logic behind not able to assign a static value to a non static variable

[Pratik D mehta]

class sample18
{
    static int a =10;
}
public class sample17
{
    int b = 5;
public static void main(String args[])
    {
    sample18 s1 = new sample18();
    b = sample18.a;//non-static variable cannot be accessed from a static context
    //I dont get the logic
      
}
}    

I dont get the logic behind this
I think it should be possible to assign a value of static variable to a non-static variable .

[Vishal Hegde]

Pratik D mehta wrote:

class sample18
{
    static int a =10;
}
public class sample17
{
    int b = 5;
public static void main(String args[])
    {
    sample18 s1 = new sample18();
    b = sample18.a;//non-static variable cannot be accessed from a static context
   

[Vishal Hegde]

Hi,

b is not a static variable while a is thats why you are getting that error
\
Try decalring b in main and see the differnce

[Pratik D mehta]

Vishal Hegde wrote:Hi,

b is not a static variable while a is thats why you are getting that error
\
Try decalring b in main and see the differnce

Yes vishal I understand that , if declared in main it will behave as a local variable , but
I just wanted to understand
why we cant assign the nonstatic variable a static value , rather than remembering it .

I assume its just a part of java , maybe there is no logic.

[Raymond Tong]

class sample18  
{  
	static int a =10;  
}  
class sample17  
{  
	int b = 5;  
	public static void main(String args[])  
	{  
		sample18 s1 = new sample18();  
		//b = sample18.a; // this won't compile
		sample17 s2 = new sample17();
		s2.doSomething();
		(new sample17()).b = sample18.a; // this would
	}
	public void doSomething() {
		b = sample18.a; // this would
	}
}

Because you don't hold any instance of sample17, you could not access its instance variable.

[Vishal Hegde]

Raymond Tong wrote:

class sample18  
{  
	static int a =10;  
}  
class sample17  
{  
	int b = 5;  
	public static void main(String args[])  
	{  
		sample18 s1 = new sample18();  
		//b = sample18.a; // this won't compile
		sample17 s2 = new sample17();
		s2.doSomething();
		(new sample17()).b = sample18.a; // this would
	}
	public void doSomething() {
		b = sample18.a; // this would
	}
}

Because you don't hold any instance of sample17, you could not access its instance variable.

Pratik , i hope your query is resolved, Thats what i meant when i was making it local(Subconciously...)

Thnks Raymond You rock Cheers (FYI: I dont drink beer )

[Pratik D mehta]

Thank you vishal and raymond so there is no logic behind - that you cannot give a static variable value to the instance variable directly .

Like for example Change ( the instance variable ) so that from that statement onwards when every new object is created it will get the new value .

It is just the way how java works .

class sample18
{
     int a =10;
}
public class sample17
{
     
    int b = 5;
public static void main(String args[])
    {
    
    sample17 s1 = new sample17();
    sample17 s2 = new sample17();
    //up to here the value of instance variable b for every object created of class Sample17 is 5 
    
    // Now suppose I want to change the value of b so that every object created after 
    // this will get a different value of b
    
    //This can only be done if b is static ? Am I right
    // This is how java works .
    
    b=a;//not possible
    b=10;// not possible
    
    //yup  
    
    // Please give a nodd on this , Thankyou
      
}
}    

[Raymond Tong]

Let me use another example with more meaningful names to help you understand.

class Person {
	static int totalAsset = 0;
	
	String name;
	int age = 10;
	
	Person(String theName) {
		name = theName;
		totalAsset = totalAsset + 500;
	}
}
public class Test {
	static int newAge = 20;
	static int newAsset = 700;
	
	public static void main(String[] args) {
		Person joe = new Person("Joe");
		Person tim = new Person("Tim");
		// now you have Joe and Tim, they should have different age (instance variable)
		// total asset should be shared (static variable)
		joe.age = newAge;
		// Joe's age has been changed, Tim's age remain unchanged
		System.out.println("Joe's age is = " + joe.age); //20
		System.out.println("Tim's age is = " + tim.age); //10
		
		System.out.println("Total asset = " + Person.totalAsset); //1000
		Person.totalAsset = newAsset;
		// the total asset been updated
		// change applied to all instances
		System.out.println("Total asset = " + Person.totalAsset); //700
		System.out.println("Total asset = " + joe.totalAsset); //700
		System.out.println("Total asset = " + tim.totalAsset); //700
	}
}

[David Newton]

Pratik D mehta wrote:so there is no logic behind - that you cannot give a static variable value to the instance variable directly .

Of course there's logic behind it--there's no instance variable available to assign a value to.

public class Foo {
    public String iString = "wat";  // Instance variable
    public String sString = "ohai"; // Class variable (static)
    
    public void instanceMethod() {
        S.o.p(iString);      // We're in an instance, so can access its properties.
        S.o.p(sString);      // Instances can access class properties, too.
    }

    public static void main(String[] args) { // Class method (static)
        S.o.p(sString);
        S.o.p(iString);      // Exception: there's no instance of Foo, so we can't access instance properties.
        
        Foo f = new Foo();
        S.o.p(f.iString);    // *Now* we have an instance: now there are instance properties.
        f.iString = sString; // ...But they must be accessed through a reference to the instance.
        S.o.p(f.iString);
        
        f.instanceMethod();  // Instance methods can access both instance and class properties.
    }
}

Short version: You have to have an instance before you can access an instance's properties.

[Pratik D mehta]

yup thank you david and raymond .