在Map值中强制两种类型-Java泛型

Question

I have a situation, I need to use Map , and put String as key and String or Long as value.

As they don't fall into same hierarchy, I think I can't use wildcards.

So is there any way to enforce compiler to accept either String or Long as value or I will have to go with Map without generics .

Answer 1

Two possible solutions either use Map<String, Object> or create a class like

public class DualValue {
    private final String stringValue;
    private final Long   longValue;

    public DualValue(final String stringValue) {
        this.stringValue = stringValue;
        this.longValue = null;
    }

    public DualValue(final Long longValue) {
        this.stringValue = null;
        this.longValue = longValue;
    }

    public String getStringValue() {
        return stringValue;
    }

    public Long getLongValue() {
        return longValue;
    }

    public boolean isString() {
        return stringValue != null;
    }

    public boolean isLong() {
        return longValue != null;
    }

    // next two are optional but should be implemented,
    // if you ever want to use this class as key of a Map

    @Override
    public boolean equals(Object o) {
        if (this == o) {
            return true;
        }
        if (o == null || getClass() != o.getClass()) {
            return false;
        }

        DualValue dualValue = (DualValue) o;

        if (longValue != null ? !longValue.equals(dualValue.longValue) : dualValue.longValue != null) {
            return false;
        }
        if (stringValue != null ? !stringValue.equals(dualValue.stringValue) : dualValue.stringValue != null) {
            return false;
        }

        return true;
    }

    @Override
    public int hashCode() {
        int result = stringValue != null ? stringValue.hashCode() : 0;
        result = 31 * result + (longValue != null ? longValue.hashCode() : 0);
        return result;
    }
} 

and use Map<String, DualValue> . You are right that the compiler is not able to merge two destinct class-hirachies with a pure genric approach in another way than using Object which is common parent to every Java class. Even if it could you would probably have to check by using value instanceof String or value instanceof Long and cast to one of those if you would want to call any functions on the value not present in Object .

Answer 2

我认为您只需要存储一个对象，这与不带泛型的对象存储几乎一样（当然，仍然会检查String键，并且不会收到编译器警告）。

Answer 3

将long转换为字符串，然后可以得到<String,String>的映射