Why do instance variables have default values in java? [duplicate]

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.