Returning String Methods in Java?

Question

I'm in a beginning programming class, and a lot of this had made sense to me up until this point, where we've started working with methods and I'm not entirely sure I understand the "static," "void," and "return" statements.

For this assignment in particular, I thought I had it all figured out, but it says it "can not find symbol histogram" on the line in the main method, although I'm clearly returning it from another method. Anyone able to help me out?

Assignment: "You see that you may need histograms often in writing your programs so you decide for this program to use your program 310a2 Histograms. You may add to this program or use it as a class. You will also write a class (or method) that will generate random number in various ranges. You may want to have the asterisks represent different values (1, 100, or 1000 units). You may also wish to use a character other than the asterisk such as the $ to represent the units of your graph. Run the program sufficient number of times to illustrate the programs various abilities.

Statements Required: output, loop control, decision making, class (optional), methods.

Sample Output:

Sales for October

Day Daily Sales Graph

2 37081 *************************************

3 28355 ****************************

4 39158 ***************************************

5 24904 ************************

6 28879 ****************************

7 13348 ************* "

Here's what I have:

import java.util.Random;

public class prog310t
{ 
  public static int randInt(int randomNum) //determines the random value for the day
  {   
    Random rand = new Random();

    randomNum = rand.nextInt((40000 - 1000) + 1) + 10000;

    return randomNum;
  }

  public String histogram (int randomNum) //creates the histogram string
  {
    String histogram = "";
    int roundedRandom = (randomNum/1000);
    int ceiling = roundedRandom;
    for (int k = 1; k < ceiling; k++)
    {
      histogram = histogram + "*";
    }

    return histogram;   
  }

  public void main(String[] Args)
  {
    System.out.println("Sales for October\n");

    System.out.println("Day        Daily          Sales Graph");

    for (int k = 2; k < 31; k++)
    {
      if (k == 8 || k == 15 || k == 22 || k == 29)
      {
        k++;
      }

      System.out.print(k + "         ");

      int randomNum = 0;

      randInt(randomNum);

      System.out.print(randomNum + "       ");

      histogram (randomNum);

      System.out.print(histogram + "\n");
    }
  }
}

Edit: Thanks to you guys, now I've figured out what static means. Now I have a new problem; the program runs, but histogram is returning as empty. Can someone help me understand why? New Code:

import java.util.Random;

public class prog310t
{ 
  public static int randInt(int randomNum) //determines the random value for the day
  {   
    Random rand = new Random();

    randomNum = rand.nextInt((40000 - 1000) + 1) + 10000;

    return randomNum;
  }

  public static String histogram (int marketValue) //creates the histogram string
  {
    String histogram = "";
    int roundedRandom = (marketValue/1000);
    int ceiling = roundedRandom;
    for (int k = 1; k < ceiling; k++)
    {
      histogram = histogram + "*";
    }

    return histogram;   
  }

  public static void main(String[] Args)
  {
    System.out.println("Sales for October\n");

    System.out.println("Day        Daily          Sales Graph");

    for (int k = 2; k < 31; k++)
    {
      if (k == 8 || k == 15 || k == 22 || k == 29)
      {
        k++;
      }

      System.out.print(k + "         ");

      int randomNum = 0;

      int marketValue = randInt(randomNum);

      System.out.print(marketValue + "       ");

      String newHistogram = histogram (randomNum);

      System.out.print(newHistogram + "\n");
    }
  }


}

Answer 1

You're correct that your issues are rooted in not understanding static. There are many resources on this, but suffice to say here that something static belongs to a Class whereas something that isn't static belogns to a specific instance. That means that

public class A{
    public static int b;

    public int x;
    public int doStuff(){
        return x;
    }

    public static void main(String[] args){
        System.out.println(b); //Valid. Who's b? A (the class we are in)'s b.
        System.out.println(x); //Error. Who's x? no instance provided, so we don't know.
        doStuff(); //Error. Who are we calling doStuff() on? Which instance?

        A a = new A();
        System.out.println(a.x); //Valid. Who's x? a (an instance of A)'s x.
    }
}

So related to that your method histogram isn't static, so you need an instance to call it. You shouldn't need an instance though; just make the method static:

Change public String histogram(int randomNum) to public static String histogram(int randomNum).

With that done, the line histogram(randomNum); becomes valid. However, you'll still get an error on System.out.print(histogram + "\n");, because histogram as defined here is a function, not a variable. This is related to the return statement. When something says return x (for any value of x), it is saying to terminate the current method call and yield the value x to whoever called the method.

For example, consider the expression 2 + 3. If you were to say int x = 2 + 3, you would expect x to have value 5 afterwards. Now consider a method:

public static int plus(int a, int b){
    return a + b;
}

And the statement: int x = plus(2, 3);. Same here, we would expect x to have value 5 afterwards. The computation is done, and whoever is waiting on that value (of type int) receives and uses the value however a single value of that type would be used in place of it. For example:

int x = plus(plus(1,2),plus(3,plus(4,1)); -> x has value 11.

Back to your example: you need to assign a variable to the String value returned from histogram(randomNum);, as such:

Change histogram(randomNum) to String s = histogram(randomNum).

This will make it all compile, but you'll hit one final roadblock: The thing won't run! This is because a runnable main method needs to be static. So change your main method to have the signature:

public static void main(String[] args){...}

Then hit the green button!