Simple HashSet. Save 25% memory for you.
'in.srain.cube:simple-hashset:1.0.1'
<dependency>
<groupId>in.srain.cube</groupId>
<artifactId>simple-hashset</artifactId>
<version>1.0.1</version>
</dependency>
<dependency org="in.srain.cube" name="simple-hashset" rev="1.0.1"/>
"in.srain.cube", "simple-hashset", "1.0.1"