Java Strings Interview Questions

Strings appear in almost every Java interview, both as theory and as live coding. This topic covers what interviewers ask beyond the basics: how many objects new String creates, intern(), why + is slow inside loops, null-safe conversion with String.valueOf, isEmpty versus isBlank, the traps of split(), why String makes a good HashMap key and why passwords belong in a char[].

It also explains how switch on Strings works internally, Compact Strings in Java 9 and the old substring memory leak, and finishes with popular coding problems: anagrams, reversing the words of a sentence, string rotation and the longest common prefix. Every answer includes a complete program with its real output and a ready-to-use interview answer.

• How many objects are created by String s = new String("hello")?
• What does String.intern() do, and when should you use it?
• Why is using + to concatenate Strings in a loop slow, and what should you use instead?
• What is the difference between String.valueOf(), toString() and concatenating with "" when converting to a String?
• What is the difference between isEmpty() and isBlank(), and how do you safely check for null or empty Strings?
• What are the common pitfalls of String.split() in Java?
• Why is String a good choice for a HashMap key?
• Why is a char[] preferred over a String for storing passwords?
• How does a switch statement on Strings work internally?
• What are Compact Strings in Java 9?
• Why could substring() cause a memory leak in older Java versions, and is it still a problem?
• How do you check whether two strings are anagrams in Java?
• How do you reverse the order of words in a sentence in Java?
• How do you check whether one string is a rotation of another?
• How do you find the longest common prefix of an array of strings?