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Multiple if statements with single else statement
(2 answers)
Closed 6 years ago.
I want to fill the acts array with values of some enum. While iterating I want to input commands from console, but my if statements don't find any match and I always get the output "Incorrect".
My code:
Action[] acts = new Action[n];
for(int i = 0; i < n; i++) {
System.out.println("Enter act: ");
Scanner in1 = new Scanner(System.in);
String s = in1.next();
acts[i] = new Action();
if (s.equals("rotate_forw"))
acts[i].type = ActionType.RotF;
if (s.equals("rotate_back"))
acts[i].type = ActionType.RotB;
if (s.equals("shift_forw"))
acts[i].type = ActionType.ShiftF;
if (s.equals("shift_back"))
acts[i].type = ActionType.ShiftB;
else
System.out.println("Incorrect");
}
Your else clause applies only to the last if statement, so you get the "Incorrect" output whenever s.equals("shift_back") is false.
Your statements should be replaced with a single if-else-if...-else statement, so that "Incorrect" is only printed if all the conditions are false :
Action[] acts = new Action[n];
for(int i = 0; i < n; i++) {
if (s.equals("rotate_forw"))
acts[i].type = ActionType.RotF;
else if (s.equals("rotate_back"))
acts[i].type = ActionType.RotB;
else if (s.equals("shift_forw"))
acts[i].type = ActionType.ShiftF;
else if (s.equals("shift_back"))
acts[i].type = ActionType.ShiftB;
else
System.out.println("Incorrect");
}
You should also consider what you want to assign to acts[i].type when the input is incorrect. Perhaps you should throw an exception in this case.
While #Eran's answer is correct, I'd like to suggest a different approach that encapsulates the enum with the translation from the external coding. Consider this:
public class EnumDemo
{
public static enum ActionType
{
Incorrect(""),
RotF("rotate_forw"),
RotB("rotate_back"),
ShiftF("shift_forw"),
ShiftB("shift_back");
private String code;
private ActionType(String code)
{
this.code = code;
}
public static ActionType fromString(String code)
{
return Arrays.stream(ActionType.values())
.filter(v->v.code.equals(code))
.findFirst()
.orElse(ActionType.Incorrect);
}
}
public static void main(String[] args)
{
String[] testData = {
"rotate_forw",
"rotate_back",
"shift_forw",
"shift_back",
"junk",
null };
Arrays.stream(testData)
.forEach(t->System.out.printf("\"%s\" -> ActionType.%s\n", t, ActionType.fromString(t)));
}
}
This uses the fact that enum constants can have associated data. I've added an instance variable code to hold the external encoding of each enum value. Then I added a static fromString(String code) method to the enum that looks up the provided code in the list of values. For 4 possibilities a simple linear search, equivalent to your if-then-else cascade, works fine. If there were dozens or more I'd set up a Map<String,ActionType> to handle the conversion.
The search using streams bears some explanation.
First create a Stream of enum values
Filter it to contain only enum values whose code matches the desired code (there should be only one)
Pick off the first entry, which comes back in a Optional. If nothing was found (i.e. the code is invalid) the Optional will be empty.
Use the orElse method to return the value if it exists or ActionType.Incorrect if not.
At first glance this might look inefficient since one expects that the filter() predicate has to scan the entire stream even if the desired element occurs early. This is a really nifty feature of Streams -- all intermediate streams are "lazy", so the filter won't iterate over the entire list if it finds the desired entry early. See this question for details.
Output:
"rotate_forw" -> ActionType.RotF
"rotate_back" -> ActionType.RotB
"shift_forw" -> ActionType.ShiftF
"shift_back" -> ActionType.ShiftB
"junk" -> ActionType.Incorrect
"null" -> ActionType.Incorrect
The last testcase shows the code is null-safe.
The biggest advantage is that the mapping is in the same place as the enum itself, so you won't have to hunt for the code when you add or remove an enum value. Also you can't forget to define the mapping since it's required by the enum's constructor.
Related
I couldn't wrap my head around writing the below condition using Java Streams. Let's assume that I have a list of elements from the periodic table. I've to write a method that returns a String by checking whether the list has Silicon or Radium or Both. If it has only Silicon, method has to return Silicon. If it has only Radium, method has to return Radium. If it has both, method has to return Both. If none of them are available, method returns "" (default value).
Currently, the code that I've written is below.
String resolve(List<Element> elements) {
AtomicReference<String> value = new AtomicReference<>("");
elements.stream()
.map(Element::getName)
.forEach(name -> {
if (name.equalsIgnoreCase("RADIUM")) {
if (value.get().equals("")) {
value.set("RADIUM");
} else {
value.set("BOTH");
}
} else if (name.equalsIgnoreCase("SILICON")) {
if (value.get().equals("")) {
value.set("SILICON");
} else {
value.set("BOTH");
}
}
});
return value.get();
}
I understand the code looks messier and looks more imperative than functional. But I don't know how to write it in a better manner using streams. I've also considered the possibility of going through the list couple of times to filter elements Silicon and Radium and finalizing based on that. But it doesn't seem efficient going through a list twice.
NOTE : I also understand that this could be written in an imperative manner rather than complicating with streams and atomic variables. I just want to know how to write the same logic using streams.
Please share your suggestions on better ways to achieve the same goal using Java Streams.
It could be done with Stream IPA in a single statement and without multiline lambdas, nested conditions and impure function that changes the state outside the lambda.
My approach is to introduce an enum which elements correspond to all possible outcomes with its constants EMPTY, SILICON, RADIUM, BOTH.
All the return values apart from empty string can be obtained by invoking the method name() derived from the java.lang.Enum. And only to caver the case with empty string, I've added getName() method.
Note that since Java 16 enums can be declared locally inside a method.
The logic of the stream pipeline is the following:
stream elements turns into a stream of string;
gets filtered and transformed into a stream of enum constants;
reduction is done on the enum members;
optional of enum turs into an optional of string.
Implementation can look like this:
public static String resolve(List<Element> elements) {
return elements.stream()
.map(Element::getName)
.map(String::toUpperCase)
.filter(str -> str.equals("SILICON") || str.equals("RADIUM"))
.map(Elements::valueOf)
.reduce((result, next) -> result == Elements.BOTH || result != next ? Elements.BOTH : next)
.map(Elements::getName)
.orElse("");
}
enum
enum Elements {EMPTY, SILICON, RADIUM, BOTH;
String getName() {
return this == EMPTY ? "" : name(); // note name() declared in the java.lang.Enum as final and can't be overridden
}
}
main
public static void main(String[] args) {
System.out.println(resolve(List.of(new Element("Silicon"), new Element("Lithium"))));
System.out.println(resolve(List.of(new Element("Silicon"), new Element("Radium"))));
System.out.println(resolve(List.of(new Element("Ferrum"), new Element("Oxygen"), new Element("Aurum")))
.isEmpty() + " - no target elements"); // output is an empty string
}
output
SILICON
BOTH
true - no target elements
Note:
Although with streams you can produce the result in O(n) time iterative approach might be better for this task. Think about it this way: if you have a list of 10.000 elements in the list and it starts with "SILICON" and "RADIUM". You could easily break the loop and return "BOTH".
Stateful operations in the streams has to be avoided according to the documentation, also to understand why javadoc warns against stateful streams you might take a look at this question. If you want to play around with AtomicReference it's totally fine, just keep in mind that this approach is not considered to be good practice.
I guess if I had implemented such a method with streams, the overall logic would be the same as above, but without utilizing an enum. Since only a single object is needed it's a reduction, so I'll apply reduce() on a stream of strings, extract the reduction logic with all the conditions to a separate method. Normally, lambdas have to be well-readable one-liners.
Collect the strings to a unique set. Then check containment in constant time.
Set<String> names = elements.stream().map(Element::getName).map(String::toLowerCase).collect(toSet());
boolean hasSilicon = names.contains("silicon");
boolean hasRadium = names.contains("radium");
String result = "";
if (hasSilicon && hasRadium) {
result = "BOTH";
} else if (hasSilicon) {
result = "SILICON";
} else if (hasRadium) {
result = "RADIUM";
}
return result;
i have used predicate in filter to for radium and silicon and using the resulted set i am printing the result.
import java.util.ArrayList;
import java.util.List;
import java.util.Set;
import java.util.stream.Collectors;
public class Test {
public static void main(String[] args) {
List<Element> elementss = new ArrayList<>();
Set<String> stringSet = elementss.stream().map(e -> e.getName())
.filter(string -> (string.equals("Radium") || string.equals("Silicon")))
.collect(Collectors.toSet());
if(stringSet.size()==2){
System.out.println("both");
}else if(stringSet.size()==1){
System.out.println(stringSet);
}else{
System.out.println(" ");
}
}
}
You could save a few lines if you use regex, but I doubt if it is better than the other answers:
String resolve(List<Element> elements) {
String result = elements.stream()
.map(Element::getName)
.map(String::toUpperCase)
.filter(str -> str.matches("RADIUM|SILICON"))
.sorted()
.collect(Collectors.joining());
return result.matches("RADIUMSILICON") ? "BOTH" : result;
}
I have a java class with 3 boolean property like this
boolean isActive;
boolean isEnable;
boolean isNew;
every property is related to an enum (e.g. ACTIVE,ENABLE,NEW).
I want to have 2 lists of enum. One which has only the enums related to true property value and one for the false one.
just to be clear. using if-else statement I could have
Set<FlagEnum> flagSet = new HashSet<>();
Set<FlagEnum> falseFlagSet = new HashSet<>();
if (object.isActive()) {
flagSet.add(ACTIVE);
} else {
falseFlagSet.add(ACTIVE);
}
if (object.isEnable()) {
flagSet.add(ENABLE);
} else {
falseFlagSet.add(ENABLE);
}
if (object.isNew()) {
flagSet.add(NEW);
} else {
falseFlagSet.add(NEW);
}
is there a way to avoid all these if-else?
I tried with something like
Map<boolean, List<Pair<boolean, FlagEnum>>> res = Stream.of(
new Pair<>(object.isActive(), ACTIVE),
new Pair<>(object.isNew(), NEW),
new Pair<>(object.isEnable(), ENABLE))
.collect(Collectors.partitioningBy(Pair::getKey));
but the resulted structure is an additional complexity which I would like to avoid.
In my real case, I have more than 15 boolean properties...
You can simplify this in various ways. Which of them make sense, depends on your exact requirements.
You can derive the falseFlagSet trivially from the flagSet using EnumSet.complementOf after populating the flagSet:
EnumSet<FlagEnum> falseFlagSet = EnumSet.complementOf(flagSet);
This assumes that all FlagEnum values have corresponding flags. If that's not the case then you need to construct a EnumSet with all enums that have flags and subtract flagSet from that using removeAll.
#1 already removes the need for the else in your cascade, simplifying the code to
if (object.isActive()) {
flagSet.add(ACTIVE);
}
if (object.isEnable()) {
flagSet.add(ENABLE);
}
if (object.isNew()) {
flagSet.add(NEW);
}
If you have enough different flags, then you can create a mapping from getter method to FlagEnum value like this:
Map<Function<YourClass,Boolean>,FlagEnum> GETTERS = Map.of(
YourClass::isActive, FlagEnum.ACTIVE,
YourClass::isNew, FlagEnum.NEW,
YourClass::isEnable, FlagEnum.ENABLE);
Then you can use this to make the whole process data-driven:
EnumSet<FlagEnum> getFlagSet(YourClass yourObject) {
EnumSet<FlagEnum> result = EnumSet.noneOf(FlagEnum.class);
for (Map.Entry<Function<YourClass,Boolean>, FlagEnum> getter : GETTERS.entrySet()) {
if (getter.getKey().apply(yourObject)) {
result.add(getter.getValue());
}
}
return result;
}
If the number of flags is very big, then you could switch entirely to reflection and detect the flags and matching getters dynamically using string comparison, but I would not suggest that approach. If you need something like that then you probably should switch to a framework that supports that kind of feature and not implement it yourself.
That last two obviously only makes sense when the number of flags is big. If it's actually just 3 flags, then I wouldn't mind and just have 3 simple if statements.
As a slight tangent: GETTERS above should definitely be an immutable map (wrap it in Collections.unmodifiableMap or use something like Guava ImmutableMap) and it could be argued that the same applies to the return value of the getFlagSet method. I've left those out for succinctness.
You can use a private helper method for this.
private void addFlagSet(boolean condition, FlagEnum flagEnum,
Set<FlagEnum> flagSet, Set<FlagEnum> falseFlagSet) {
Set<FlagEnum> chosenFlagSet = condition ? flagSet: falseFlagSet;
chosenFlagSet.add(flagEnum);
}
Call it as:
addFlagSet(object.isActive(), FlagEnum.ACIVE, flagSet, falseFlagSet);
addFlagSet(object.isNew(), FlagEnum.NEW, flagSet, falseFlagSet);
addFlagSet(object.isEnable(), FlagEnum.ENABLE, flagSet, falseFlagSet);
You could probably use Reflection to get all methods, then check if a getReturnType() == boolean.class. Problem is the connection between the method's name and the enum. If every single one is named like the method without the 'is', you could use FlagEnum.valueOf() to retrieve the enum value from the method name and use it.
I think this could be the easiest and clearest way to do what I need
Map<Boolean, Set<FlagEnum>> flagMap = new HashMap<>();
flagMap.computeIfAbsent(object.isActive(), h -> new HashSet()).add(ACTIVE);
flagMap.computeIfAbsent(object.isEnabled(), h -> new HashSet()).add(ENABLE);
flagMap.computeIfAbsent(object.isNew(), h -> new HashSet()).add(NEW);
//to get TRUE set simply :
flagMap.get(true);
what do you think?
I have a homework to do in java about ArrayList and Generics types.
I have 2 classes :
-> CoupeDeA
-> TableauPartiel
CoupeDeA is just a describer from where to where an array is cut.
(It contains only two private integer variables "begin" and "end")
TableauPartiel is the class where the ArrayList is.
My problem is I need to create a method in TableauPartiel like this :
public TableauPartiel<E> coupe(CoupeDeA coupe)
And the TableauPartiel returned needs to be a reference of my intial TableauPartiel. Example :
Integer[] arr = {8,7,6,5};
TableauPartiel<E> tab = new TableauPartiel<>(arr);
TableauPartiel<E> tab2 = tab.coupe(1,3);
tab2.set(1,45);
This code is supposed to set 45 at index 1 of my tab2 and at the same time set 45 at index 2.
But I tried many different ways and I managed to get the sublist, but it is not pointing to my original ArrayList.
For example, I tried something like this :
private ArrayList<E> tableau;
...
public TableauPartiel<E> coupe(Coupe coupe)
throws IndexOutOfBoundsException {
if (coupe.getBegin() >= 0 && coupe.getEnd() <= tableau.size()) {
TableauPartiel<E> tab = new TableauPartiel<>((E[]) new Object[coupe.getEnd()-coupe.getBegin()]);
for (int i = 0; i < coupe.getEnd()-coupe.getBegin(); ++i) {
tab.set(i, this.get(coupe.getBegin()+i));
}
return tab;
} else {
throw new IndexOutOfBoundsException();
}
}
How can I do to get a sublist which refers to his original ArrayList?
I've found a solution for my code with the subList method and by switching the signature of my ArrayList to List but my teacher doesn't want us to use subList finally.
Here is my code with the subList method :
TableauPartiel<E> tab;
if (coupe.getDebut() >= 0 && coupe.getFin() <= taille()) {
if (coupe.getFin() == -1)
tab = new TableauPartiel<>(tableau.subList(coupe.getDebut(),taille()));
else
tab = new TableauPartiel<>(tableau.subList(coupe.getDebut(),coupe.getFin()));
return tab;
} else {
throw new IndexOutOfBoundsException();
}
}
Few small things first:
stick to English words in your code. Especially in names of classes, functions, variables, etc - names have to reveal intentions (without Google Translate). Best not to obtain a bad habit by letting yourself do otherwise.
I am not so sure how your Coupe is expected to work (is 0 a legal min number or 1?) but coupe.getEnd() <= tableau.size() might get out of hand
Now my suggestion for the solution:
I suggest you modify your TableauPartiel class to have start and end integer fields in addition to private ArrayList<E> tableau; reference you already have. Maybe add a new 'copy constructor' accepting an instance of
TableauPartiel (from which you can copy reference to tableau) and two int values indicating which part of the original tableau you can use (trick here is to also look at start and end values of the object you're 'sublisting' from). That way, when you're calling #coupe you can check for validity of the input numbers (as you already do) and simply return a new TableauPartiel object with a reference to this and method params - start and end values. Add some indexes manipulation logic using those start and end to whatever methods your TableauPartiel has and you should be good to go.
I'm iterating a set object to find a particular value. Is there any short way to fetch instead of iterating it? Here is my code
for(Tree t : assignedTrees) {
println t.treeName;
}
The above code will return the expected value.
assignedTrees is the set object
Set<Tree> assignedTrees = new HashSet<Tree>()
println assignedTrees will return
[Tree{id=null, treeName=Mango}]
Can I fetch the treeName instead of iterating?
You can fetch an object from a set by calling mySet.get(object). However, in your case you wish to fetch an object based on one of its attributes. The best way to do this is with a map - e.g.
Map<String, Tree> trees = new HashMap<>();
trees.put(treeObject.treeName, treeObject);
Tree myTree = trees.get("myTreeName");
Note that if you're putting your own objects into sets or maps, you must override the equals and hashcode methods, or very strange things will happen.
In general you can use lambda to find any/first element that fullfils any condition. For example:
Set<Integer> coolStrings = new HashSet<String>();
coolStrings.add("HEHE")
coolStrings.add("NOPE")
coolStrings.add("JP2GMD")
coolStrings.add("1234")
try{
String nice =
coolStrings.stream().filter(
(str) -> { return str.equals("JP2GMD") ||
str.equals("2137"); }
}).findFirst().get();
)
System.out.println("Yay, i found a REALLY cool string! : " + nice);
}
catch(NoSuchElementException e){
System.out.println("Not even one awesome string was found :(");
}
It will print "JP2GMD"
(I didn't compile it, there might be some minor syntax errors)
Working with Stream class is extremally handy (as for java standards)
Current assignment needs me to write a program to read a file with instructions in a very tiny and basic programming language (behaves a little like FORTRAN) and execute those instructions. Basically it's a simple interpreter for the language I guess. It's completely linear, with statements all being defined in sequence and it only has String and integer variables. There are 8 keywords and 4 arithmetic operators I would need to find and define if they exist within the source file, and each line must start off with one of the reserved words.
A program in this language might look something like this:
#COMMENTS
LET.... (declares variables with values)
INTEGER myINT
STRING myString
CALCULATE...
PRINT
PRINTLN
END
Can I use a switch block instead of if-loops to find and then execute all these? My concern is that switches don't work with Strings in Java 6, which is what I'm supposed to be using, but I don't see how to easily assign various int values so the switch block would work. Thanks in advance for any suggestions and advice!
If your language is so simple that every statement begins in its own line and is identified by one word only, then (as Gray pointed out in another comment) you can split the words in each line, then compare the first word against a map. However, I would suggest, instead of mapping the words to ints and then doing one big switch, to map them into objects instead, like this (suggested by Dave Newton):
interface Directive {
public void execute(String line);
}
class LetDirective implements Directive {
public void execute(String line) { ...handle LET directive here... }
}
...define other directives in the same way...
Then define the map:
private Map<String, Directive> directives = new HashMap<String, Directive>();
directives.put("LET", new LetDirective());
...
Then in your parsing method:
int firstSpace = line.indexOf(' ');
String command = line;
if (firstSpace > 0)
command = line.substring(0, firstSpace);
Directive directive = directives.get(command.toUpperCase());
if (directive != null)
directive.execute(line);
else
...show some error...
Each directive would have to parse the rest of the line on its own and handle it correctly inside its execute() method.
The benefit of this over a switch is that you can handle a larger amount of commands without ending up with one gigantic method, but instead with one smaller method per each command.
If you are talking about converting strings to integers then you could do it with an Java enumerated type:
private enum ReservedWord {
LET,
...
}
// skip blank lines and comments
String[] tokens = codeLine.split(" ");
ReservedWord keyword;
try {
keyword = ReservedWord.valueOf(tokens[0]);
} catch (IllegalArgumentException e) {
// spit out nice syntax error message
}
You could also put the processing of the line inside of the enum as a method if you'd like. You could also do it with a Map:
private final Map<String, Integer> reservedWords = new HashMap<String, Integer>();
private final int RESERVED_WORD_LET 1
...
{
reservedWords.put("LET", RESERVED_WORD_LET);
...
}
// skip blank lines and comments
String[] tokens = codeLine.split(" ");
Integer value = reservedWords.get(tokens[0]);
if (value == null) // handle error... ;
switch (value) {
case 1:
// LET
...
}