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Your first Java program
Quiz — print and println
How a Java program runs
Quiz — what javac does
Compile errors
Quiz — a capital letter
The main method
Quiz — why static

Variables
Quiz — a variable with no value
Primitive types
Quiz — a big number
var and final
Quiz — var keeps its type
Default values
Quiz — a field with no value

Arithmetic and integer division
Quiz — 7 / 2 and −7 % 3
Type casting
Quiz — (int) 9.99
Overflow
Quiz — MAX_VALUE + 1
Strings and numbers: + and format
Quiz — the order of string +

Comparisons and logic
Quiz — && stops early
if, else and ?:
Quiz — if without braces
switch and break
Quiz — switch without break
The arrow switch
Quiz — a switch expression without default

while and do-while
Quiz — do-while with a false condition
The for loop
Quiz — i++ and ++i
break and continue
Quiz — continue and break
Nested loops and labels
Quiz — a labeled break

Your own methods
Quiz — return not on every path
Parameters are copies
Quiz — a method changes its parameter
Method overloading
Quiz — which method runs
Recursion
Quiz — a recursive sum

Arrays
Quiz — an index out of bounds
Looping over an array and Arrays
Quiz — writing to the for-each variable
An array is a reference
Quiz — two references to an array
Two-dimensional arrays
Quiz — the sizes of a 2D array

Strings and their methods
Quiz — substring
Strings are immutable
Quiz — toUpperCase without assignment
== and equals for strings
Quiz — new String and ==
StringBuilder
Quiz — append and reverse
The char type
Quiz — 'a' + 'b'
Exercise: run-length encoding

Class and object
Quiz — two variables, one object
Constructors and this
Quiz — new without arguments
Methods and encapsulation
Quiz — access to private
static: shared by all
Quiz — an object counter
toString and null
Quiz — a field of null

Inheritance
Quiz — the order of constructors
Overriding and polymorphism
Quiz — whose method runs
Overloading or overriding
Quiz — overloading by the declared type
abstract and final
Quiz — new on an abstract class
Casting and instanceof
Quiz — a wrong cast

Interfaces
Quiz — an interface method not written
default methods
Quiz — interface or abstract class
Enums
Quiz — ordinal
Records
Quiz — a record and equals

Wrapper classes
Quiz — parseInt("12a")
Autoboxing and unboxing
Quiz — unboxing null
The Integer cache and ==
Quiz — two Integers and ==
equals in your own class
Quiz — equals not overridden

What an exception is
try and catch
Quiz — the order of catch blocks
finally
Quiz — try, catch and finally
Checked and unchecked
Quiz — throws forgotten
throw and your own exceptions
Quiz — throw or throws
try-with-resources
Quiz — the order resources close
Exercise: age check

Generics: List<String>
Quiz — List<int>
ArrayList
Quiz — remove(1) on a List<Integer>
Iterating and immutable lists
Quiz — add to List.of
HashSet, equals and hashCode
Quiz — equals without hashCode
HashMap
Quiz — put with the same key
Comparable and sorting
Quiz — compareTo and sorting
Exercise: word frequency

Lambda expressions
Quiz — a lambda changes a variable
Function, Predicate and method references
Quiz — andThen
Comparator
Quiz — reversed
Stream API: filter, map, collect
Quiz — filter and map
Lazy streams
Quiz — a stream with no terminal operation
Optional
Quiz — orElse

Process and thread
Quiz — process and thread
Starting a thread
Quiz — run instead of start
Data races and synchronized
Quiz — two threads and count++

Final project: a library
Quiz — what the project prints
Where to go next
Quiz — everything together

Courses/Java Core

Quiz · Interfaces, enums and records · lesson 4 of 8

Quiz — interface or abstract class

What can an abstract class have that an interface cannot?

← Backdefault methods Next →Enums

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