java - 如何正确比较 Java 中的两个整数？

Question

我知道，如果您将装箱的原始整数与常量进行比较，例如:

Integer a = 4;
if (a < 5)

a 将自动被拆箱，并且可以进行比较。

但是，当您比较两个装箱的 Integers 并想要比较相等或小于/大于时会发生什么？

Integer a = 4;
Integer b = 5;

if (a == b)

上面的代码会检查它们是否是同一个对象，还是会在这种情况下自动拆箱？

怎么样:

Integer a = 4;
Integer b = 5;

if (a < b)

?

Answer 1

否，Integer、Long 等之间的 == 将检查 引用相等性 - 即

Integer x = ...;
Integer y = ...;

System.out.println(x == y);

这将检查 x和 y引用相同的对象而不是相等的对象。

所以

Integer x = new Integer(10);
Integer y = new Integer(10);

System.out.println(x == y);

保证打印false . “小”自动装箱值的实习可能会导致棘手的结果:

Integer x = 10;
Integer y = 10;

System.out.println(x == y);

这将打印 true , 由于拳击规则 (JLS section 5.1.7)。它仍然使用引用相等，但引用真正是相等的。

If the value p being boxed is an integer literal of type int between -128 and 127 inclusive (§3.10.1), or the boolean literal true or false (§3.10.3), or a character literal between '\u0000' and '\u007f' inclusive (§3.10.4), then let a and b be the results of any two boxing conversions of p. It is always the case that a == b.

我个人会使用:

if (x.intValue() == y.intValue())

或

if (x.equals(y))

如您所说，对于包装器类型(Integer、Long 等)和数字类型(int、long 等)之间的任何比较，包装器类型值是未装箱 并且测试应用于所涉及的原始值。

这发生在二进制数字提升 (JLS section 5.6.2) 中。查看每个运算符(operator)的文档以查看它是否已应用。例如，来自 == 的文档和 != (JLS 15.21.1):

If the operands of an equality operator are both of numeric type, or one is of numeric type and the other is convertible (§5.1.8) to numeric type, binary numeric promotion is performed on the operands (§5.6.2).

对于 < , <= , >和 >= (JLS 15.20.1)

The type of each of the operands of a numerical comparison operator must be a type that is convertible (§5.1.8) to a primitive numeric type, or a compile-time error occurs. Binary numeric promotion is performed on the operands (§5.6.2). If the promoted type of the operands is int or long, then signed integer comparison is performed; if this promoted type is float or double, then floating-point comparison is performed.

请注意，在两种类型都不是数字类型的情况下，这些都不被视为的一部分。