Module 1 · Java from zero Lesson 3 of 15
Strings and StringBuilder
A String is immutable: every "change" builds a new object. That single fact is behind half the Java questions in a fresher interview.
The concept
A String in Java is immutable: every method that appears to change it returns a NEW string. That makes strings safe to pass around and it also means concatenating inside a loop builds a new string each round, which is how a fresher program becomes slow on a real file.
Use StringBuilder when you are assembling text in a loop — .append() then .toString() — and the built-in String methods the rest of the time: length, charAt, substring, indexOf, split, replace, trim, toUpperCase, and String.format or %s for output. equalsIgnoreCase is what you want for comparing user input.
String s = "Hello";
System.out.println(s.length()); // 5
System.out.println(s.toUpperCase()); // HELLO
System.out.println(s.substring(1, 3)); // el
System.out.println(s.contains("ell")); // true
// Building many Strings in a loop? StringBuilder avoids the copies.
StringBuilder sb = new StringBuilder();
for (int i = 0; i < 3; i++) sb.append(i).append("-");
System.out.println(sb.toString());Common mistakes
The mistake is == for string comparison, and the second is string concatenation inside a loop over ten thousand rows. Both are common enough that interviewers ask about them by name, so learn the reason rather than the rule.
Worked example
Concatenating in a loop of 10000 rows creates 10000 discarded Strings; StringBuilder appends into one buffer and is the version to use. equals() compares content, == compares references — a distinction that shows up in every written test.
Your practice task
Reverse a sentence, count the words and replace one word — first with String methods, then with StringBuilder, and note the difference in readability.
Do this before moving on. Reading a solution teaches the shape of it; typing it teaches you.
Key takeaway
A String in Java is immutable: every method that appears to change it returns a NEW string.
Keep your place in this course
A free account saves which lessons you have finished and unlocks the certificate. The lesson itself stays free to read.
Create free account or sign inCourse complete
You finished every lesson.
Your certificate is ready, and the projects you built are the proof to attach to an application.
Almost there
15 lessons left of 15
Finish every lesson to unlock the certificate. Each one ends in a practice task — do them and the final project brief writes itself.