Effective Java

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ITEM 40: CONSISTENTLY USE THE OVERRIDE ANNOTATION
189
equals
method is not of type 
Object
, so 
Bigram
inherits the 
equals
method from
Object
. This 
equals
method tests for object 
identity
, just like the 
==
operator.
Each of the ten copies of each bigram is distinct from the other nine, so they are
deemed unequal by 
Object.equals
, which explains why the program prints 
260
.
Luckily, the compiler can help you find this error, but only if you help it by
telling it that you intend to override 
Object.equals
. To do this, annotate
Bigram.equals
with 
@Override
, as shown here:
@Override public boolean equals(Bigram b) {
return b.first == first && b.second == second;
}
If you insert this annotation and try to recompile the program, the compiler
will generate an error message like this:
Bigram.java:10: method does not override or implement a method 
from a supertype
@Override public boolean equals(Bigram b) {
^
You will immediately realize what you did wrong, slap yourself on the forehead,
and replace the broken 
equals
implementation with a correct one (Item 10):
@Override public boolean equals(