jshell> /open Queue.java
jshell> Integer i;
jshell> String s;
jshell> boolean b;
jshell> Queue<Integer> q = new Queue<Integer>(2);
jshell> b = q.enq(4);
jshell> b
b ==> true
jshell> b = q.enq(8);
jshell> b
b ==> true
jshell> b = q.enq(0);
jshell> b
b ==> false
jshell> s = q.deq();
|  Error:
|  incompatible types: java.lang.Integer cannot be converted to java.lang.String
|  s = q.deq();
|      ^-----^
jshell> i = q.deq();
jshell> i
i ==> 4
jshell> i = q.deq();
jshell> i
i ==> 8
jshell> i = q.deq();
jshell> i
i ==> null
jshell> q.enq("hello");
|  Error:
|  incompatible types: java.lang.String cannot be converted to java.lang.Integer
|  q.enq("hello");
|        ^-----^
jshell> 