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POWhen Does HashMap.values()/HashMap.keySet() Return By Reference And When Does It Return By Value?
 

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  1. POWhen Does HashMap.values()/HashMap.keySet() Return By Reference And When Does It Return By Value?
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    <p>The code below indicates that values are being returned by reference:</p> <pre><code>public class Playground { public static void main(String[] args) { Map&lt;Integer, vinfo&gt; map = new HashMap&lt;Integer, vinfo&gt;(); map.put(1, new vinfo(true)); map.put(2, new vinfo(true)); map.put(3, new vinfo(true)); for(vinfo v : map.values()){ v.state = false; } printMap(map); } public static void printMap(Map m){ Iterator it = m.entrySet().iterator(); while(it.hasNext()){ Map.Entry pairs = (Map.Entry) it.next(); Integer n = (Integer) pairs.getKey(); vinfo v = (vinfo) pairs.getValue(); System.out.println(n + "=" + v.state); } } } class vinfo{ boolean state; public vinfo(boolean state){ this.state = state; } } </code></pre> <p>Output:<br> 1=false<br> 2=false<br> 3=false<br></p> <p>In the code below, they are being returned by value. However; I am using the Integer object.</p> <pre><code>public class Playground { public static void main(String[] args) { Map&lt;Integer, Integer&gt; map = new HashMap&lt;Integer, Integer&gt;(); map.put(1, 1); map.put(2, 1); map.put(3, 1); for(Integer n : map.values()){ n+=1; } printMap(map); } public static void printMap(Map m){ Iterator it = m.entrySet().iterator(); while(it.hasNext()){ Map.Entry pairs = (Map.Entry) it.next(); Integer n = (Integer) pairs.getKey(); Integer n2 = (Integer) pairs.getValue(); System.out.println(n + "=" + n2); } } } </code></pre> <p>Output:<br> 1=1<br> 2=1<br> 3=1<br></p> <p>How can I know when I am directly changing the value (or key for that matter) or I have to do a full .remove() and .put().</p>
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    1. COWhat do you mean by "values" and "returned"? You may be misunderstanding how Java handles objects; objects are always passed (and returned) by reference. In the second example, `getValue()` is returning a reference to an `Integer` object. Also, what do you mean by "directly changing the value"? Any operations you carry out on the result of `get(key)` or `getValue()` are carried out on the actual object whose reference is stored in the `Map`.
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    2. COWhy do you think your code is showing anything about return by reference vs return by value? Where is your code changing any key and/or value?
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    3. COUhm, returning by reference is when you get back the memory location of the actual object, instead of a copy of it. Changes to such a return are therefore reflected in the actual object. So in the first example, we are changing the map's values (proven by changing true to false). In the second, we are adding +1 but nothing changes when you print the map again.
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