Java泛型+ Builder模式

Question

我如何在start()下面打电话？

package com.example.test;

class Bar {}

public class Foo<K>
{
    final private int count;
    final private K key;

    Foo(Builder<K> b)
    {
        this.count = b.count;
        this.key = b.key;
    }

    public static class Builder<K2>
    {
        int count;
        K2 key;

        private Builder() {}
        static public <K3> Builder<K3> start() { return new Builder<K3>(); }
        public Builder<K2> setCount(int count) { this.count = count; return this; }
        public Builder<K2> setKey(K2 key) { this.key = key; return this; }
        public Foo<K2> build() { return new Foo(this); }
    }

    public static void main(String[] args)
    {
        Bar bar = new Bar();
        Foo<Bar> foo1 = Foo.Builder.start().setCount(1).setKey(bar).build();
        // Type mismatch: cannot convert from Foo<Object> to Foo<Bar>

        Foo<Bar> foo2 = Foo.Builder<Bar>.start().setCount(1).setKey(bar).build();
        // Multiple markers at this line
        // - Bar cannot be resolved
        // - Foo.Builder cannot be resolved
        // - Syntax error on token ".", delete this token
        // - The method start() is undefined for the type Foo<K>
        // - Duplicate local variable fooType mismatch: cannot convert from Foo<Object> to Foo<Bar>

        Foo<Bar> foo3 = Foo<Bar>.Builder.start().setCount(1).setKey(bar).build();
        // Multiple markers at this line
        // - Foo cannot be resolved
        // - Syntax error on token ".", delete this token
        // - Bar cannot be resolved     
    }
}

Answer 1

你很亲密:

Foo.Builder.<Bar> start().setCount(1).setKey(bar).build();

干杯! :)

PS如果编译器无法自己推断方法的类型参数,可以通过调用强制它obj.<Type> method(...).

您可能想要使用的PPS:

public Foo<K2> build() {
    return new Foo<K2>(this);
}

避免使用原始类型.

Answer 2

Andrei的方法没问题,但是大多数程序员可能会对相当未知的语法感到困惑.使用这种方式可能更容易:

static public <K3> Builder<K3> start(Class<K3> cls) { return new Builder<K3>(); }

Foo<Bar> foo1 = Foo.Builder.start(Bar.class).setCount(1).setKey(bar).build();

该类仅传递给泛型类型的帮助.它不漂亮,但至少语法是常识.

另一种选择是立即开始使用泛型类型的对象:

Foo<Bar> foo1 = Foo.Builder.startWithKey(bar).setCount(1).build();

Answer 3

如果您是lombok项目的粉丝并使用其注释@Builder来实现构建器模式，那么您可以像编写一样删除所有不必要的代码，并用更少的代码来完成。

例子：

@Builder
public class ErrorResponse<T> {
    private String message;
    private List<String> reasons;
    private List<String> details;
    private T data;
}

你可以像这样初始化它：

ErrorResponse<MyData> myDataError = ErrorResponse.<MyData>builder()
.message("My message")
.build();