TYPE COERCION AND CONVERSION

Transcription

1 TYPE COERCION AND CONVERSION Type coercion is the automatic conversion of a datum from one data type to another within an expression. Coercion occurs because the datum is stored as one data type but its context requires a different data type. In this statement, the constant 1 is an integer but its context requires a double value. double x = 1; Many programming languages, including Java, allow type coercion as a convenience to the programmer. In line 3 the value of count is automatically converted to double before executing the / double average, sum; int count; average = sum / count; Although hidden within a Java statement, type conversions are real machine operations performed on the data as the program executes. average = sum / count; What Happens at Run Time Fetches the value of sum Fetches the value of count Converts the value of count to floating-point Performs floating-point division Stores result into memory allocated to average Type Coercion and Conversion Page 1

2 To explicitly specify the conversion of data types within a Java program, use a type cast. Java Type Cast Syntax ( data type ) datum datum is some variable, constant or expression whose data type you want converted. data type is what you want it to be converted to. The datum is converted to the equivalent value in data type but itself is left unchanged. int m; m = (int) 10.5; m 10 int code = (int) 'A'; code 65 char letter = (char) 66; letter B int sum, count; double avg; avg = sum / count; int sum, count; double avg; avg = (double) sum / (double) count; sum 20 count 8 avg 2 sum 20 count 8 avg 2.5 When type casts and other operators are mixed within the same expression, the type casts are executed first, unless you change the order using parentheses. char small, cap; cap = 'A'; small = (char) ( (int) cap + 32 ); cap small A a Type Coercion and Conversion Page 2

3 Numeric Data Conversions This figure shows proper data type conversions between Java s numeric data types. char byte short int long Java will automatically coerce data in the direction of the arrows, either solid or dashed. float double int a = 'A'; long b = 'A'; double c = 'A'; a 65 b 65 c 65.0 Coercions that traverse a dotted arrow are legal but may result in loss of precision, meaning that the converted value may not be equal to the starting value. float d = 1_234_567_890; float e = 2_147_483_650L; double f = L; d e f The compiler will flag as an error any statement that tries to coerce data against the direction of the arrows. The diagnostic says possible loss of precision. int g = L; float h = ; Cannot convert the 64-bit L to a 32-bit int. Cannot convert the 64-bit to a 32-bit float. Type Coercion and Conversion Page 3

4 You can use a type cast to specify a data conversion against the direction of an arrow. The compiler won t complain, but beware that your code may not run correctly. This code is fine since 2 divides evenly into 10. double a = 10.0, b = 2.0; double c = (int)(a / b); This code is potentially wrong because 2 doesn t divide evenly into 11 and the fraction is truncated. double a = 11.0, b = 2.0; double c = (int)(a / b); Numeric Coercions for Binary Operators If one operand of a binary operator is double and the other isn t, then the one that isn t is converted to double before executing the operation. This code works fine since the integer is converted to double. double x = * 1.0; If neither operand is double, but one is float and the other isn t, then the one that isn t is converted to float before executing the operation. This code doesn t work because the conversion of the integer to float causes loss of precision. double x = * 1.0F; Type Coercion and Conversion Page 4

5 If neither operand is floating point then both operands are converted to int before executing the operation. byte a, b, c; c = a * b; byte a, b; short c; c = a * b; byte a, b; int c; c = a * b; Operands a and b are converted to int. The 32-bit result cannot be converted to 8 bits. Operands a and b are converted to int. The 32-bit result cannot be converted to 16 bits. OK. Type Coercion and Conversion Page 5

6 Exercises What is the final value of x? 1. int x; x = 7 / 2; 2. int x; x = (double) 7 / (double) 2; 3. int x; x = (double) 7 / 2; 4. double x; x = (double) 7 / (double) 2; 5. double x; x = (double) 7 / 2; 6. double x; x = (double) (7 / 2); 7. What is wrong with this statement? Correct it. percent = 1/100 * value; Type Coercion and Conversion Page 6