从字符串转换为通用类型 [英] Conversion from string to generic type

问题描述

我需要将字符串转换为变量值。我发现仅针对C＃的解决方案。我需要用Java。

I need to convert string to variable value. I found solution only for C#. I need it in Java.

public class Property<T> {

    T value;

    public void setValue(String input){
        if(value instanceof String){
           value= input; // value is type of T not type of string (compilation error)
                         // incompatible types: String cannot be converted to T
        }
        if(value instanceof int){
           //parse string
        }
        if(value instanceof boolean){
           //parse string
        }
        ...
    }
}

推荐答案

那不是它的工作原理。但是，您可以使用多态来获得有用的结果。

That is not how it works. You can, however, use polymorphism, to achieve a useful result.

基本通用（和摘要）property

Base generic (and abstract) property

public abstract class Property<T> {
    T value;
    public abstract void setValue(String input);
}

字符串属性

public class StringProperty extends Property<String> {
    @Override
    public void setValue(String input) {
        this.value = input;
    }
}

整数属性

public class IntegerProperty extends Property<Integer> {
    @Override
    public void setValue(String input) {
        this.value = Integer.valueOf(input);
    }
}

不确定您的实际目标是什么，但是这种方法可能有效。

Not sure what your actual goal is, but this approach might work.

请注意，由于类型擦除， T 的输入实例将失败。

Note, that input instanceof T will fail, because of type erasure. It's not gonna work.

要详细说明您的方法，这可以工作-但这很丑。

To elaborate more on your approach, this would work - but it's UGLY.

丑陋而不是丑陋很方便。

Ugly and not very convenient. No idea why you'd want it, tbh.

class Property<T> {

    public T value;
    private final Class<T> clazz;

    public Property(Class<T> clazz) {
        super();
        this.clazz = clazz;
    }       

    @SuppressWarnings("unchecked")
    public void setValue(String input) {
        if (clazz.isAssignableFrom(String.class)) {
            value = (T) input;
        } else if (clazz.isAssignableFrom(Integer.class)) {
            value = (T) Integer.valueOf(input);
        } else if (clazz.isAssignableFrom(Boolean.class)) {
            value = (T) Boolean.valueOf(input);
        } else if (clazz.isAssignableFrom(Double.class)) {
            value = (T) Double.valueOf(input);
        } else {
            throw new IllegalArgumentException("Bad type.");
        }
    }
}

用法如下：

Property<String> ff = new Property<>(String.class);
ff.setValue("sdgf");

Property<Integer> sdg = new Property<>(Integer.class);
sdg.setValue("123");

System.out.println(ff.value);
System.out.println(sdg.value);

---

具有 Reflection

显然，可以找出用于实例化属性的参数。

---

Solution with Reflection

Apparently, it's possible to figure out the parameter used to instantiate property.

这个小小的魔术公式可以为您提供以下信息：

This little magic formula gives you just that:

(Class<?>) getClass().getTypeParameters()[0].getBounds()[0]

我什至不知道如何找到它。好吧，我们开始：

I don't even know how I managed to find it. Well, here we go:

class Property<T> {

    T value;    

    @SuppressWarnings("unchecked")
    public void setValue(String input)
    {
        // BEHOLD, MAGIC!
        Class<?> clazz = (Class<?>) getClass().getTypeParameters()[0].getBounds()[0];

        if (clazz.isAssignableFrom(String.class)) {
            value = (T) input;
        } else if (clazz.isAssignableFrom(Integer.class)) {
            value = (T) Integer.valueOf(input);
        } else if (clazz.isAssignableFrom(Boolean.class)) {
            value = (T) Boolean.valueOf(input);
        } else if (clazz.isAssignableFrom(Double.class)) {
            value = (T) Double.valueOf(input); 
        } else {
            throw new IllegalArgumentException("Bad type.");
        }
    }
}

不要看我，我不会用那个。我有一些常识。

And don't look at me, I wouldn't use that. I have some common sense.

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