Is there any particular relation between class instance (e.g variables that declared by static/final) and static methods(e.g class methods) ?
I hop you understand what i mean.
4 Answers
I think what you talk about is static variable instead of class instance (no this term), one association between static variable and static method I can think of is that, in your static method you can only access static variables rather than instance variables.
Comments
Static and non-static methods/classes are entirely different beasts. Let's use an example of object-oriented programming. Say I have a class called "Person", with a constructor called Person(), with the instance variables declared as myAge (set as 0), and a method called Birthday():
int myAge;
public Person(){
myAge = 0;
}
public void Birthday(){
myAge += 1;
}
In my main method, this class would be used like this:
Person Sajjad = new Person();
Sajjad.Birthday();
When I create a new person, which is you, your age is 0, because our constructor's default values are 0 for myAge. Once I apply the Birthday() method on you, your age goes up by one, through the increment.
As for static methods, there's no object-oriented principles in it. There are uses for it.. I usually use it for simple math.
public static double addValues(double a, double b){
return a + b;
}
In my main:
int sum;
sum = addValues(1, 2);
System.out.println(sum);
Output:
3
See how above there's no need to declare a new objects? Static methods are easier to prototype, but in the long run I definitely go with object-oriented principles because it makes maintaining code in the long run so much easier. Also, I don't need to clutter my main method with unnecessary lines of code.
P.S. If the code is wrong, it's really just some pseudo code I whipped up.
Comments
Class instances are objects stored in memory and referenced through a variable of the Type of the class. Each element has its own state and its own methods. Ex: if we have a Apple class and a getColor non-static element:
Apple a = new Apple("green");
Apple b = new Apple("red");
a.getColor();//green
b.getColor();//red
Static methods/variables are elements related to that class and acceded via the name of the class. You can't access static elements from an instance. Ex: if we have a Apple class and a size static element:
Apple.tree; //Apple Tree
Here's class Apple
class Apple {
public static String tree = "Apple Tree"; // Class related
private String color; //object instance related
public Apple(String color) {
this.color = color;
}
public String getColor() {
return this.color;
}
}
So, having this class color will depend on the instance.
1 Comment
theGreenCabbage
I think OP is asking about Java instead of C#. The lack of brackets after
Apple.size makes me think you're referring to object properties instead of what OP is trying to learn.Access:
class instance (i.e variables that declared by static/final) and static methods can work together without explicit need of object reference. Whereas to access member variable (aka instance variables) you need to access via object reference.
class Test {
static String str = "asdf";
int num = 10;
static void methodA() {
System.out.println("Static variable direct access: " + str);
System.out.println("Member variable access : " + new Test().num);
}
}
Concurrency:
Other difference is during concurrent access or thread-safety logic, class-level-lock can be taken via class-variables in static-methods and object-level-lock can be taken via member-variables in member-methods though you can mix-n-match ofcourse.
class Test {
static Object CLASS_LOCK = new Object();
Object MEMBER_LOCK = new Object();
Test sharedTest = new Test(); // or created in different manner
static void methodA() {
synchronized (CLASS_LOCK) {
// .. thread-safe code
}
synchronized (sharedTest.MEMBER_LOCK) {
// .. thread-safe code
}
}
}
finalmodifier has nothing to do with if a variable is astatic(or class) variable or if it's an instance variable.