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Classes, Desing Patterns, Principles, Generics and Collections

Classes, Desing Patterns, Principles, Generics and Collections

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About these flashcards

This flashcard set delves into advanced Java programming concepts, focusing on collections, methods, and object-oriented principles. It covers topics like enums, generic types, and various collection classes such as ArrayList, Map, and Set. The flashcards explore method signatures, static and instance methods, and the use of interfaces. Programmers looking to master Java's advanced features and best practices will find this set particularly useful for understanding and applying these concepts in their code.

Flashcards

124

Students

2

Language

English

Category

Computer Science

Level

Other

Created / Updated

31.01.2019 / 08.01.2020

In a Comparator or Comparable, a.id - b.id sorts in ___ order and b.id - a.id sorts in ___ order.

In a Comparator or Comparable, a.id - b.id sorts in ascending order and b.id - a.id sorts in descening order.

Do compareTo and equals have to be consistent?

The should be consistend if, and only if, x.equals(y) == true. In this case, x.compareTo(y) should return 0

In which package is Comparable?

java.lang

in which package is Comparator?

java.util

How can you search in a Collection of Objects?

Use Collections.binarySearch. Note that, as with Arrays.binarySearch, the given list must be sorted!

For which classes can TreeSet be used?

You can add only objects that implement the Comparable interface!

How can you build a Comparator over multiple fields?

Uing the Comparator static helper fuctions, eg:

Comparator<Squirrel> c = Comparator.comparing(s -> s.getSpices()).thenComparingInts(s -> s.getWeight));

What kind of Method-References exist?

Static Methods
Instance Methods on a particular instance
Instance methods on an instance to be determined at runtime
Constructors

What does a static method reference to Collections.sort look like?

Consumer<List<Integer>> methodRef1 = Colelctions::sort;

Consumer<List<Integer>> asLambda = l -> Colelctions.sort(l);

What does a method reference to the instance method str.startsWith look like?

Predicate<String> methodRef = str::startsWith;

Predicate<String> asLambda = s -> str.startsWith(s);

What does a method reference to the instance method isEmpty of a yet unknown instance look like?

Predicate<String> methodRef = String::isEmpty;

Predicate<String> asLambda = s -> s.isEmpty();

What does a method reference to the the constructor of ArrayList look like?

Supplier<ArrayList> constRef = ArrayList::new

Supplier<ArrayList> constRef = () -> new ArrayList();

what is the signature of removeIf? How does it behave?

boolean removeIf(Predicate<? super E> filter)

Iterates through the list and applies the Predicate to each element. If the Predicate returns true, it will be removed.

returns true if any elements were removed

what is the signature of replaceAll? How does it behave?

void replaceAll(UnaryOperator<E> o)

Calls the UnaryOperator for every Element and replaces the value with the returned value

what is the signature of forEach?

void forEach(Consumer<? super E> action)

What are new methods on Map in Java8?

putIfAbsent
merge
computeIfPresent
computeIfAbsent

What is the signature of Map.merge?

V merge(K key, V value, BiFunction<? super V,? super V,? extends V> remappingFunction)

What does the Map.merge method do?

merge(key, value,function)

If the the value for the given key is null or not present, insert the provided value.
Otherwise, replace the value for the given key with the result of function
- function takes the value from the map as the first argument and the new value as the second one.

Map<String, String> leMap = new HashMap<String, String>();

String result;
result = leMap.merge("key", "v1", (k1, k2) -> k1 + k2);
result = leMap.merge("key", "v2", (k1, k2) -> k1 + k2);
result = leMap.merge("key", "v3", (k1, k2) -> k1 + k2);

System.out.println(result);

When defining an enum, where do the static method values() and valueOf() come from?

The compiler generates it on the enum class

When defining an enum, where do the methods name() and ordinal() come from?

All enums implicitly extend the enum in which these methods are defined

What is the Result of `null instanceOf X` ?

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False, an instanceOf check with null always results in false

When are instanceOf checks executed?

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For Classes: At Compiletime
For Interfaces: At Runtime

What is virtual method invocation?

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It basically means that Jva will look at subclasses when finding the right method to call (override)

What's the goal of @override?

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It is helpful because the compiler can tell you when you've messd up

What does the equals signature look like?

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public boolean equals(Object obj)

What is the equals contract?

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"Should not change at random"
- reflexive: x.equals(x) == true
- symmetric: x.equals(y) == y.equals(x)
- transitive: x.equals(y) == y.equals(z) == z.equals(x)
- consistent: x.equals(y) == x.equals(y) (does not change when x and y are not modified)
An object an null are not equal
- x.equals(null) must return false

What is the contract of hashCode?

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The result must not change -> derive it from members that do not change (eg. an id)
The hasCode() can use a subset of equals
- two equal objects must return the same hashcode
the hash code can be a constant, altough it is not efficient

What does the hashCode signature look like?

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public int hashCode()

How is an enum declared?

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By using the enum keyword instead of the class keyword

Are Enum values comparable with ==

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yes, think of them as static final constants

What Methods does an enum provide?

Static:
- MyEnum.values(): Returns an array containing the constants of this enum
- MyEnum.valueOf("string"): Create an enum from a string
Instance:
- MyEnum.FIRST.name(): Returns the name of this enum constant, exactly as declared in its enum declaration
- MyEnum.FIRST.ordinal(): ordinal of this enumeration constant

What happens when an illegal string value is providee to the Enum valueOf method?

An IllegalArgumentException is Thrown

Can a class or enum extend an enum?

No

What needs special attention when implementing a switch statement with an enum

The enum type is implied in the case statements and it is not allowed to type it!

switch(myEnum){
case ONE: // NOT MyEnum.ONE
// ...
}

Can a constructor of an enum be public?

No, it wont' compile!

What is the output of this program?

public class Main {

enum LeEnum {
    ONE, TWO, THREE;
    LeEnum(){
      System.out.println(this.name());
    }
}

public static void main(String... args) {
    System.out.println("a");
    LeEnum one = LeEnum.ONE;
    System.out.println("b");
}
}

the first time that we ask for any of the enum values, Java constructs all of the enum values

a
ONE
TWO
THREE
b

Can an enum declare an abstract method?

Yes, but each value must provide an implementation:

public enum Season {
Winter {
pubic void print(){}
}
pubic abstract void print(){}
}

What is a Nested Class? What kind of nested classes do exist?

A class that is defined within another class, incluiding:

Member inner class
local inner class
anonymous inner class
nested static class

What is an inner class?

A netsted class that is not static

Member inner class
local inner class
anonymous inner class

Where is a member inner class declared?

At the member level.

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Flashcards

What is the signature of Map.merge?

V merge(K key, V value, BiFunction<? super V,? super V,? extends V> remappingFunction)

What does the Map.merge method do?

merge(key, value,function)

If the the value for the given key is null or not present, insert the provided value.
Otherwise, replace the value for the given key with the result of function
- function takes the value from the map as the first argument and the new value as the second one.

Map<String, String> leMap = new HashMap<String, String>();

String result;
result = leMap.merge("key", "v1", (k1, k2) -> k1 + k2);
result = leMap.merge("key", "v2", (k1, k2) -> k1 + k2);
result = leMap.merge("key", "v3", (k1, k2) -> k1 + k2);

System.out.println(result);

When defining an enum, where do the static method values() and valueOf() come from?

The compiler generates it on the enum class

When defining an enum, where do the methods name() and ordinal() come from?

All enums implicitly extend the enum in which these methods are defined

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