规格
Java语言规范8的Example 3.10.5-1. String Literals告诉我们:
该程序由编译单元组成(第7.3节):
package testPackage;
class Test {
public static void main(String[] args) {
String hello = "Hello",lo = "lo";
System.out.print((hello == "Hello") + " ");
System.out.print((Other.hello == hello) + " ");
System.out.print((other.Other.hello == hello) + " ");
System.out.print((hello == ("Hel"+"lo")) + " ");
System.out.print((hello == ("Hel"+lo)) + " ");
System.out.println(hello == ("Hel"+lo).intern());
}
}
class Other { static String hello = "Hello"; }
和编译单位:
package other;
public class Other { public static String hello = "Hello"; }
产生输出:
true true true true false true
现实
但编译并运行oracle jdk 1.8.0_65(win)输出
true true true true true true
问题1
为什么会有所不同?
(我有一个猜测,并会发布答案)
问题2
这是规范或编译器或解释器中的错误吗?
如果有的话,在哪里举报?
版本
"c:\Program Files\Java\jdk1.8.0_65\bin\java.exe" -version
java version "1.8.0_65"
Java(TM) SE Runtime Environment (build 1.8.0_65-b17)
Java HotSpot(TM) 64-Bit Server VM (build 25.65-b01,mixed mode)
"c:\Program Files\Java\jdk1.8.0_65\bin\javac.exe" -version
javac 1.8.0_65
最佳答案
回答问题1
javac编译器在编译期间进行优化.
编译器将此识别为常量表达式:
String lo = "lo";
并得出结论,这也必须是一个常量表达式:
"Hel" + lo
因此假设整个项是interned作为字符串值常量表达式,我们将一个实习字符串与一个相等的实习字符串进行比较.因此,我们比较相同的引用并获得真实,并且可以将完整检查预先评估为真.
证据
>字节代码(javap -c)向我们显示我们有一些预先计算.来自1.,4.和5. print的表达式只是替换为“true”.
>以下代码解除了优化:
资源
public class StringTest {
public static void main(final String[] args) {
final String hello = "Hello",lo = "lo";
String myLo = "";
if (Math.random() < 10) {
myLo = "lo";
}
System.out.print((hello == "Hello") + " ");
System.out.print((Other.hello == hello) + " ");
// System.out.print((other.Other.hello == hello) + " "); // same package
System.out.print((hello == ("Hel" + "lo")) + " ");
System.out.print((hello == ("Hel" + lo)) + " ");
System.out.print((hello == ("Hel" + myLo)) + " ");
System.out.println(hello == ("Hel" + lo).intern());
}
}
输出:
true true true true false true