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Casting lets you convert primitive values from one type to another. We looked at primitive casting earlier in this chapter, in the assignments section, but now we re going to take a deeper look. Object casting is covered in 5. Casts can be implicit or explicit. An implicit cast means you don t have to write code for the cast; the conversion happens automatically. Typically, an implicit cast happens when you re doing a widening conversion. In other words, putting a smaller thing (say, a byte) into a bigger container (like an int). Remember those possible loss of precision compiler errors we saw in the assignments section Those happened when you tried to put a larger thing (say, a long) into a smaller container (like a short). The large-value-into-small-container conversion is referred to as narrowing and requires an explicit cast, where you tell the compiler that you re aware of the danger and accept full responsibility. First we ll look at an implicit cast:
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int a = 100; long b = a; // Implicit cast, an int value always fits in a long
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An explicit casts looks like this:
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float a = 100.001; int b = (int)a; // Explicit cast, a float can lose info as an int
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Integer values may be assigned to a double variable without explicit casting, because any integer value can fit in a 64-bit double. The following line demonstrates this:
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double d = 100L; // Implicit cast
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In the preceding statement, a double is initialized with a long value (as denoted by the L after the numeric value). No cast is needed in this case because a double can hold every piece of information that a long can store. If, however, we want to assign a double value to an integer type, we re attempting a narrowing conversion and the compiler knows it:
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class Casting { public static void main(String [] args) { int x = 3957.229; // illegal } }
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3: Operators and Assignments
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If we try to compile the preceding code, the following error is produced:
%javac Casting.java Casting.java:3: Incompatible type for declaration. Explicit cast needed to convert double to int. int x = 3957.229; // illegal 1 error
In the preceding code, a floating-point value is being assigned to an integer variable. Because an integer is not capable of storing decimal places, an error occurs. To make this work, we ll cast the floating-point number into an integer:
class Casting { public static void main(String [] args) { int x = (int)3957.229; // legal cast System.out.println("int x = " + x); } }
When you a cast a floating-point number to an integer type, the value loses all the digits after the decimal. Running the preceding code will produce the following output:
%java Casting int x = 3957
We can also cast a larger number type, such as a long, into a smaller number type, such as a byte. Look at the following:
class Casting { public static void main(String [] args) { long l = 56L; byte b = (byte)l; System.out.println("The byte is " + b); } }
The preceding code will compile and run fine. But what happens if the long value is larger than 127 (the largest number a byte can store) Let s modify the code and find out:
class Casting { public static void main(String [] args) { long l = 130L; byte b = (byte)l;
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System.out.println("The byte is " + b); } }
The code compiles fine, and when we run it we get the following:
%java Casting The byte is -126
You don t get a runtime error, even when the value being narrowed is too large for the type. The bits to the left of the lower 8 just go away. As we saw in the assignments section, if the leftmost bit in the byte now happens to be a 1, the 1 is no longer part of the value and instead becomes the sign bit for the new byte.
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