Encapsulation for ICSE Class 10: who can see what, and for how long.

package school;
public class Locker {
    private int code = 4821;
    int row = 3;                  // no keyword: default
    protected String owner = "Tara";
    public int number = 17;
}

Each member is then printed from Office (package school), SportsLocker (package other, extends Locker) and Visitor (package other, unrelated).

school/Office.java:5: error: code has private access in Locker
other/SportsLocker.java:5: error: code has private access in Locker
other/SportsLocker.java:6: error: row is not public in Locker; cannot be accessed from outside package
other/Visitor.java:6: error: code has private access in Locker
other/Visitor.java:7: error: row is not public in Locker; cannot be accessed from outside package
other/Visitor.java:8: error: owner has protected access in Locker

Every line that is not listed compiled. public number was readable from all three places; private code from none of them.

class Ticket {
    static int sold = 0;   // class variable
    int seat;              // instance variable

    Ticket(int s) {        // s: argument variable
        seat = s;
        sold++;
    }

    int price(int rows) {  // rows: argument variable
        int base = 150;    // local variable
        return base + rows * 20;
    }

    public static void main(String[] args) {
        Ticket a = new Ticket(12);
        Ticket b = new Ticket(30);
        System.out.println(a.seat + " " + b.seat + " " + Ticket.sold);
        System.out.println(a.price(3));
    }
}

// Output:
12 30 2
210

Two objects, so two seats but one sold counter. base and rows vanish when price returns.

class Shadow {
    int marks = 40;

    void update(int marks) {
        marks = marks + 5;
        System.out.println(marks + " " + this.marks);
    }

    void fix(int marks) {
        this.marks = marks;
    }

    public static void main(String[] args) {
        Shadow s = new Shadow();
        s.update(70);
        s.fix(62);
        System.out.println(s.marks);
    }
}

// Output:
75 40
62

Inside update, plain marks means the parameter, so the member stays 40. this.marks always means the member, which is how fix reaches it.

int sum = 0;
for (int i = 1; i <= 5; i++)
    sum += i;
System.out.println(sum + " " + i);   // line 6

// javac says:
Loop.java:6: error: cannot find symbol
  symbol:   variable i

i was declared in the for header, so it ends with the loop. Declare it before the loop if it is needed afterwards.

void show(int x) {
    int x = 5;                       // line 3
    System.out.println(x);
}

// javac says:
Dup.java:3: error: variable x is already defined in method show(int)

A local variable may hide a data member, but it may not reuse the name of a parameter or another local in the same method.