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Within a block, variables can be declared at any point, but are valid only after they are declared Thus, if you define a variable at the start of a method, it is available to all of the code within that method Conversely, if you declare a variable at the end of a block, it is effectively useless, because no code will have access to it For example, this fragment is invalid because count cannot be used prior to its declaration:
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// This fragment is wrong! count = 100; // oops! cannot use count before it is declared! int count;
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Here is another important point to remember: variables are created when their scope is entered, and destroyed when their scope is left This means that a variable will not hold its value once it has gone out of scope Therefore, variables declared within a method will not hold their values between calls to that method Also, a variable declared within a block will lose its value when the block is left Thus, the lifetime of a variable is confined to its scope If a variable declaration includes an initializer, then that variable will be reinitialized each time the block in which it is declared is entered For example, consider the next program
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// Demonstrate lifetime of a variable class LifeTime { public static void main(String args[]) { int x; for(x = 0; x < 3; x++) { int y = -1; // y is initialized each time block is entered Systemoutprintln("y is: " + y); // this always prints -1 y = 100; Systemoutprintln("y is now: " + y); } } }
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The output generated by this program is shown here: y y y y y y is: -1 is now: 100 is: -1 is now: 100 is: -1 is now: 100
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As you can see, y is reinitialized to 1 each time the inner for loop is entered Even though it is subsequently assigned the value 100, this value is lost One last point: Although blocks can be nested, you cannot declare a variable to have the same name as one in an outer scope For example, the following program is illegal:
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// This program will not compile class ScopeErr { public static void main(String args[]) {
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D a t a Ty p e s , Va r i a b l e s , a n d A r r a y s
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int bar = 1; { // creates a new scope int bar = 2; // Compile-time error bar already defined! } } }
Type Conversion and Casting
If you have previous programming experience, then you already know that it is fairly common to assign a value of one type to a variable of another type If the two types are compatible, then Java will perform the conversion automatically For example, it is always possible to assign an int value to a long variable However, not all types are compatible, and thus, not all type conversions are implicitly allowed For instance, there is no automatic conversion defined from double to byte Fortunately, it is still possible to obtain a conversion between incompatible types To do so, you must use a cast, which performs an explicit conversion between incompatible types Let s look at both automatic type conversions and casting
Java s Automatic Conversions
When one type of data is assigned to another type of variable, an automatic type conversion will take place if the following two conditions are met:
The two types are compatible The destination type is larger than the source type
When these two conditions are met, a widening conversion takes place For example, the int type is always large enough to hold all valid byte values, so no explicit cast statement is required For widening conversions, the numeric types, including integer and floating-point types, are compatible with each other However, there are no automatic conversions from the numeric types to char or boolean Also, char and boolean are not compatible with each other As mentioned earlier, Java also performs an automatic type conversion when storing a literal integer constant into variables of type byte, short, long, or char
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