Java makes every effort to ensure that all variables are properly initialized before they are used. If a variable is declared as a "local" variable within a method, the compiler enforces this by issuing a compile-time error if the variable is used without being initialized. For example, consider the following code:
```java
void f() {
int i;
i++;
}
```
In this case, the compiler will throw an error indicating that `i` may not have been initialized. While the compiler could automatically assign a default value to `i`, doing so might mask a programmer's mistake. By requiring explicit initialization, the compiler helps catch potential bugs early in the development process.
However, things are slightly different when dealing with primitive data types that are defined as instance variables (data members) of a class. Since these variables can be accessed and modified by any method of the class, it's not always practical to force the programmer to initialize them explicitly before they are used. On the other hand, allowing them to hold garbage values would be unsafe.
To address this, Java guarantees that all primitive data type members of a class are automatically initialized to default values. This ensures that even if the programmer forgets to assign a value, the variable will still have a predictable state.
You can observe these default values in the following small program:
```java
// IniTIalValues.java
// Shows default initial values
class Measurement {
boolean t;
char c;
byte b;
short s;
int i;
long l;
float f;
double d;
void print() {
System.out.println("Data type Initial value: boolean " + t +
" char " + c +
" byte " + b +
" short " + s +
" int " + i +
" long " + l +
" float " + f +
" double " + d);
}
}
public class InitialValues {
public static void main(String[] args) {
Measurement d = new Measurement();
d.print(); // You can also write: new Measurement().print();
}
}
```
When you run this program, the output will be:
```
Data type Initial value: boolean false char 0 byte 0 short 0 int 0 long 0 float 0.0 double 0.0
```
Note that the `char` value appears as a blank because its default value is the null character (`\u0000`).
As you'll learn later, the same principle applies to object references. When an object reference is declared as an instance variable and not explicitly initialized, it will automatically be set to `null`. This behavior helps prevent undefined behavior and ensures that the program can safely check for null before using the reference.
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