问题
I have a number of functions:
String first(){}
String second(){}
...
String default(){}
Each can return a null value, except the default. each function can take different parameters. For example, first could take no arguments, second could take in a String, third could take three arguments, etc. What I'd like to do is something like:
ObjectUtils.firstNonNull(first(), second(), ..., default());
The problem is that because of the function call, this does eager evaluation. Where'd I'd like to exit early, say after the second function (because the function calls can be expensive, think API calls, etc). In other languages, you can do something similar to this:
return first() || second() || ... || default()
In Java, I know I can do something like:
String value;
if (value = first()) == null || (value = second()) == null ...
return value;
That's not very readable IMO because of all the == null checks.ObjectUtils.firstNonNull() creates a collection first, and then iterates, which is okay as long as the function gets evaluated lazily.
Suggestions? (besides doing a bunch of ifs)
回答1:
String s = Stream.<Supplier<String>>of(this::first, this::second /*, ... */)
.map(Supplier::get)
.filter(Objects::nonNull)
.findFirst()
.orElseGet(this::defaultOne);
It stops on the first non-null value or else sets the value which is returned from defaultOne. As long as you stay sequential, you are safe. Of course this requires Java 8 or later.
The reason why it stops on the first occurrence of a non-null value is due how the Stream handles each step. The map is an intermediate operation, so is filter. The findFirst on the other side is a short-circuiting terminal operation. So it continues with the next element until one matches the filter. If no element matches an empty optional is returned and so the orElseGet-supplier is called.
this::first, etc. are just method references. If they are static replace it with YourClassName::first, etc.
Here is an example if the signature of your methods would differ:
String s = Stream.<Supplier<String>>of(() -> first("takesOneArgument"),
() -> second("takes", 3, "arguments")
/*, ... */)
.map(Supplier::get)
.filter(Objects::nonNull)
.findFirst()
.orElseGet(this::defaultOne);
Note that the Supplier is only evaluated when you call get on it. That way you get your lazy evaluation behaviour. The method-parameters within your supplier-lambda-expression must be final or effectively final.
回答2:
This can be done pretty cleanly with a stream of Suppliers.
Optional<String> result = Stream.<Supplier<String>> of(
() -> first(),
() -> second(),
() -> third() )
.map( x -> x.get() )
.filter( s -> s != null)
.findFirst();
The reason this works is that despite appearances, the whole execution is driven by findFirst(), which pulls an item from filter(), which lazily pulls items from map(), which calls get() to handle each pull. findFirst() will stop pulling from the stream when one item has passed the filter, so subsequent suppliers will not have get() called.
Although I personally find the declarative Stream style cleaner and more expressive, you don't have to use Stream to work with Suppliers if you don't like the style:
Optional<String> firstNonNull(List<Supplier<String>> suppliers {
for(Supplier<String> supplier : suppliers) {
String s = supplier.get();
if(s != null) {
return Optional.of(s);
}
}
return Optional.empty();
}
It should be obvious how instead of returning Optional you could equally return a String, either returning null (yuk), a default string, or throwing an exception, if you exhaust options from the list.
回答3:
It isn't readable because you are dealing with a bunch of separate functions that don't express any kind of connection with each other. When you attempt to put them together, the lack of direction is apparent.
Instead try
public String getFirstValue() {
String value;
value = first();
if (value != null) return value;
value = second();
if (value != null) return value;
value = third();
if (value != null) return value;
...
return value;
}
Will it be long? Probably. But you are applying code on top of a interface that's not friendly toward your approach.
Now, if you could change the interface, you might make the interface more friendly. A possible example would be to have the steps be "ValueProvider" objects.
public interface ValueProvider {
public String getValue();
}
And then you could use it like
public String getFirstValue(List<ValueProvider> providers) {
String value;
for (ValueProvider provider : providers) {
value = provider.getValue();
if (value != null) return value;
}
return null;
}
And there are various other approaches, but they require restructuring the code to be more object-oriented. Remember, just because Java is an Object-Oriented programming language, that doesn't mean it will always be used in an Object-Oriented manner. The first()...last() method listing is very not-object oriented, because it doesn't model a List. Even though the method names are expressive, a List has methods on it which permit easy integration with tools like for loops and Iterators.
回答4:
If you are using java 8 you can convert these function calls to lambdas.
public static<T> T firstNonNull(Supplier<T> defaultSupplier, Supplier<T>... funcs){
return Arrays.stream(funcs).filter(p -> p.get() != null).findFirst().orElse(defaultSupplier).get();
}
If you don't want the generic implementation and use it only for Strings go on and just replace T with String:
public static String firstNonNull(Supplier<String> defaultSupplier, Supplier<String>... funcs){
return Arrays.stream(funcs).filter(p -> p.get() != null).findFirst().orElse(defaultSupplier).get();
}
And then call it like:
firstNonNull(() -> getDefault(), () -> first(arg1, arg2), () -> second(arg3));
P.S. btw default is a reserved keyword, so you cannot use it as a method name :)
EDIT: ok, the best way to do this would be to return Optional, then you don't need to pass default supplier separetely:
@SafeVarargs
public static<T> Optional<T> firstNonNull(Supplier<T>... funcs){
return Arrays.stream(funcs).filter(p -> p.get() != null).map(s -> s.get()).findFirst();
}
回答5:
If you want to package it up into a utility method, you'll have to wrap each function up into something that defers execution. Perhaps something like this:
public interface Wrapper<T> {
T call();
}
public static <T> T firstNonNull(Wrapper<T> defaultFunction, Wrapper<T>... funcs) {
T val;
for (Wrapper<T> func : funcs) {
if ((val = func.call()) != null) {
return val;
}
}
return defaultFunction.call();
}
You could use java.util.concurrent.Callable instead of defining your own Wrapper class, but then you'd have to deal with the exception that Callable.call() is declared to throw.
This can then be called with:
String value = firstNonNull(
new Wrapper<>() { @Override public String call() { return defaultFunc(); },
new Wrapper<>() { @Override public String call() { return first(); },
new Wrapper<>() { @Override public String call() { return second(); },
...
);
In Java 8, as @dorukayhan points out, you can dispense with defining your own Wrapper class and just use the Supplier interface. Also, the call can be done much more cleanly with lambdas:
String value = firstNonNull(
() -> defaultFunc(),
() -> first(),
() -> second(),
...
);
You can also (as @Oliver Charlesworth suggests) use method references as shorthand for the lambda expressions:
String value = firstNonNull(
MyClass::defaultFunc,
MyClass::first,
MyClass::second,
...
);
I'm of two minds as to which is more readable.
Alternatively, you can use one of the streaming solutions that many other answers have proposed.
回答6:
Just make a class with one function like this:
class ValueCollector {
String value;
boolean v(String val) { this.value = val; return val == null; }
}
ValueCollector c = new ValueCollector();
if c.v(first()) || c.v(second()) ...
return c.value;
回答7:
You can accomplish this via reflection:
public Object getFirstNonNull(Object target, Method... methods) {
Object value = null;
for (Method m : methods) {
if ( (value = m.invoke(target)) != null) {
break;
}
}
return value;
}
来源：https://stackoverflow.com/questions/43280207/return-first-non-null-value