带有boolean的java.lang.NullPointerException [英] java.lang.NullPointerException with boolean

问题描述

我根据另一个问题编写了一个简单的简单代码，这里是：

I wrote a realy simple code based on another question and here it is:

它会抛出一个错误

java.lang.NullPointerException第5行和第17行

java.lang.NullPointerException line 5 and 17

我不知道我在做什么我做错了。

I don't know what I'm doing wrong.

 public class Main {

    public static String bool(Boolean param){
        if(param == true){    (line 5)
            return "a";
        }else if(param == false){
            return "b";
        }
        return "c";

    }

    public static void main(String[] args){

        System.out.println(bool(true));
        System.out.println(bool(null)); (line 17)
        System.out.println(bool(false));


    }
}

推荐答案

null 无法自动取消装箱到原始布尔值值，这就是当您尝试将其与 true 进行比较。在

null cannot be auto-unboxed to a primitive boolean value, which is what happens when you try to compare it with true. In

param == true

true 的类型是 boolean ，因此左手操作数也必须是a 布尔值。您传入 Boolean ，这是一个对象，但可以自动取消装箱到 boolean 。

The type of true is boolean, therefore the left-hand operand must also be a boolean. You are passing in a Boolean, which is an object, but can be auto-unboxed to boolean.

因此这相当于

param.booleanValue() == true

显然，如果 param 是 null ，上面抛出 NullPointerException 。

Clearly, if param is null, the above throws NullPointerException.

为了避免隐藏的陷阱自动取消装箱，您可以改为使用布尔对象：

To avoid the hidden pitfalls of auto-unboxing, you could instead work with the Boolean objects:

if (Boolean.TRUE.equals(param))
  return "a";
if (Boolean.FALSE.equals(param))
  return "b";
return "c";

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