为什么将Integer与int进行比较可以在Java中抛出NullPointerException？

问题描述

观察这种情况让我非常困惑：

It was very confusing to me to observe this situation:

Integer i = null;
String str = null;

if (i == null) {   //Nothing happens
   ...
}
if (str == null) { //Nothing happens

}

if (i == 0) {  //NullPointerException
   ...
}
if (str == "0") { //Nothing happens
   ...
}

所以，as我认为首先执行装箱操作（即java尝试从 null 中提取int值），并且比较操作具有较低的优先级，这就是抛出异常的原因。

So, as I think boxing operation is executed first (i.e. java tries to extract int value from null) and comparison operation has lower priority that's why the exception is thrown.

问题是：为什么在Java中以这种方式实现？为什么拳击优先于比较参考？或者为什么他们没有在拳击前对 null 实施验证？

The question is: why is it implemented in this way in Java? Why boxing has higher priority then comparing references? Or why didn't they implemented verification against null before boxing?

当 NullPointerException 被包装的基元抛出时，它看起来不一致，并且不会抛出 true 对象类型。

At the moment it looks inconsistent when NullPointerException is thrown with wrapped primitives and is not thrown with true object types.

推荐答案

简答

关键点是：

The Short Answer

The key point is this:

• == 两种参考类型之间总是参考比较
  
  
  
  
  • 通常情况下，例如使用 Integer 和 String ，您要使用等于
  
  
  • == between two reference types is always reference comparison
    • More often than not, e.g. with Integer and String, you'd want to use equals instead
    
    
    • 引用类型将进行拆箱转换
    
    
    • 拆箱 null 总是抛出 NullPointerException
    
    
    • The reference type will be subjected to unboxing conversion
    • Unboxing null always throws NullPointerException

    以上语句适用于任何给定的有效 Java代码。根据这种理解，您提供的代码段中没有任何不一致。

    The above statements hold for any given valid Java code. With this understanding, there is no inconsistency whatsoever in the snippet you presented.

    以下是相关的JLS部分：

    Here are the relevant JLS sections:

    这解释了以下内容：

    Integer i = null;
    String str = null;
    
    if (i == null) {   // Nothing happens
    }
    if (str == null) { // Nothing happens
    }
    if (str == "0") {  // Nothing happens
    }
    

    两个操作数是引用类型，这就是 == 是引用相等比较的原因。

    Both operands are reference types, and that's why the == is reference equality comparison.

    这也解释了以下内容： / p>
    
    

    This also explains the following:

    System.out.println(new Integer(0) == new Integer(0)); // "false"
    System.out.println("X" == "x".toUpperCase()); // "false"
    

    对于 == 为了数字相等，至少有一个操作数必须是数字类型：

    注意二进制数字促销执行价值集转换和拆箱转换。

    Note that binary numeric promotion performs value set conversion and unboxing conversion.

    这解释了：

    Integer i = null;
    
    if (i == 0) {  //NullPointerException
    }
    

    以下摘录自 Effective Java 2nd Edition，第49项：首选原语为盒装基元：

    有些地方你别无选择但要使用盒装基元，例如泛型，但你应该认真考虑是否有合理使用盒装基元的决定。

    There are places where you have no choice but to use boxed primitives, e.g. generics, but otherwise you should seriously consider if a decision to use boxed primitives is justified.

    
    
    • 
      
      
      
      
      • 数字类型是整数类型和浮点类型。
      
      
      • JLS 4.2. Primitive Types and Values
        • "The numeric types are the integral types and the floating-point types."
        
        
        • 一种类型据说是可兑换到数字类型如果是数字类型，或者它是可以通过取消装箱转换转换为数字类型的引用类型。
        
        
        • 取消装箱转换会转换[ ...]从类型整数到键入 int
        
        
        • 如果 r 是 null ，取消装箱转换会抛出 NullPointerException
        
        
        • "A type is said to be convertible to a numeric type if it is a numeric type, or it is a reference type that may be converted to a numeric type by unboxing conversion."
        • "Unboxing conversion converts [...] from type Integer to type int"
        • "If r is null, unboxing conversion throws a NullPointerException"
        
        
        
        
        
        
        
        
        • When comparing two Integers in Java does auto-unboxing occur?
        • Why are these == but not equals()?
        • Java: What’s the difference between autoboxing and casting?
        
        
        
        
        • （是的！这个盒子是未装箱的，而不是其他方式！）
        
        
        • （!!!）
        
        
        • （不幸的是，是的）
        
        
        
        
        • What is the difference between an int and an Integer in Java/C#?
        • Is it guaranteed that new Integer(i) == i in Java? (YES!!! The box is unboxed, not other way around!)
        • Why does int num = Integer.getInteger("123") throw NullPointerException? (!!!)
        • Java noob: generics over objects only? (yes, unfortunately)
        • Java String.equals versus ==

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