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A final variable is a constant despite being called a variable Its value cannot be changed once it has been initialized Instance and static variables can be declared final Note that the keyword final can also be applied to local variables, including method parameters Declaring a variable final has the following implications: A final variable of a primitive data type cannot change its value once it has been initialized A final variable of a reference type cannot change its reference value once it has been initialized This effectively means that a final reference will always refer to the same object However, the keyword final has no bearing on whether the state of the object denoted by the reference can be changed or not Final static variables are commonly used to define manifest constants (also called named constants), eg, IntegerMAX_VALUE, which is the maximum int value Variables defined in an interface are implicitly final (see Section 76, p 309) Note that a final variable need not be initialized in its declaration, but it must be initialized in
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410: OTHER MODIFIERS FOR MEMBERS
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the code once before it is used These variables are also known as blank final variables For a discussion on final parameters, see Section 37, p 89 A final method in a class is complete (that is, has an implementation) and cannot be overridden in any subclass (see Section 72, p 288) Final variables ensure that values cannot be changed and final methods ensure that behavior cannot be changed Final classes are discussed in Section 48, p 136 The compiler may be able to perform code optimizations for final members, because certain assumptions can be made about such members In Example 414, the class Light defines a final static variable at (1) and a final method at (2) An attempt to change the value of the final variable at (3) results in a compile-time error The subclass TubeLight attempts to override the final method setWatts() from the superclass Light at (4), which is not permitted The class Warehouse defines a final local reference aLight at (5) The state of the object denoted by the reference tableLight is changed at (6), but its reference value cannot be changed as attempted at (7) Another final local reference streetLight is declared at (8), but it is not initialized The compiler reports an error when an attempt is made to use this reference at (9)
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Example 414 Accessing Final Members class Light { // Final static variable (1) final public static double KWH_PRICE = 325; int noOfWatts; // Final instance method final public void setWatts(int watt) { noOfWatts = watt; } public void setKWH() { // KWH_PRICE = 410; } (2)
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// (3) Not OK Cannot be changed
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} //______________________________________________________________________________ class TubeLight extends Light { // Final method in superclass cannot be overridden // This method will not compile /* public void setWatts(int watt) { // (4) Attempt to override noOfWatts = 2*watt; } */ } //______________________________________________________________________________
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150 public class Warehouse { public static void main(String[] args) {
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final Light tableLight = new Light();// (5) Final local variable tableLightnoOfWatts = 100; // (6) OK Changing object state tableLight = new Light(); // (7) Not OK Changing final reference // (8) Not initialized // (9) Not OK
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final Light streetLight; // streetLightnoOfWatts = 2000; } }
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abstract Methods
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An abstract method has the following syntax:
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abstract <accessibility modifier> <return type> <method name> (<parameter list>) <throws clause>;
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An abstract method does not have an implementation; ie, no method body is defined for an abstract method, only the method header is provided in the class declaration The keyword abstract is mandatory in the header of an abstract method declared in a class Its class is then incomplete and must be explicitly declared abstract (see Section 48, p 135) Subclasses of an abstract class must then provide the method implementation; otherwise, they must also be declared abstract The accessibility of an abstract method declared in a class cannot be private, as subclasses would not be able to override the method and provide an implementation See Section 48, where Example 411 also illustrates the use of abstract methods Only an instance method can be declared abstract Since static methods cannot be overridden, declaring an abstract static method makes no sense A final method cannot be abstract (ie, cannot be incomplete) and vice versa The keyword abstract can only be combined with accessibility modifiers public or private Methods specified in an interface are implicitly abstract (see Section 76, p 309), and the keyword abstract is seldom specified in their method headers These methods can only have public or package accessibility
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