如何使用2种不同类型的HashMaps定义2个构造函数?

时间:2013-09-26 15:33:41

标签: java constructor hashmap polymorphism

我有一个包含2个HashMap字段的类,如下所示 -

HashMap<String, Integer> map1;
HashMap<String, String> map2;

现在,我想只传递构造函数中的一个映射,即map1或map2的类型。但是,我无法使用不同类型的HashMap定义2个不同的构造函数。这是一个解决方法吗?

6 个答案:

答案 0 :(得分:26)

一些选择:

1)一个构造函数,它接受两个映射,并且在传递null时是安全的。

public MyClass( Map<String, Integer> map1, Map<String, String> map2 ) {
    if ( map1 != null ) { this.map1 = map1; }
    if ( map2 != null ) { this.map2 = map2; }
}

2)每张地图的设定者

public MyClass {
    private Map<String, Integer> map1;
    private Map<String, String> map2;
    public void setMap1( Map<String, Integer> map1 ) {
        this.map1 = map1;
    }
    public void setMap2( Map<String, String> map2 ) {
        this.map2 = map2;
    }
}

3)一个构建器,允许您区分地图并正确构造对象(调用setter)

public MyClass {
    private Map<String, Integer> map1;
    private Map<String, String>  map2;
    // pretend you don't want people to be able to swap out the map after construction so you protect the setter here.
    protected void setMap1( Map<String, Integer> map1 ) {
        this.map1 = map1;
    }
    protected void setMap1( Map<String, String> map2 ) {
        this.map2 = map2;
    }
    // getters for the maps and other properties
    public static Builder builder() {
        return new Builder();
    }
    public static class Builder {
        private Map<String, Integer> map1;
        private Map<String, String> map2;
        public Builder withMap1( Map<String, Integer> map ) {
            map1 = map;
            return this;
        }
        public Builder withMap2( Map<String, String> map ) {
            map2 = map;
            return this;
        }
        public MyClass build() {
            MyClass c = new MyClass();
            // possibly conditional code that inspects the maps for specific values or validity
            c.setMap1( map1 );
            c.setMap2( map2 );
            // initialization of other fields
            return c;
        }
    }

    public static void main( String[] args ) {
        // sample usage
        MyClass instance1 = MyClass.builder().withMap1(myMap1).build();
        MyClass instance2 = MyClass.builder().withMap2(myMap2).build();
        MyClass instance3 = MyClass.builder().withMap1(myMap1).withMap2(myMap2).build();
    }
}

4)静态工厂(如下面的Evgeniy Dorofeev所指出的)

public MyClass {
    private Map<String, Integer> map1;
    private Map<String, String> map2;
    // other properties

    private MyClass() {}

    public static MyClass withMap1(Map<String, Integer> map ) {
        MyClass c = new MyClass();
        c.map1 = map;
        return c;
    }
    public static MyClass withMap2(Map<String, String> map ) {
        MyClass c = new MyClass();
        c.map2 = map;
        return c;
    }
    // getters and setters
}

答案 1 :(得分:13)

你不能:在编译阶段,泛型被剥离:在两种情况下,编译的代码只看到HashMap<Object, Object>

此过程的技术名称为 type erasure 。见http://docs.oracle.com/javase/tutorial/java/generics/erasure.html

在很多方面,Java泛型都不如C ++模板。

答案 2 :(得分:8)

作为一种解决方法,您可以使用具有不同名称的静态工厂方法

答案 3 :(得分:1)

public class MyObject<T> {
    public MyObject(Map<String, T> map) {
         // process map
    }
}

然后你可以用:

创建你的对象
new MyObject<Integer>(map1);
new MyObject<String>(map2);

问题是:你想用MyObject中的通用地图做什么......?

另一种解决方案是:

public class MyObject {
    public <T> MyObject(Map<String, T> map) {
        // process map
    }
}

这更容易使用,因为在编译时推断出类型参数T:

new MyObject(map1);
new MyObject(map2);

但是,您无法在运行时确定T的具体类型......

答案 4 :(得分:0)

HashMap map1和HashMap map2完全不同。它们不可互换。因此,您需要定义一个传递两个哈希映射的构造。

  public MyClass(HashMap<String, Integer> map1, HashMap<String, String> map2){
        ............
  }

答案 5 :(得分:0)

我同意digitaljoel提供的答案......但是,作为第3个选项提供的构思模式可能很少有改进。 Builder Pattern的一个主要优点是它从父类中删除了setter函数,客户端现在调用构建器类的类似setter的函数来设置父类的实例变量。

public class MyClass{
    private Map<String, Integer> map1;
    private Map<String, String> map2;

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

    public static class Builder {
        private Map<String, Integer> map1;
        private Map<String, String> map2;

        public Builder withMap1(Map<String, Integer> map) {
            map1 = map;
            return this;
        }

        public Builder withMap2(Map<String, String> map) {
            map2 = map;
            return this;
        }

        public MyClass build() {
            return new MyClass(this);
        }

    }

    public MyClass(Builder b) {
        map1 = b.map1;
        map2 = b.map2;
    }

    public static void main(String[] args) {
        // sample usage
        MyClass instance1 = MyClass.builder().withMap1(myMap1).build();
        MyClass instance2 = MyClass.builder().withMap2(myMap2).build();
        MyClass instance3 = MyClass.builder().withMap1(myMap1).withMap2(myMap2).build();
    }
}