Why is this string on the heap and not in the string constant pool in java

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Hello everyone,
Please the code below returns false meaning that the 2 String references below are pointing to 2 different objects in memory.

String str1 = "GoodMorning";
String str2 = "Good";
String str3 = "Morning";
str2 += str3; // Line 4
System.out.println("str2 == str1"); // output is false.

Question - Why does java saves Line 4 on the heap and not in the String Constant Pool(SCP) when concatenating two String literals that are in the SCP ?
Thanks for any help in advance.

What I have tried:

System.out.println("str2 == str1"); // output is false.

Posted 17-Feb-23 18:08pm

UT7

Updated 17-Feb-23 20:12pm

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OriginalGriff · Accepted Answer · 2023-02-17T20:12:00

Simple: they are separate strings.
Strings str1 and str3 are "pointing at" constants because they are using literal strings whose value is absolutely known at compile time. These are allocated in the string constant pool.

And remember, string are immutable: once created, they cannot be changed.

str2 is originally set to a literal in the pool, but you then concatenate two strings together: this means a new string is created on the heap, and both the original strings are copied into it. It is not a literal anymore, it is a variable whose value is set at runtime.

Strings whose values are only known at run time are created on the heap, not the pool.