title: Why EXPAND_FACTOR is 2 in ImmutableCollections.java
body: <p>Below code comes from Java 11, java.util.ImmutableCollections.java, line 783.</p>
  
  <pre><code> static final int EXPAND_FACTOR = 2;
  
   MapN(Object... input) {
              if ((input.length &amp; 1) != 0) { // implicit nullcheck of input
                  throw new InternalError("length is odd");
              }
              size = input.length &gt;&gt; 1;
  
              int len = EXPAND_FACTOR * input.length;
              len = (len + 1) &amp; ~1; // ensure table is even length
              table = new Object[len];
  
  </code></pre>
  
  <p>Why the <code>len</code> is twice <code>input.length</code>? I think the table doesn't need to store elements more than <code>input.length</code>. </p>