Why do instance variables have default values in java?

Question

Why do variables declared in a class have default values, but the variables declared inside methods, said to be "local variables", don't have default values in Java?

For example

class abc
{
   int a;

   public static void main(String ss[])
   {
        int b;

          abc aa=new abc();
          System.out.println(aa.a);
          System.out.println(b);
    }
 }

In this above example variable a has default value of 0 but variable b gives error that it might not have been initialized.

Answer 1

All member variable have to load into heap so they have to initialized with default values when an instance of class is created. In case of local variables, they don't get loaded into heap they are stored in stack until they are being used before java 7, so we need to explicitly initialize them. Now the "Java Hotspot Server Compiler" performs "escape analysis" and decides to allocate some variables on the stack instead of the heap.

Answer 2

Local variables Initialization

Variables declared in methods and in blocks are called local variables. Local variable are not initialized when they are created at method invocation. Therefore, a local variable must be initialized explicitly before being used. Otherwise the compiler will flag it as error when the containing method or block is executed.

Example:

public class SomeClassName{

public static void main(String args[]){
int total;
System.out.println("The incremented total is " + total + 3); //(1)
}
}

The compiler complains that the local variable total used in println statement at (1) may not be initialized. Initializing the local variable total before usage solves the problem:

public class SomeClassName{

public static void main(String args[]){
int total = 45; //Local variable initialized with value 45 System.out.println("The incremented total is " + total+ 3); //(1)
}
}

Fields initialization

If no initialization is provided for an instance or static variable, either when declared or in an initializer block, then it is implicitly initialized with the default value of its type. An instance variable is initialized with the default value of its type each time the class is instantiated, that is for every object created from the class. A static variable is initialized with the default value of its type when the class is first loaded.

Answer 3

As local variables are allocated on stack, memory chunk for a local variable is allocated when it is assigned with a value.

Take simple example

class Abc {
   int i = -111;
   int e;

   int doSomething() {
        int a = 10;
        int b = a + i;    
        int c = b + 100;

        Abc d = new Abc();

        e = b + c + d.a;

        return e + 1000;
    }
 }

and the bytecode from javap -c Abc

Compiled from "Abc.java"
class Abc {
  int i;
  int e;

  Abc();
    Code:
       0: aload_0
       1: invokespecial #1                  // Method java/lang/Object."<init>":()V
       4: aload_0
       5: bipush        -111
       7: putfield      #2                  // Field i:I
      10: return

  int doSomething();
    Code:
       0: bipush        10
       2: istore_1
       3: iload_1
       4: aload_0
       5: getfield      #2                  // Field i:I
       8: iadd
       9: istore_2
      10: iload_2
      11: bipush        100
      13: iadd
      14: istore_3
      15: new           #3                  // class Abc
      18: dup
      19: invokespecial #4                  // Method "<init>":()V
      22: astore        4
      24: aload_0
      25: iload_2
      26: iload_3
      27: iadd
      28: aload         4
      30: getfield      #2                  // Field i:I
      33: iadd
      34: putfield      #5                  // Field e:I
      37: aload_0
      38: getfield      #5                  // Field e:I
      41: sipush        1000
      44: iadd
      45: ireturn
}

When a method is inovked a memory space in the stack called current frame is allocated

If you look carefully even int a=-111; assignment happens in an implicit init function Abc() !

       int a = -111;

       5: bipush        -111
       7: putfield      #2                  // Field a:I

As field variable e is not assigned any value it will be 0 if primitive or null if a Object reference

And if you look at doSomething()

        int a = 10;
        0: bipush        10

for a local to be used the initial value needs to be pushed into stack in this case 10 . without this 'push' [initialization] a's value is not accessible to subsequent statements (as the value is not on the stack). once the value is pushed to stack other operations like iadd istore etc are carried out on the stack

below statement actually creates an object on the heap space and invokes init method. This is where un initialized variables like 'e' gets default values

      15: new           #3                  // class Abc
      18: dup

I leave further bytecode comparison upto you ;) but I hope it is clear

Answer 4

tl;dr: It was more or less an arbitrary choice

If you ask me, it was a mistake that Java has default values for instance variables. The compiler should have forced the programmer to initialize it before like it is the case for local variables.

The rationale behind the default values is safety. When an object is instantiated, a chunk of memory will be allocated for the object which contains where the instance variables are pointing to etc. The Java designers decided it would be a good idea to wipe this part of memory with zeros and nulls. This way you will never read garbage that happened to be there before the object was allocated. They could have forced initialization; there is nothing fundamental about the choice. It probably made things easy to implement and made enough sense to the designers of Java.

In case of local variables, the designers chose to force initialization (or perhaps it's more accurate to say they chose to not do any kind of initialization when a local variable is only declared, and thus the most logical behavior of the compiler was to force initialization of the variable before use).