Class as the basis of all computation, explained for ICSE Class 10.

int p = 5;
int q = p;          // q gets a copy of 5
q = 8;
System.out.println(p + " " + q);

Mug m1 = new Mug();
Mug m2 = m1;        // m2 refers to the same mug
m2.ml = 250;
System.out.println(m1.ml);

Mug m3 = new Mug();
Mug m4 = new Mug();
m3.ml = 300;
m4.ml = 300;
System.out.println((m3 == m4) + " " + (m3.ml == m4.ml));

// Output
5 8
250
false true

Copying a primitive copies the value. Copying a class variable copies only the reference, so both names reach one object. == on two objects asks "same object?", not "same contents?".

import java.util.Scanner;
class WaterTank {
    int capacity;          // instance variable: litres the tank holds
    int level;             // instance variable: litres in it now
    static int tanks = 0;  // class variable: tanks made so far

    WaterTank(int cap) {
        capacity = cap;
        level = 0;
        tanks++;
    }

    void fill(int litres) {
        level = level + litres;
        if (level > capacity) {
            System.out.println("Overflow of " + (level - capacity) + " litres");
            level = capacity;
        }
    }

    void use(int litres) {
        if (litres > level)
            System.out.println("Only " + level + " litres available");
        else
            level = level - litres;
    }

    void display() {
        double pct = level * 100.0 / capacity;
        System.out.println(level + " of " + capacity + " litres (" + pct + "%)");
    }

    public static void main(String args[]) {
        Scanner sc = new Scanner(System.in);
        System.out.print("Capacity of tank A and tank B: ");
        WaterTank a = new WaterTank(sc.nextInt());
        WaterTank b = new WaterTank(sc.nextInt());
        System.out.print("Litres to pour into A, then into B: ");
        a.fill(sc.nextInt());
        b.fill(sc.nextInt());
        System.out.print("Litres to draw from A: ");
        a.use(sc.nextInt());
        a.display();
        b.display();
        System.out.println("Tanks created: " + WaterTank.tanks);
    }
}

Each tank keeps its own level, but tanks is one counter for the class, read through the class name in main. Multiplying by 100.0 makes the percentage a double.

import java.util.Scanner;
class Batsman {
    String name;              // instance: player's name
    int runs;                 // instance: runs scored
    int balls;                // instance: balls faced
    static int teamRuns = 0;  // class variable: shared by all players

    Batsman() {
        name = "";
        runs = 0;
        balls = 0;
    }

    void accept(Scanner sc) {
        System.out.print("Name, runs and balls: ");
        name = sc.next();
        runs = sc.nextInt();
        balls = sc.nextInt();
        teamRuns = teamRuns + runs;
    }

    double strikeRate() {
        if (balls == 0)
            return 0.0;
        return Math.round(runs * 10000.0 / balls) / 100.0;
    }

    void display() {
        System.out.println(name + "\t" + runs + "\t" + balls + "\t" + strikeRate());
    }

    public static void main(String args[]) {
        Scanner sc = new Scanner(System.in);
        Batsman p1 = new Batsman();
        Batsman p2 = new Batsman();
        Batsman p3 = new Batsman();
        p1.accept(sc);
        p2.accept(sc);
        p3.accept(sc);
        System.out.println("Name\tRuns\tBalls\tStrike rate");
        p1.display();
        p2.display();
        p3.display();
        System.out.println("Team total = " + Batsman.teamRuns);
    }
}

One Scanner is made in main and passed to accept, so all three objects read from the same keyboard. A method that returns a value (strikeRate) is called inside display.