What is the easiest way to iterate over all the key/value pairs of a java.util.Map in Java 5 and higher?

Question

What is the easiest way to iterate over all the key/value pairs of a java.util.Map in Java 5 and higher?

Nones

Answer 1

Assuming K is your key type and V is your value type:

for (Map.Entry<K,V> entry : map.entrySet()) {
  K key = entry.getKey();
  V value = entry.getValue();
  // do stuff
}

This version works with every Java version that supports generic (i.e. Java 5 and up).

Java 10 simplified that by letting variable types be inferred using var:

for (var entry : map.entrySet()) {
  var key = entry.getKey();
  var value = entry.getValue();
  // do stuff
}

While the previous two options are basically equivalent at the bytecode level, Java 8 also introduced forEach which uses lambdas^* and works slightly difference. It invokes the lambda for each entry, which might add some overhead of those calls, if it's not optimized away:

map.forEach((k, v) -> { 
  // do stuff 
});

^{* technically the BiConsumer can also be implemented using a normal class, but it's intended for and mostly used with lambdas.}

Chemisette

Answer 2

While Joachim Sauer is correct for Java 5, there is a simpler option available starting at Java 10:

for (var entry : map.entrySet()) {
    // Do something with entry.getKey() and entry.getValue();
}

and Java 8 introduced:

map.forEach((k, v) -> {
    // Do something with k, v
});

Mayemayeda

BTW 1st snippet similar to already suggested one, in accepted answer (from 2009, >24 years ago) but with less details (as posted here, works only with Java 10 or later; question asking for Java 5); (ii) 2nd will not work with Java 5, 6, or 7 :-/ – Coronagraph

@Coronagraph "in Java 5 and higher". 1st is more readable. 2nd will for newer versions. – Mayemayeda

so it will work with Java 5? is my comment wrong? (I think it isn't bad to give some advice to someone reading this answer... even if Java 5 is very old, Java 8 is still being used) – Coronagraph

@Coronagraph is right. Your first answer is less detailed, and your second one is incorrect! The .forEach() method was only made available in Java 8 or newer as part of the Java 8 Collections Improvements. – Asynchronism

While the question may mention Java 5, it also mentioned higher versions. Since this was the oldest question with the highest number of votes, it was the first I was offered when doing a search. Although this answer does not provide a Java 5 answer, it does answer the part of the question related to both easier and higher, since higher versions of Java do provide an easier option than Java 5 does. I can't help but think that the effort which went into criticising this answer, could have, instead, gone into improving it. – Beffrey