String and == Puzzles

35 min · Intermediate

Back to syllabus (Output Prediction & Debugging Rounds)

== on Strings asks "is this the same object?", while equals asks "is this the same text?". String puzzles are built to make those two answers differ, using the string pool, compile-time constants, immutable methods and wrapper caches.

Topics in this lesson

Compile-time constant folding vs runtime concatenation
final String variables and the pool
replace, substring, toUpperCase without assignment
Wrapper == puzzles mixed with Strings
A checklist for any string puzzle

Interview questions this lesson answers

For String a = "Hi"; String b = new String("Hi");, what do a == b and a.equals(b) give?
For String c = "Hi"; String d = "Hi";, what does c == d give, and why?
Can new String("Hi") == new String("Hi") ever be true?
What does System.out.println("Ja" + "va" == "Java") print?
With String s = "Ja";, what does s + "va" == "Java" give?
With final String s = "Ja";, what does s + "va" == "Java" give?
Is every final String a compile-time constant?
How many String objects does String s = "a" + "b" + "c"; create at run time?
What does ("Ja" + new String("va")).intern() == "Java" give?
What does String s = "Hi"; String t = s; s += "!"; System.out.println(t); print?

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