带有布尔值的 java.lang.NullPointerException

java.lang.NullPointerException with boolean

提问by Alejandro Garcia

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

我根据另一个问题编写了一个非常简单的代码，这里是：

It throws me an error

它给我一个错误

java.lang.NullPointerException line 5 and 17

java.lang.NullPointerException 第 5 行和第 17 行

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

我不知道我做错了什么。

 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));


    }
}

回答by Marko Topolnik

nullcannot be auto-unboxed to a primitive booleanvalue, which is what happens when you try to compare it with true. In

null无法自动拆箱为原始boolean值，当您尝试将其与true. 在

param == true

The type of trueis 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.

trueis的类型boolean，因此左边的操作数也必须是 a boolean。您传入的是Boolean，这是一个对象，但可以自动拆箱为boolean。

Therefore this is equivalent to

因此这相当于

param.booleanValue() == true

Clearly, if paramis null, the above throws NullPointerException.

显然，如果param是null，则上面会抛出NullPointerException。

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

为了避免自动拆箱的隐藏陷阱，您可以改为使用Boolean对象：

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

回答by Mureinik

Your code compares a java.lang.Booleaninstance with a primitive boolean, which means unboxing the java.lang.Boolean. Since nullcan't be unboxed, a NullPointerExceptionis thrown.

您的代码将java.lang.Boolean实例与原语进行比较boolean，这意味着将java.lang.Boolean. 由于null无法拆箱，NullPointerException因此抛出 a 。

You could work around this by using the built in constants Boolean.TRUEand Boolean.FALSE:

您可以通过使用内置常量Boolean.TRUE和来解决这个问题Boolean.FALSE：

public static String bool(Boolean param) {
    if (Boolean.TRUE.equals(param)) {
        return "a";
    } else if (Boolean.FALSE.equals(param)) {
        return "b";
    }
    return "c";
}

回答by Govind

So your program must be something like this.

所以你的程序一定是这样的。

public class BooleanBug {

    public static String bool(Boolean param) {
        if ((null != param) && param.booleanValue() == true) {
            return "a";
        } else if ((null != param) && param.booleanValue() == false) {
            return "b";
        }
        return "c";

    }

    public static void main(String[] args) {

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

    }
}

回答by Alex

You used the Booleaninstead of boolean. Booleanis a class, which means you can assign objects to it. In your case, you passed in a null, which is then assigned to param. You then tried to use param, which of course resulted in a NullPointerException.

您使用了Boolean而不是boolean。Boolean是一个类，这意味着您可以为其分配对象。在您的情况下，您传入了 a null，然后将其分配给 param。然后您尝试使用 param，这当然会导致NullPointerException.

You can:

你可以：

• Get rid of the line bool(null)
• Change Booleanto booleanin the parameters for bool()
• Add an else if for when paramis null

• 摆脱线 bool(null)
• 更改Boolean到boolean在参数bool()
• 添加一个 else if for when paramisnull