盒装布尔值的相等性

Question

小问题：是否保证此代码始终打印 true？

Boolean b1 = true;
Boolean b2 = true;
System.out.println(b1 == b2);

布尔值的装箱似乎总是产生相同的布尔对象，但我在 JLS 中找不到太多关于装箱布尔相等性的信息。相反，它甚至似乎暗示装箱应该创建新对象，甚至可能导致 OOM 异常。

你有什么想法？

Answer 1

来自Java Language Specification on Boxing Conversion

Boxing conversion converts expressions of primitive type to corresponding expressions of reference type. Specifically, the following nine conversions are called the boxing conversions:

From type boolean to type Boolean

[...]

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.

这个比较简单implemented as

/**
 * The {@code Boolean} object corresponding to the primitive
 * value {@code true}.
 */
public static final Boolean TRUE = new Boolean(true);

/**
 * The {@code Boolean} object corresponding to the primitive
 * value {@code false}.
 */
public static final Boolean FALSE = new Boolean(false);

public static Boolean valueOf(boolean b) {
    return (b ? TRUE : FALSE);
}

Answer 2

是的。编译器自动翻译这个：

Boolean b1 = true;

进入这个：

Boolean b1 = Boolean.valueOf(true);

这总是 returns 两个常量之一 Boolean.TRUE 或 Boolean.FALSE.

盒装布尔值的相等性

Equality of boxed boolean

java

primitive

autoboxing