String intern

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本文通过六个示例说明了Java中字符串常量池的工作原理，包括不同情况下字符串字面量如何引用相同的String对象，以及运行时计算的字符串与通过intern方法获取的字符串之间的区别。

https://javaranch.com/journal/200409/Journal200409.jsp#a1

Example 3.10.5-1. String Literals

The program consisting of the compilation unit (§7.3):

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”; }
and the compilation unit:

package other;
public class Other { public static String hello = “Hello”; }
produces the output:

true true true true false true

This example illustrates six points:

Literal strings within the same class (§8) in the same package (§7) represent references to the same String object (§4.3.1).

Literal strings within different classes in the same package represent references to the same String object.

Literal strings within different classes in different packages likewise represent references to the same String object.

Strings computed by constant expressions (§15.28) are computed at compile time and then treated as if they were literals.

Strings computed by concatenation at run time are newly created and therefore distinct.

The result of explicitly interning a computed string is the same string as any pre-existing literal string with the same contents.