A for-loop to iterate over an enum in Java
JavaLoopsFor LoopEnumsJava Problem Overview
I have an enum
in Java for the cardinal and intermediate directions:
public enum Direction {
NORTH,
NORTHEAST,
EAST,
SOUTHEAST,
SOUTH,
SOUTHWEST,
WEST,
NORTHWEST
}
How can I write a for
loop that iterates through each of these enum
values?
Java Solutions
Solution 1 - Java
.values()
You can call the values()
method on your enum.
for (Direction dir : Direction.values()) {
// do what you want
}
This values()
method is implicitly declared by the compiler. So it is not listed on Enum
doc.
Solution 2 - Java
All the constants of an enum type can be obtained by calling the implicit public static T[] values()
method of that type:
for (Direction d : Direction.values()) {
System.out.println(d);
}
Solution 3 - Java
You can do this as follows:
for (Direction direction : EnumSet.allOf(Direction.class)) {
// do stuff
}
Solution 4 - Java
Streams
Prior to Java 8
for (Direction dir : Direction.values()) {
System.out.println(dir);
}
Java 8
We can also make use of lambda and streams (Tutorial):
Stream.of(Direction.values()).forEachOrdered(System.out::println);
Why forEachOrdered
and not forEach
with streams ?
The behaviour of forEach
is explicitly nondeterministic where as the forEachOrdered
performs an action for each element of this stream, in the encounter order of the stream if the stream has a defined encounter order. So forEach
does not guarantee that the order would be kept.
Also when working with streams (especially parallel ones) keep in mind the nature of streams. As per the doc:
Stream pipeline results may be nondeterministic or incorrect if the behavioral parameters to the stream operations are stateful. A stateful lambda is one whose result depends on any state which might change during the execution of the stream pipeline.
Set<Integer> seen = Collections.synchronizedSet(new HashSet<>());
stream.parallel().map(e -> { if (seen.add(e)) return 0; else return e; })...
Here, if the mapping operation is performed in parallel, the results for the same input could vary from run to run, due to thread scheduling differences, whereas, with a stateless lambda expression the results would always be the same.
Side-effects in behavioral parameters to stream operations are, in general, discouraged, as they can often lead to unwitting violations of the statelessness requirement, as well as other thread-safety hazards.
Streams may or may not have a defined encounter order. Whether or not a stream has an encounter order depends on the source and the intermediate operations.
Solution 5 - Java
If you don't care about the order this should work:
Set<Direction> directions = EnumSet.allOf(Direction.class);
for(Direction direction : directions) {
// do stuff
}
Solution 6 - Java
Java8
Stream.of(Direction.values()).forEach(System.out::println);
from Java5+
for ( Direction d: Direction.values()){
System.out.println(d);
}
Solution 7 - Java
More methods in java 8:
EnumSet
with forEach
Using EnumSet.allOf(Direction.class).forEach(...);
Arrays.asList
with forEach
Using Arrays.asList(Direction.values()).forEach(...);
Solution 8 - Java
we can use a filter(JAVA 8) like this.
Stream.of(Direction.values()).filter(name -> !name.toString().startsWith("S")).forEach(System.out::println);