Java practise exam question

 return name + "(" + age + ")";

to

return name + " (" + age + ")";

That's fine, but I would have written the last method as return (age >= 18);

I don't think it's a good idea to write toString method with arguments - doing so you will not override the default Object.toString and will have no benefits from using it outside your code. For example, when outputting elements of a collection which contain Person won't display neither age nor name, but something like Person@a3f33f (default implementation). At least provide toString with no arguments as well

Your code is almost fine. I would change two things:

The following is a little shorter :)

public boolean canVote()
{
    return age >= 18;
}

Your toString(true) would return "Sue(21)" rather than the "Sue (21)" (with space) which is asked for. My toString would look like this:

public String toString(boolean full)
{
    String str = name;
    if(full) {
       str += " (" + age + ")";
    }

    return str;     
 }

If the age is implemented as a primitive integer there is no need to initialize it to 0 because all primitive number instance variables are initialized to 0 by default. Anyway if you had used a wrapper instead (Integer) the initialization to 0 is required.

Take a look at the "Default Values" section:

http://download.oracle.com/javase/tutorial/java/nutsandbolts/datatypes.html

In addition to the other comments, on your constructor, having an argument called "st" isn't particularly useful to the reader as it doesn't tell them what it's for. From a code style perspective might be better to have an argument called name, and explicitly set this.name = name in the body.

How about adding a constant for 18, say MIN_AGE_TO_VOTE?

And:

return name + (full ? " (" + age + ")" : "");

Less readable, I agree :-)

Use String.format instead of string concatenation:
return name + "(" + age + ")"; becomes

return String.format("%s (%s)", name, age);

1. The name field could be final. Making a variable final relieves the programmer of excess mental juggling - he/she doesn't have to scan through the code to see if the variable has changed. (From @nerdytenor's answers.)

2. You should check the argument in the constructor. Does it make sense to create a Person object with a name which is null or an empty String? If not, check it and throw a NullPointerException or an IllegalArgumentException. (Effective Java, Second Edition, Item 38: Check parameters for validity)

3. A note about the specification:

   Flag arguments are ugly. Passing a boolean into a function is a truly terrible practice. It immediately complicates the signature of the method, loudly proclaiming that this function does more than one thing. It does one thing if the flag is true and another if the flag is false!

   Source: Clean Code by Robert C. Martin, Chapter 3: Functions.

   This should be two methods: toString() and toFullString(), for example, without any parameter.

Looks fine, only canVote is maybe a bit clumsy:

   public boolean canVote()
   {
        return age >= 18;
   }

I would see if you can make the code less verbose.

public boolean canVote() {
    return age >= 18;
}

This is also less verbose

public int incrementAge() {
  return ++age;
}

public Person(String st)
{
  name = st;
  age = 0;
}

st is not a good name for a variable or parameter, because it does not tell you what it contains. It implies that it means "string", but even that is not clear. Call it name instead. Use this.name = name to distinguish member and parameter names.

Put the opening brackets on the first line of the block, not at the beginning of the next line, since it is common practise in Java (even though I find it prettier if it is on the next line, since I code C# more than Java). Other Java developers reading your code will be used to it.

Others have already pointed it out: Write if (full) instead of if (full == true). That is clearer and easier to read. You can use the ternary operator for a one liner:

return full ? name + "(" + age + ")" : name;.

Your incrementAge() looks fine.

   public boolean canVote()
   {
     if(age >= 18)
     {
        return true;
     }
     else
     {
        return false;
     }
   }

Your indentation is weird. Use 4 spaces (less common tabs with width of 4 spaces) for indentation. You have sometimes 3 spaces, sometimes 2.

Since age >= 18 is a boolean expression, you can return the result just like this:

return age >= 18;

Use spaces after if and for (also while and similar) keywords. That makes the code clearer and more readable:

if (someCondition) {
    doThis();
}