Calendar implementation: trouble to add days to date

My task is:
Implement the method public void advance() that moves the date by one day. In this exercise we assume that each month has 30 day. NB! In certain situations you need to change the values of month and year.

I have done this:

public class SimpleDate {
private int day;
private int month;
private int year;
public SimpleDate(int day, int month, int year) {
this.day = day;
this.month = month;
this.year = year;
}
@Override
public String toString() {
return this.day + "." + this.month + "." + this.year;
}
public boolean before(SimpleDate compared) {
if (this.year < compared.year) {
return true;
}
if (this.year == compared.year && this.month < compared.month) {
return true;
}
if (this.year == compared.year && this.month == compared.month &&
this.day < compared.day) {
return true;
}
return false;
}
public void advance(){
if(this.day<30){
this.day += 1;
}    
if(this.day==30 && this.month==12){
this.day = 1;
this.month=1;
this.year +=1;
}
if(this.day==30){
this.day = 1;
this.month +=1;
}
}
public void advance(int howManyDays){
if(howManyDays < 30){
if(this.day<30 && (this.day + howManyDays < 30)){
this.day += howManyDays;
}
if(this.day==30 && this.month==12){
this.day = howManyDays;
advance();
}
if(this.day==30){
this.day = howManyDays;
this.month +=1;
} 
}
if(howManyDays== 30 && this.month==12){
this.day = howManyDays;
advance();
}
if(howManyDays == 30){
this.month += 1;
}
}
public SimpleDate afterNumberOfDays(int days){
SimpleDate obj = new SimpleDate(days, this.month,this.year);
return obj;
}
}

But when I test it with:

SimpleDate date = new SimpleDate(30, 12, 2011);  date.advance(3);

Now the date should be 3.1.2012.
expected:[3.1.2012] but was:[4.12.2011]

This happens because you set the date and than call advance(). So the day changed to 3 but now you advance is called with the date 3.12.2011 which results in 4.12.2011 as it should.

You can solve it in 2 ways:

inefficient way

Just call advance, howManyDays times:

public void advance(int howManyDays){
for(int i = 1; i <= howManyDays; i++)
advance();
}

Efficient way

this is a general way that will work for every amount of days (even large number of days):

public void advance(int howManyDays){
if(howManyDays + this.day <= 30){
this.day += howManyDays;
return;
}
int monthsToAdd = (howManyDays + this.day) / 30;
int yearsToAdd = (monthsToAdd + this.month) / 12;
int day = (howManyDays + this.day) % 30;
int month = (monthsToAdd + this.month) % 12;
if(month == 0) month = 12;
this.day = day;
this.month = month;
this.year += yearsToAdd;
}
public void advance(){
advance(1);
}

I recommend just simplifying your approach on this with while loops instead of the multiple and nested if statements...

public void advance(int howManyDays) {
this.day += howManyDays;
//Subtracts out the days till within range, incrementing months
while(this.day > 30) {
this.day -= 30;
this.month++;
}
//Subtracts out the months till within range, incrementing years
while(this.month > 12) {
this.month -= 12;
this.year++;
}
}

In your test case, you pass in this block:

        if(this.day==30 && this.month==12){
this.day = howManyDays;
advance();
}

When entering this block, you have day==30, month==12, howManyDays==3.

• You set day to howManyDays, so now day==3.
• You call advance(), as day is 3, day is incremented then the advance()method returns.
• There is no other valid ifcondition, advance(3)method returns.

Only the dayfield has changed.

Call advance()before setting the day and it should works.

        if(this.day==30 && this.month==12){
advance(); // go to the next day, change the month/year if necessary
this.day = howManyDays-1; // advance() already goes one day forward
}

There are other issues with your code but I let you find them and fix them. (ex: what if dayis 27 and howManyDays is 3 ?)

I strongly suggest you to learn to use a step-by-step debugger to handle those easy logic issues. That's a valuable and very useful skill.