java - Java 中的常量串联

Question

如果我有常量

public static final String PREFIX = "meter.";

public static final String READING = "reading";

public static final String LEVEL = "level";

我有一个代码

run() {
dao.set("meter.reading", x);
dao.set("meter.level", y);
}

考虑到 run() 每天会被调用数百万次，

按如下方式编写上述代码是否会因串联而降低我的性能？我想维护与前缀分开的常量，因为它们在各种上下文中使用和不使用前缀

run() {
dao.set(PREFIX+READING, x);
dao.set(PREFIX+LEVEL, y);
}

Answer 1

这不会降低性能，因为您使用的是编译时常量表达式。

PREFIX+READING

编译器会为您连接它，并且它的性能与您尝试过的字符串文字相同。

JLS, Section 15.28 ，定义常量表达式:

A compile-time constant expression is an expression denoting a value of primitive type or a String that does not complete abruptly and is composed using only the following:

(其他选项)

• The unary operators +, -, ~, and ! (but not ++ or --) (§15.15.3, §15.15.4, §15.15.5, §15.15.6)

(其他选项)

• Simple names (§6.5.6.1) that refer to constant variables (§4.12.4).

(其他选项)

Compile-time constant expressions of type String are always "interned" so as to share unique instances, using the method String.intern.