Java: static

1. static keyword can be used in the declaration of variables and methods.
2. A variable declared with static is called class variable.
3. A method declared with static is called class method.
4. Variables/methods without static keyword are called instance variable and instance method.
5. the static keyword cannot be used in class declaration.

Purpose of class variable and methods are:

• class variable has only one copy (1 value). All objects (instances)'s value of the static variable will be the same. Instance variable has a copy/value for each object.
• class method do not need to be instantiated before call. They are called directly, for example: MyClass.f(4)

Following are details.

Class Variable vs Instance Variable

“static” Keyword = Class Variable

Variables can be declared with the static keyword.

static int y = 0;

When a variable is declared with the keyword static, its called a class variable.

• For a class variable, all instances of the class share the same copy of the variable.
• A class variable can be accessed directly with the class, without the need to create a instance. By the syntax class_name.static_var_name.

No “static” Keyword = Instance Variable

Without the static keyword, it's called instance variable, and each instance of the class has its own copy of the variable.

// example of instance vs class variable

class T2 {
    int x = 0; // instance variable
    static int y = 0; // class variable

    void setX (int n) { x = n;}
    void setY (int n) { y = n;}

    int getX () { return x;}
    int getY () { return y;}
}

class T1 {
    public static void main(String[] arg) {
        T2 b1 = new T2();
        T2 b2 = new T2();

        b1.setX(9);
        b2.setX(10);

        // each b1 and b2 has separate copies of x
        System.out.println( b1.getX() );
        System.out.println( b2.getX() );

        // class variable can be used directly without a instance of it.
        //(if changed to T2.x, it won't compile)
        System.out.println( T2.y );
        T2.y = 7;
        System.out.println( T2.y );

        // class variable can be manipulated thru methods as usual
        b1.setY(T2.y+1);
        System.out.println( b1.getY() );
    }
}

Instance Method vs Class Methods

Methods can also be declared with the keyword static.

When a method is declared static, it can be used without having to create a object first.

For example, you can define a collection of math functions in a class, all static, using them like functions.

Methods declared with “static” keyword are called class methods. Otherwise they are instance methods.

// A class with a static method
class M2 { static int triple (int n) {return 3*n;}}

class M1 {
    public static void main(String[] arg) {

        // calling static methods without creating a instance
        System.out.println( M2.triple(4) );

        // calling static methods thru a instance is also allowed
        M2 m1 = new M2();
        System.out.println( m1.triple(5) );
    }
}

Static Methods Cannot Access Non-Static Variables

Methods declared with static cannot access variables declared without static.

The following gives a compilation error, unless x is also static.

class V2 {
    int x = 3;
    static int returnIt () { return x;}
}

class V1 {
    public static void main(String[] arg) {
        System.out.println( V2.returnIt() );
    }
}

~ $ javac V1.java
V1.java:3: error: non-static variable x cannot be referenced from a static context
    static int returnIt () { return x;}
                                    ^
1 error
~ $