General questions related to Java

Question

Good evening,I would like to ask you some questions related to Java :
1) class A {
public int x=1;
public int y=5;
public A() { y=6; }
public A(int a) { x=a; }
public A(int a, int b) { x=a; y=b; }
class B extends A {
public B() {}
public B(int a) { super(a); }
public B(int a, int b) { super(a,b); }
public B(int a, int b, int c) { x=a+b+c; }
public B(int a, int b, int c, int d) {
super(a,b); y=c+d; }
public class Test {
public static void main(String[] args) {
B b4 = new B(1,1,1);
System.out.println("b4.x: "+b4.x+" b4.y: "+b4.y);
I cannot understand why y = 6.
As i see it,i find it more logical that y = 5.
Could anybody tell me why y becomes 6?
How did we get into the constructor of A : public A() in order y to be changed?
2) Well in an exercise i have to handle out we have to find the a,b,c of a program .Unfortunately i dunno about them,Can anybody explain to me what do they mean or attach me a link (e.g. a tutorial) in order to figure out?
a)pre condition in Java
b)post condition in Java
c)invariant condition
3)
3) Fraction apple = new Fraction (1, 2);
Fraction peach = new Fraction (4, 5);
Fraction pear = apple;
peach.halve();
pear.halve();
System.out.println(apple);
System.out.println(peach);
System.out.println(pear);
It's the first time i see an object as a parameter in System.out.println.
CAn anybody tell me when i can pass an object as an argument in System.out.println?
Thanks in advance!

Answer 1

How did we get into the constructor of A : public A() in order y to be changed?if you don't explicitly call a super constructor, the empty super constructor is called for you. If no empty accessible super constructor is available and no explicit call from subclass is made - compile error.
CAn anybody tell me when i can pass an object as an argument in System.out.println?Whenever you want. Object.toString() will be called and the resulting String will be used. If the object is null, the String "null" will be used instead.