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The "Why" behind PMD's StringInstantiation rule

Along the lines of an existing thread, The “Why” behind PMD's rules, I'm trying to figure out the meaning of one particular PMD rule : String and StringBuffer Rules.StringInstantiation.

This rule states that you shouldn't explicitly instantiate String objects. As per their manual page :

Avoid instantiating String objects; this is usually unnecessary since they are immutable and can be safely shared.

This rule is defined by the following Java class:net.sourceforge.pmd.lang.java.rule.strings.StringInstantiationRule

Example(s):

private String bar = new String("bar"); // just do a String bar = "bar";

http://pmd.sourceforge.net/pmd-5.0.1/rules/java/strings.html

I don't see how this syntax is a problem, other than it being pointless. Does it affect overwhole performance ?

Thanks for any thought.

Solution

  • With String foo = "foo" there will be on instance of "foo" in PermGen space (This is referred to as string interning). If you were to later type String bar = "foo" there would still only be one "foo" in the PermGen space.

    Writing String foo = new String( "foo" ) will also create a String object to count against the heap.

    Thus, the rule is there to prevent wasting memory.

    Cheers,