Member initialization
Java goes out of its way to guarantee that variables are properly initialized before they are used. In the case of a method’s local variables, this guarantee comes in the form of a compile-time error. So if you say:
void f() {
int i;
i++; // Error -- i not initialized
}
you’ll get an error message that says that i might not have been initialized. Of course, the compiler could have given i a default value, but an uninitialized local variable is probably a programmer error, and a default value would have covered that up. Forcing the programmer to provide an initialization value is more likely to catch a bug.
If a primitive is a field in a class, however, things are a bit different. As you saw in the Everything Is an Object chapter, each primitive field of a class is guaranteed to get an initial value. Here’s a program that verifies this, and shows the values:
//: initialization/InitialValues.java
// Shows default initial values.
import static net.mindview.util.Print.*;
public class InitialValues {
boolean t;
char c;
byte b;
short s;
int i;
long l;
float f;
double d;
InitialValues reference;
void printInitialValues() {
print("Data type Initial value");
print("boolean " + t);
print("char [" + c + "]");
print("byte " + b);
print("short " + s);
print("int " + i);
print("long " + l);
print("float " + f);
print("double " + d);
print("reference " + reference);
}
public static void main(String[] args) {
InitialValues iv = new InitialValues();
iv.printInitialValues();
/* You could also say:
new InitialValues().printInitialValues();
*/
}
} /* Output:
Data type Initial value
boolean false
char [ ]
byte 0
short 0
int 0
long 0
float 0.0
double 0.0
reference null
*///:~
You can see that even though the values are not specified, they automatically get initialized (the char value is a zero, which prints as a space). So at least there’s no threat of working with uninitialized variables.
When you define an object reference inside a class without initializing it to a new object, that reference is given a special value of null.
