Double与Double之间的差异比较 [英] Difference between double and Double in comparison

问题描述

我知道Double是包装类，并且包装double数字.今天，我看到了另一个主要区别:

I know that Double is a a wrapper class, and it wraps double number. Today, I have seen another main difference :

double a = 1.0;
double b = 1.0;
Double c = 1.0;
Double d = 1.0;
System.out.println(a == b);  // true
System.out.println(c == d);  // false

我真奇怪!!!

因此，如果我们每次使用Double，则必须执行以下操作:

So, if we use Double, each time, we must do something like this :

private static final double delta = 0.0001;
System.out.println(Math.abs(c-d) < delta); 

我无法解释Double为何直接比较错误.请为我解释.

I cannot explain why Double make directly comparison wrong. Please explain for me.

推荐答案

c和d从技术上讲是两个不同的对象，并且==运算符仅比较引用.

c and d are technically two different objects and == operator compares only references.

c.equals(d)

更好，因为它比较值而不是引用.但是仍然不理想.直接比较浮点值应始终将一些误差(ε)考虑在内(Math.abs(c - d) < epsilon).

is better as it compares values, not references. But still not ideal. Comparing floating-point values directly should always take some error (epsilon) into account (Math.abs(c - d) < epsilon).

请注意:

Integer c = 1;
Integer d = 1;

在这里进行比较会产生true，但这比较复杂(Integer内部缓存，在

here comparison would yield true, but that's more complicated (Integer internal caching, described in JavaDoc of Integer.valueOf()):

此方法将始终缓存-128至127(包括)范围内的值，并且可能会缓存该范围之外的其他值.

This method will always cache values in the range -128 to 127, inclusive, and may cache other values outside of this range.

为什么valueOf()?因为此方法隐式用于实现自动装箱:

Why valueOf()? Because this method is implicitly used to implement autoboxing:

Integer c = Integer.valueOf(1);
Integer d = Integer.valueOf(1);

另请参见

• Java中的奇怪整数拳击
• 如何在Java中正确比较两个整数?

See also

• Weird Integer boxing in Java
• How to properly compare two Integers in Java?

这篇关于Double与Double之间的差异比较的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！