Effective Java

Item 6:
Avoid creating unnecessary objects
It is often appropriate to reuse a single object instead of creating a new function-
ally equivalent object each time it is needed. Reuse can be both faster and more
stylish. An object can always be reused if it is immutable (Item 17).
As an extreme example of what not to do, consider this statement:
String s = new String("bikini");
// DON'T DO THIS!
The statement creates a new 
String
instance each time it is executed, and none
of those object creations is necessary. The argument to the 
String
constructor
(
"bikini"
) is itself a 
String
instance, functionally identical to all of the objects
created by the constructor. If this usage occurs in a loop or in a frequently invoked
method, millions of 
String
instances can be created needlessly.
The improved version is simply the following:
String s = "bikini";
This version uses a single 
String
instance, rather than creating a new one
each time it is executed. Furthermore, it is guaranteed that the object will be
reused by any other code running in the same virtual machine that happens to
contain the same string literal [JLS, 3.10.5].
You can often avoid creating unnecessary objects by using 
static factory meth-
ods 
(Item 1) in preference to constructors on immutable classes that provide both.
For example, the factory method 
Boolean.valueOf(String)
is preferable to the
constructor 
Boolean(String)
, which was deprecated in Java 9. The constructor
must
create a new object each time it’s called, while the factory method is never
required to do so and won’t in practice. In addition to reusing immutable objects,
you can also reuse mutable objects if you know they won’t be modified.
Some object creations are much more expensive than others. If you’re going
to need such an “expensive object” repeatedly, it may be advisable to cache it for
reuse. Unfortunately, it’s not always obvious when you’re creating such an object.
Suppose you want to write a method to determine whether a string is a valid
Roman numeral. Here’s the easiest way to do this using a regular expression: