Final (Java): Difference between revisions

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Latest revision as of 16:08, 25 August 2025

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{{Short description|Keyword in the Java programming language}}

{{lowercase title}}

In the [[Java (programming language)|Java programming language]], the <code>'''final'''</code> [[Keyword (computing)|keyword]] is used in several contexts to define an entity that can only be assigned once.

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Example:

<syntaxhighlight lang="java">

public final class MyFinalClassFinalClass {...}

// ...

}

// Forbidden

public class ThisIsWrong extends MyFinalClass {...} // forbidden

public class DerivedClass extends FinalClass {

// ...

}

</syntaxhighlight>

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Example:

<syntaxhighlight lang="java">

public class Base {

public final void m2m1() { ... }

{

public final void m1m2() { ... }

public final void m2() {...}

public static void m3() { ... }

public static final void m4() { ... }

}

public class Derived extends Base {

public void m2m1() { ... } // forbiddenOK, overriding Base#m1()

{

public void m1m2() { ... } // OK, overriding Base#m1()forbidden

public void m2() {...} // forbidden

public static void m3() { ...} } // OK, hiding Base#m3()

public static void m4() { ...} } // forbidden

}

</syntaxhighlight>

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public class Sphere {

// piPi is a universal constant, about as constant as anything can be.

public static final double PI = 3.141592653589793;

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Sphere(double x, double y, double z, double r) {

radius = r;

xPos = x;

yPos = y;

zPos = z;

}

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final boolean hasTwoDigits;

if (number >= 10 && number < 100) {

hasTwoDigits = true;

}

if (number > -100 && number <= -10) {

hasTwoDigits = true; // compile-error because the final variable might already be assigned.

}

</syntaxhighlight>

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if (number % 2 == 0) {

isEven = true;

}

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if (number % 2 == 0) {

isEven = true;

}

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==C/C++ analog of final variables==

{{further|const (computer programming)}}

In [[C (programming language)|C]] and [[C++]], the analogous construct is the <code>[[const (computer programming)|const]]</code> [[keyword (computer programming)|keyword]]. This differs substantially from <code>final</code> in Java, most basically in being a [[type qualifier]]: <code>const</code> is part of the ''[[data type|type]],'' not only part of the identifier (variable). This also means that the constancy of a value can be changed by casting (explicit type conversion), in this case known as "const casting". Nonetheless, casting away constness and then modifying the object results in [[undefined behavior]] if the object was originally declared <code>const</code>. Java's <code>final</code> is a strict rule such that it is impossible to compile code that directly breaks or bypasses the final restrictions. Using [[Reflection (computer programming)#Java|reflection]], however, it is often possible to still modify final variables. This feature is mostly made use of when [[Serialization|deserializing]] objects with final members.

Further, because C and C++ expose pointers and references directly, there is a distinction between whether the pointer itself is constant, and whether the data pointed to by the pointer is constant. Applying <code>const</code> to a pointer itself, as in <code>SomeClass * const ptr</code>, means that the contents being referenced can be modified, but the reference itself cannot (without casting). This usage results in behaviour which mimics the behaviour of a <code>final</code> variable reference in Java. By contrast, when applying const to the referenced data only, as in <code>const SomeClass * ptr</code>, the contents cannot be modified (without casting), but the reference itself can. Both the reference and the contents being referenced can be declared as <code>const</code>.

In C++, the <code>final</code> keyword is used to denote that a function cannot be further overridden. It is also used similarly to Java to declare a class as final (cannot be extended).

<syntaxhighlight lang="C++">

// final in a class declaration declares that a class cannot be extended

class Z final : public X, public Y {

public:

// final in a method signature declares that a method cannot be overridden further

void someOperation() override final {

// do something here

}

};

</syntaxhighlight>

==C# analogs for final keyword ==

[[C Sharp (programming language)|C#]] can be considered as similar to Java, in terms of its language features and basic syntax: Java has JVM, C# has .Net Framework; Java has bytecode, C# has MSIL; Java has no pointers (real memory) support, C# is the same.

Regarding the final keyword, C# has two related keywords: