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34_[자바] 초기화 1 (필드)

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.

[Thinking in Java]

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