How to know that a method has been called and returning value of a method

Question

Hi, everyone! I have two questions. One is about making judgment about whether a method has been called or not; another one is about how to return "String value+newline character+String value" with a return statement.
Here are the two original problems that I tried to solve.
Write a class definition of a class named 'Value' with the following:
a boolean instance variable named 'modified', initialized to false
an integer instance variable named 'val'
a constructor accepting a single paramter whose value is assigned to the instance variable 'val'
a method 'getVal' that returns the current value of the instance variable 'val'
a method 'setVal' that accepts a single parameter, assigns its value to 'val', and sets the 'modified' instance variable to true, and
a boolean method, 'wasModified' that returns true if setVal was ever called.
And I wrote my code this way:
public class Value
boolean modified=false;
int val;
public Value(int x)
{val=x;}
  public int getVal()
  {return val;}
   public void setVal(int y)
    val = y;
    modified = true;
     public boolean wasModified()
      if(val==y&&modified==true)
      return true;
}I tried to let the "wasModified" method know that the "setVal" has been called by writing:
if(val==y&&modified==true)
or
if(x.setVal(y))
I supposed that only when the "setVal" is called, the "modified" variable will be true(it's false by default) and val=y, don't either of this two conditions can prove that the method "setVal" has been called?
I also have some questions about the feedback I got
class Value is public, should be declared in a file named Value.java
public class Value
cannot find symbol
symbol  : variable y
location: class Value
if(val==y&&modified==true)
*^*
*2 errors*
I gave the class a name Value, doesn't that mean the class has been declared in a file named Value.java*?
I have declared the variable y, why the compiler cann't find it? is it because y has been out of scale?
The other problem is:
Write a class named  Book containing:
Two instance variables named  title and  author of type String.
A constructor that accepts two String parameters. The value of the first is used to initialize the value of  title and the value of the second is used to initialize  author .
A method named  toString that accepts no parameters.  toString returns a String consisting of the value of  title , followed by a newline character, followed by the value of  author .
And this is my response:
public class Book
String title;
String author;
  public Book(String x, String y)
   { title=x; author=y; }
   public String toString()
   {return title;
    return author;
}I want to know that is it ok to have two return statements in a single method? Because when I add the return author; to the method toString, the compiler returns a complain which says it's an unreachable statement.
Thank you very much!

Answer 1

Lets take this slow and easy. First of all, you need to learn how to format your code for readability. Read and take to heart
{color:0000ff}http://java.sun.com/docs/codeconv/html/CodeConvTOC.doc.html{color}
Now as to your first exercise, most of it is OK but not this:   public boolean wasModified()
      if (val == y && modified == true)
            return true;
y being a parmeter to the setValue method exists only within the scope of that method. And why would you want to test that anyways? If modified evaluates to true, that's all you need to know that the value has been modified. So you could have   public boolean wasModified()
      if (modified == true)
            return true;
   }But even that is unnecessarily verbose, as the if condition evaluates to true, and the same is returned. So in the final analysis, all you need is   public boolean wasModified()
      return modified;
   }And a public class has to be declared in a file named for the class, yes.
As for your second assignment, NO you cannot "return" two variables fom a method. return means just that: when the return statement is encountered, control returns to the calling routine. That's why the compiler is complaining that the statement following the (first) return statement is unreachable.
Do you know how to string Strings together? (it's called concatenation.) And how to represent a newline in a String literal?
db