Java Interview Questions
The Java an interviewer actually probes, answered from the rules rather than from memory. The equals and hashCode contract, the string pool, references and boxing, collections, erasure, checked exceptions, dispatch, immutability, and the what-does-this-print questions behind them.
Who it is for
Preparing, and short of time.
For someone confident in the language who wants the traps, the edge cases and the interview questions.
You will get the most out of it if you have written Java recently enough to read it without effort, and have been surprised by it at least once. If you are picking the language up for the first time, start with Java Essentials and come back to this one.
It is also built for interview preparation: the questions are the ones that get asked because they separate people who have read the documentation from people who have hit the behaviour in production.
How long it takes
90 graded questions at roughly 45 seconds each is about 68 minutes of answering in total.
At the default five questions a day, and because the session mixes in your other courses, expect around 18 days — call it 3 weeks — to see everything once, plus a few weeks of thinning reviews after that before it is all mastered.
A question is mastered on the third correct answer, once the gap between reviews has grown past a week — so a course cannot be finished in a weekend however long you sit with it.
Question mix
- 60 multiple choice
- 10 concept cards
- 10 predict the output
- 10 put it in order
- 10 matching questions
Syllabus
What you will be asked about
-
01
The equals and hashCode Contract
State the contract and show what a hash-based collection does when it is broken.
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02
String Immutability and the Constant Pool
Predict when two strings are the same object and when they only look alike.
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03
== , equals and What Gets Passed
Say what a variable holds and what a method receives.
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04
Which Collection, and Why
Pick a collection from what the code needs, and know how each one bites.
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05
Generics, Erasure and What Survives
Say what the compiler checked and what is left at run time.
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06
Checked, Unchecked and the Exception Hierarchy
Say which exceptions the compiler polices and what try-with-resources does with them.
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07
Dispatch: What Is Decided When
Say which calls the compiler resolves and which the JVM does.
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08
The Four Pillars, Asked Properly
Answer the pillars question with what the compiler and the JVM actually do.
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09
Immutability and Defensive Copying
Find the reference that escapes and close it.
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10
What Does This Print, and Why
Work out the answer from the rules rather than from what the code looks like.
8 more topics, and all 90 questions
Java Interview Questions is a Pro course. One subscription opens it and every other course in the catalogue, including the ones still being written.
A question from this course
The equals and hashCode Contract
What does this print?
public class Main {
public static void main(String[] a) {
System.out.println(
"Aa".hashCode() == "BB".hashCode());
System.out.println("Aa".equals("BB"));
}
}Show the answer
D true false
String.hashCode is specified as a sum of s[i] * 31^(n-1-i), and that formula sends Aa and BB to the same number. The contract only runs one way: equal objects must agree on the hash, but a shared hash says nothing about equality. A hashCode that returns the same value for unequal objects is correct and slow, never wrong.
The rest of the track
Java at every depth
Java Essentials
The behaviour underneath the ceremony: what a variable really holds, why 1/2 is 0, and why two Strings that look identical are not always the same object. Ten spines on the code that compiles cleanly and still does something else.
Java Core
Where Java trips developers who use it every day: autoboxing that hides a NullPointerException, a ternary that quietly unboxes, and dispatch rules that pick a method before the program runs. Ten spines on the gap between what the code says and what the JVM does.
Java Data & Collections
The collections framework taken seriously: which container promises what, why remove(1) deletes a position, and how a mutated key disappears from its own HashMap. Ten spines on the containers you use every day and the contracts underneath them.
Java Errors & Edge Cases
Exceptions at the points where they stop behaving the way you assumed: a finally block that discards the exception unwinding through it, a map lookup that throws on the way into an int, a class that fails once and reports a missing definition forever after. Ten spines on the paths your happy-path code never takes.
Java Standard Library
The library people reimplement by hand: what split really returns, StringBuilder, Optional, streams and collectors, comparators, java.time, the Objects helpers and text blocks. Ten built-ins that already do the job, and one behaviour in each that is not the one you assumed.
Java Idioms & Pitfalls
The Java people write out of habit that is wrong or dated: raw types, boxed equality, catching Exception, building strings in a loop, arrays where a List belongs and getters that hand out the field. Ten habits, each one run to see what it actually does.
Java Advanced
The traps a confident Java reader still walks into: erasure, wildcard variance, dispatch decided before the program runs, and a constructor that sees a field before it is assigned. Ten spines of code that compiles cleanly and behaves otherwise.
Java Performance
What actually costs on the JVM, read off code rather than a stopwatch: the allocation that never happens, the concat loop that copies everything twice, the map that rehashes sixteen times on its way to a million entries. Ten spines on the difference between the operation you wrote and the work the VM does.
Java Testing
What a Java test actually asserts, and where that claim is weaker than it looks. Argument order and failure messages, the three kinds of equality, deltas, throwing, fixtures and static state, test doubles, and the difference between a failure and an error.
Start Java Interview Questions
in tomorrow’s session.
Enrol and this course joins your daily five, mixed in with everything else you are studying — 90 questions spread over about 18 days rather than dropped on you at once.
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