title: how to combine two ImmutableMap<String,String> in Java 8?
body: <p>I have tried this code:</p>
  
  <pre><code>    final ImmutableMap&lt;String, String&gt; map1 = ImmutableMap.of("username", userName, "email", email1, "A", "A1",
              "l", "500L");
      final ImmutableMap&lt;String, String&gt; map2 = ImmutableMap.of("b", "ture", "hashed_passwords", "12345", "e",
              "TWO", "fakeProp", "fakeVal");
  
      final ImmutableMap&lt;String, String&gt; map3 = ImmutableMap.builder().putAll(map1).putAll(map2).build();
  </code></pre>
  
  <p>but got an error:</p>
  
  <pre><code>Error:(109, 105) java: incompatible types: com.google.common.collect.ImmutableMap&lt;java.lang.Object,java.lang.Object&gt; cannot be converted to com.google.common.collect.ImmutableMap&lt;java.lang.String,java.lang.String&gt;
  </code></pre>
  
  <p>how can I cast it otherwise?</p>