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    <p>JSON is made up of name/value pairs (where the value side can be a list of things). The name part of that is a string (See: <a href="http://json.org" rel="noreferrer">http://json.org</a>)</p> <p>What you're trying to do is use an object as a name; you can't do that directly. A JSON object can't be a name for a name/value pair. </p> <p>If you read the <a href="http://google-gson.googlecode.com/svn/trunk/gson/docs/javadocs/com/google/gson/GsonBuilder.html#enableComplexMapKeySerialization()" rel="noreferrer">documentation for enableComplexMapKeySerialization</a> it explains what the resulting JSON is going to be.</p> <p>The JSON it produces (a Map as a JSON array) will deserialize perfectly back to your map. The following is a complete, working example (Java 7). </p> <p>Note that once I deserialize from JSON back to Java, I'm iterating over the map to get the keys. This is because without <code>equals()</code> and <code>hashCode()</code> being overridden in <code>Teacher</code> there's no way to create a new instance of <code>Teacher</code> and have it work as a key (only the reference values are compared). </p> <pre><code>import com.google.gson.Gson; import com.google.gson.GsonBuilder; import com.google.gson.reflect.TypeToken; import java.lang.reflect.Type; import java.util.ArrayList; import java.util.HashMap; import java.util.List; public class App { public static void main( String[] args ) { HashMap&lt;Teacher, List&lt;Student&gt;&gt; map = new HashMap&lt;&gt;(); Teacher t = new Teacher("12345", "Teacher"); Teacher t2 = new Teacher("23456", "Teacher2"); ArrayList&lt;Student&gt; list = new ArrayList&lt;&gt;(); for (int i = 0; i &lt; 3; i++) { list.add(new Student(String.valueOf(i), "Student" + String.valueOf(i))); } map.put(t, list); map.put(t2, list); GsonBuilder builder = new GsonBuilder(); Gson gson = builder.enableComplexMapKeySerialization().setPrettyPrinting().create(); Type type = new TypeToken&lt;HashMap&lt;Teacher,List&lt;Student&gt;&gt;&gt;(){}.getType(); String json = gson.toJson(map, type); System.out.println(json); System.out.println("\nAfter deserialization:"); HashMap&lt;Teacher, List&lt;Student&gt;&gt; map2 = gson.fromJson(json, type); for (Teacher t3 : map2.keySet()) { System.out.println(t3.name); for (Student s2 : map2.get(t3)) { System.out.println("\t" + s2.name); } } } } class Teacher { public String id; public String name; public Teacher(String id, String name) { this.id = id; this.name = name; } } class Student { public String id; public String name; public Student(String id, String name) { this.id = id; this.name = name; } } </code></pre> <p>Output:</p> <pre><code>[ [ { "id": "12345", "name": "Teacher" }, [ { "id": "0", "name": "Student0" }, { "id": "1", "name": "Student1" }, { "id": "2", "name": "Student2" } ] ], [ { "id": "23456", "name": "Teacher2" }, [ { "id": "0", "name": "Student0" }, { "id": "1", "name": "Student1" }, { "id": "2", "name": "Student2" } ] ] ] After deserialization: Teacher2 Student0 Student1 Student2 Teacher Student0 Student1 Student2 </code></pre> <p>If you implement <code>equals()</code> and <code>hashCode()</code> in your <code>Teacher</code> class, you would then be able to use a new instance of <code>Teacher</code> to retrieve things from the map: </p> <pre><code>class Teacher { public String id; public String name; public Teacher(String id, String name) { this.id = id; this.name = name; } @Override public int hashCode() { int hash = 3; hash = 37 * hash + Objects.hashCode(this.id); hash = 37 * hash + Objects.hashCode(this.name); return hash; } @Override public boolean equals(Object obj) { if (obj == null) { return false; } if (getClass() != obj.getClass()) { return false; } final Teacher other = (Teacher) obj; if (!Objects.equals(this.id, other.id)) { return false; } if (!Objects.equals(this.name, other.name)) { return false; } return true; } } </code></pre> <p>Once you have that, you could do:</p> <pre><code>... HashMap&lt;Teacher, List&lt;Student&gt;&gt; map2 = gson.fromJson(json, type); Teacher t = new Teacher("23456", "Teacher2"); List&lt;Student&gt; list = map2.get(t); ... </code></pre>
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    1. USBrian Roach
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    1. COthanks mate your guidance solved the problem
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    2. COThanks, I used this solution too, but beware of ClassNotFoundException on enableComplexMapKeySerialization on certain HTC devices/versions (HTC decided to include gson themselves, and it's an old version, also overriding the version included in your app).
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    3. COLovely explanation !! I was not aware of **enableComplexMapKeySerialization**
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