Question

我想创建一个参数化（通用）类MyType<T>及其构建器。构建器将有一些忽略类型T的方法和一些使用该类型的方法。但似乎使用这种模式我必须重复T声明：

                     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();

是否可以使用一些java语法技巧或其他设计模式来省略第二种类型声明并使API更加用户友好？例如：

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

或

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

我不想在构建器

中强制执行任何方法顺序

Answer 1

由于我原来的答案并没有像宣传的那样有效，我已经重新做了一些工作。（感谢shmosel发现问题，感谢Daniel Pryden建议的解决方案。）

/* 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;
    }
}

这是客户端代码：

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();

这也有效：

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

构建器模式中的java类型推断

1 个答案: