This short article introduces the definition and usage of final variables in Java and some interesting use cases.<\/p>\n
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A variable is said “final<\/strong>” when, after its declaration, its reference cannot be changed.<\/p>\n The below code will raise error during compilation because we attempt to modify a variable declared as final<\/strong>.<\/p>\n <\/p>\n The final keyword can be used in 3 situations:<\/p>\n The first case is quite straightforward, no much to say.<\/p>\n For a class attribute declaration<\/strong>, the final variable should either:<\/p>\n Any subsequent attempt to assign a new value to finalVa<\/strong>r will result in compilation error.<\/p>\n The last use case of final variable relates to method argument:<\/p>\n <\/p>\n There are some false beliefs among developers, stating that a final variable cannot be modified. It is right and wrong at the same time, depending on what we mean by “being modified<\/strong>“.<\/p>\n Let’s have a look at the definition of “final<\/strong>“: …after its declaration, its reference cannot be changed<\/strong><\/p>\n The definition clearly states that the reference of a final variable cannot be changed after its initialization. Nothing is said about its internal state. Especially if the object is mutable, its internal state can be modified even though the variable is declared “final<\/strong>“.<\/p>\n The output displays:<\/p>\n Indeed, mutability and final are two distinct notions in Java. You can have a final mutable variable as well as final immutable variable.<\/p>\n\nfinal Object finalObj = new Object();\n...\nfinalObj = new Object(); \/\/ Compilation error\n<\/pre>\n
II Usage<\/h1>\n
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\npublic class Test\n{\n\t\/\/ Immediate initialization\n\tfinal String finalVar = "test";\n}\n<\/pre>\n\n
\npublic class Test\n{\n\tfinal String finalVar;\n\n\tpublic Test(String input)\n\t{\n\t\tthis.finalVar="firstConstructor";\n\t}\n\n\tpublic Test()\n\t{\n\t\tthis.finalVar="defaultConstructor";\n\t}\n}\n<\/pre>\n\npublic class Test\n{\n\t...\n\tpublic void doSomething(final String value)\n\t{\n\t\t...\n\t\t\/\/ Compilation error, forbidden\n\t\tvalue = "anotherString";\n\t\t...\n\t}\n}\n<\/pre>\nIII Final vs immutable<\/h1>\n
\npublic class Test\n{\n\tpublic void addToList(final List<String> items)\n\t{\n\t\titems.add("dummy");\n\t}\n}\n\n...\n\tList<String> items = new ArrayList<String>();\n\titems.add("singleton");\n\n\tSystem.out.println("items size before == " + items.size());\n\n\tTest testObject = new Test();\n\ttestObject.addToList(items);\n\n\tSystem.out.println("items size after == " + items.size());\n...\n<\/pre>\n\nitems size before == 1\nitems size after == 2\n<\/pre>\n