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.
Who it is for
Knows the syntax, not yet the behaviour.
Start here. It assumes you can read the language and nothing else.
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.
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
Primitives and References
Predict which assignments copy a value and which copy an address.
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02
Integer Division and Overflow
Predict the result of integer division, remainder and overflow before running the code.
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03
char Arithmetic and Widening
Predict when a char behaves as a letter and when it behaves as a number.
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04
== and equals on Strings
Decide whether two Strings are the same object or merely the same text.
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05
Boxed Integers and the Cache
Explain why == on two Integers is true for 127 and false for 128.
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06
String Immutability
Predict what a String method returns and why the original never changes.
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07
Arrays, Defaults and Bounds
State an array's size and starting contents without running the code.
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08
The Ternary and Conditions
Evaluate a conditional expression and say which operands are never evaluated.
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09
switch and Fallthrough
Trace a switch statement through its cases and say where execution stops.
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10
What static Actually Means
Decide whether a member belongs to the class or to each object.
A question from this course
Primitives and References
What does this print?
public class Main {
public static void main(String[] args) {
int[] a = {1, 2, 3};
int[] b = a;
b[0] = 99;
System.out.println(a[0]);
}
}Show the answer
A 99
You may have expected 1, reading b = a as making a copy of the array. It copies the reference instead, so a and b are two names for one object and writing through either is visible through the other. Copying an array needs a.clone() or Arrays.copyOf.
The rest of the track
Java at every depth
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.
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.
Start Java Essentials
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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