Keith Devens .com - Weblog: Using Java 1.5's foreach with Maps

Thursday, July 7, 2005

Using Java 1.5's foreach with Maps

From the third edition of the Java specification, here's an example of how you use Java 1.5's foreach construct with a Map. That's:

for(Map.Entry<K, V> e : map.entrySet())

    System.out.println(e.getKey()+": "+e.getValue());

Of course, to iterate just over the values:

for(V e : map.values())

    System.out.println(e);

To do the same thing with the keys, use keySet() instead of values();

Update (7/22): For reference, here's a way to iterate over a map that doesn't assume generics or the types of the map's key:values:

public static void dumpMap(Map mp) {

    Iterator it = mp.entrySet().iterator();

    while (it.hasNext()) {

        Map.Entry pairs = (Map.Entry)it.next();

        System.out.println(pairs.getKey() + " = " + pairs.getValue());

    }

}

Permalink | 5 Comments | Tags: Programming | id 7076

14:21 (utc-5)
edited: 2005-07-22 09:06 (utc-5)

Comments

146.50.22.99 wrote:

Hi, Can we change the value of each entry of the Map inside the for each ? Or is it just read only ?

∴ 146.50.22.99 | 18-Nov-2005 6:49am est | #8694

What do you think? You get object references back from getKey() and getValue(). So 1. You obviously can't assign to them and modify what the map points to. 2. If there are any mutator methods on the objects you can of course call them. Java doesn't have any "const" references like C++ does.

∴ Keith | 18-Nov-2005 9:46am est | http://keithdevens.com/ | #8695

212.118.224.34 wrote:

This has been immensely helpful time and time again. Thank you so much.

∴ 212.118.224.34 | 7-Aug-2007 6:35am est | #10236

98.15.252.31 wrote:

Stephen Coulborne made a prototype compiler that allows:

for(KeyType k, ValueType v : map) { // do something with k and v. }

∴ 98.15.252.31 | 19-Apr-2008 1:25am est | #10641

98.15.252.31 wrote:

where KeyType/ValueType are replaced by their respective data type

∴ 98.15.252.31 | 19-Apr-2008 1:25am est | #10642

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