    }
}

/* contents of TypedBoxBuilder.java */
public class TypedBoxBuilder<T>
{
    private T contents;
    private Object data;

    public TypedBoxBuilder() {
    }

    public TypedBoxBuilder<T> withAnything(Object data) {
        this.data = data;
        return this;
    }

    public TypedBoxBuilder<T> withContents(T contents) {
        this.contents = contents;
        return this;
    }

    public Box<T> build() {
        return new Box<T>(contents, data);
    }

    public void setContents(T contents) {
        this.contents = contents;
    }

    public void setData(Object data) {
        this.data = data;
    }
}

And here's the client code:

Box<String> box = Box.builder() // Returns BoxBuilder
        .withBoxContent("FOO") // Returns TypedBoxBuilder<String>
        .withAnything(42) // Returns TypedBoxBuilder<String>
        .build(); // Returns Box<String>
String c = box.getContents();

And this works as well:

Box<String> box = Box.builder() // Returns BoxBuilder
        .withAnything(42) // Returns BoxBuilder
        .withBoxContent("FOO") // Returns TypedBoxBuilder<String>
        .build(); // Returns Box<String>
String c = box.getContents();

来源：https://stackoverflow.com/questions/45362662/java-type-inference-in-builder-pattern

标签

java

generics

design-patterns

type-inference

builder