1. The Main Method¶

Traditionally, the first program you write when learning a new programming language is called the hello world program. All it does is display the words “Hello, World!” on the screen. In Java, it looks like this:

public class HelloWorld
{
    public static void main(String[] args)
    {
        // generate some simple output
        System.out.println("Hello, World!");
    }
}

We know from previous modules that the single line of code inside this method will print out the phrase “Hello, World!”

This program defines one method named main() with the following signature:

public static void main(String[] args)

A method declared with this exact signature is special: it can be used as the entry point for executing a standalone Java program. The HelloWorld program is an example of a Java application program, or a Java application for short, An application program is a stand-alone program in the sense that it does not depend on any other program to run. In Java, any class you write can contain its own stand-alone program, if it provides its own declaration of a main() method with this special signature.

Every Java application program must have a main() method (with exactly that method signature) which is the place where the program begins executing when it is run. For a program that contains many classes, it is up to the programmer which class should contain the main() method. Further, when you “start” or “execute” a Java application, execution begins with the main() method rather than by creating an object–it is then up to main() to create all the objects needed by your program.

As mentioned above, the method signature for this method is very specific. This is because of the very special role this method has in the program. The method signature must be exactly:

public static void main(String[] args)

Many of the keywords we see here you’ll recognize.

• public means this method is accessible to other classes in the program.

• void means this method does not return anything.

• main is the name given to this method.

• String[] args means that this method takes in an array of Strings called args.

The String array args is actually a feature that will allow a user to pass String arguments to the program when it starts up. For the most part, we won’t worry about making use of this array in this class. However, it is still part of the method signature and can’t be considered a main method without it. When a Java program is executed in a stand-alone way using a command line, the arguments given on the command line are used to populate this array, so they are accessible inside main().

2. Check Your Understanding: Main Methods¶

3. The Static Keyword¶

In English, when we say something is static we mean it does not move or change. This is the same in Java. By contrast, a computer program is dynamic. It changes. It does things and performs certain actions.

Once put into place, we know that anything with the keyword static will mean that it will not change. The static keyword is a modifier you can place on declarations within a class, and it means that the corresponding declaration will belong to a class itself, rather than a specific object (an instance of the class).

Lets take a look at what we mean by that:

public class Ball
{
    private String color;

    public Ball()
    {
        this.color = "Red";
    }

    public void setColor(String newColor)
    {
        this.color = newColor;
    }

    public String getColor()
    {
        return this.color;
    }
}

This code declares a Ball class. When initialized, all Ball objects have their color field set to “Red”, but a setter method has been provided so we could change that to another color. The field color “belongs” to the object, since each individual instance of Ball has its own color field, and each instance can store a different value in its own color field, separate from all other Balls. To access or change this field though, we need to create a Ball object to work with.

Ball b1 = new Ball();               // creates a red Ball
b1.setColor("Blue");                // changes the color to Blue
System.out.println(b1.getColor());  // prints out the string "Blue"

Now, lets assume that all Ball objects we want to must have exactly the same diameter–a diameter of 15 centimeters. This might be a good application for using the static keyword.

public class Ball
{
    private String color;
    private static int diameter = 15;

    // ...
}

The main reason to use the static keyword is when you want to define a common property of the entire class, rather than just a single object. Here, because diameter is declared as static, there is just one diameter field stored in the class itself, and that single copy is shared by all objects created from the class. Rather than each object storing its own diameter, there is just one place where this value is stored–in the class itself. The static modifier is used when you want a single resource (or method) to be shared by all objects in the class.

Then, when accessing a static method or field, you talk about it differently than regular fields that are stored within the object itself. For example, consider a toString() method for the Ball class:

public String toString()
{
    return this.color + " ball with diameter " + Ball.diameter;
}

Here, when referring to the color field, we use this.color, where this refers to the current object, which owns the field color. We are talking about the color value stored in the current object. However, when referring to diameter, it does not belong to the current object, and instead belongs to the class. Therefore, we use the class name before the dot (.) to refer to it: Ball.diameter.

Similarly, if we had a class with a main method, we don’t need to create an object of that class to run the main(), since main() is a method that (because it is static) belongs to the class itself, rather than being invoked on an individual object:

public class HelloWorld
{
    public static void main(String[] args)
    {
        // generate some simple output
        System.out.println("Hello, World!");
    }
}

With this declaration, we can call the main() method without creating an object of type HelloWorld.

HelloWorld.main(null);

In short, methods or fields declared static belong to the class itself, rather than to any object created from that class, and all objects created from the class share the same copy of the static field(s) or method(s). Also, when referring to static fields or methods, use the class name itself followed by a dot (.), rather than using this, since they belong to the class rather than any instance of the class.

public class Ball
{
    private String color;
    private static int diameter = 15;

    public static void printData()
    {
        // This line will not work!
        String message = "This ball is " + color + " and has a diameter of "
            + diameter + " cm" ;
        System.out.println(message);
    }
}

public class HelloWorld
{
    public void print()
    {
        System.out.println("Hello, World!");
    }

    public static void main(String[] args)
    {
        // This will not work!
        print();
    }
}

public class Hello
{
    public void print()
    {
        System.out.println("Hello, World!");
    }

    public static void main(String[] args)
    {
        // This will work!
        HelloWorld hObject = new HelloWorld();
        hObject.print();
    }
}

4. Check Your Understanding: The Static Keyword¶

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6. Errors¶

There are two major kinds of errors that can occur in a program: compile-time errors and run-time errors. It is useful to distinguish among them in order to track them down more quickly.

public class HelloWorld
{
    public static void main(String[] args)
    {
        System.out.println("Hello, World!") // there's a missing semi-colon here!
    }
}

File: Hello.java [line: 5]
Error: ';' expected

public class HelloWorld
{
    public static void main(String[] args)
    {
        System.out.println("Hello, World!");
        // there's a missing curly brace here!
}

File: Hello.java [line: 7]
Error:  reached end of file while parsing

Exception in thread "main" java.lang.ArithmeticException: / by zero at HelloWorld.main(HelloWorld.java:5)

7. Throwing Exceptions¶

In Java, errors and other abnormal conditions are handled by throwing and catching exceptions. When an error or an exceptional condition is detected, you can throw an exception. This is like pulling the fire alarm.

Lets assume we had a method that divided one parameter by the other:

public double div(double num, double denom)
{
    return num / denom;
}

However, what if we couldn’t guarantee that the variable denom was going to be non-zero? We could add an if statement that will throw an exception if denom is equal to 0.

public double div(double num, double denom)
    throws Exception
{
    if (denom == 0.0)
    {
        throw new Exception("Tried to divide by 0!");
    }

    return num / denom;
}

Running div(1.0, 0.0) would cause the following text to be printed to the console:

java.lang.Exception: Tried to divide by 0!

Our message is what gets printed out.

Again, throwing an exception is like pulling a fire alarm. It brings everything to a halt. Just like a return statement, when an Exception is thrown, the code will stop running at that line.

We can see the syntax for creating a new Exception above. throw is a keyword in java for throwing exceptions.

You’ll notice the syntax after the word throw looks a lot like how we create objects–because that is exactly what it is doing. An Exception is a kind of object baked into Java. In fact, Exception is a parent class with many different subclasses. In this case, it is perhaps more accurate to use a child class called IllegalArgumentException.

public double div(double num, double denom)
    throws IllegalArgumentException
{
    if (denom == 0.0)
    {
        throw new IllegalArgumentException("Tried to divide by 0!");
    }
    return num / denom;
}

This would produce the error message:

java.lang.IllegalArgumentException: Tried to divide by 0!

You’ll also notice a change to the method signature. There’s additional text after the parameter list but before the opening curly brace. If there’s a path where an exception will be intentionally thrown, we need to add that to the method signature. You won’t be able to compile code without adding these lines to the method signature, which indicate to the caller that an exception might be thrown.

8. Check Your Understanding: Throwing Exceptions¶

9. Syntax Practice: Throwing Exceptions¶

10. Try/Catch Blocks¶

So far, we’ve seen how to throw errors intentionally. However, what if we would like code to fall back to some other case if an error is thrown? Just as we can throw an exception, we can catch one as well. Let us revisit our example:

public double div(double num, double denom)
    throws IllegalArgumentException
{
    if (denom == 0.0)
    {
        throw new IllegalArgumentException("Tried to divide by 0!");
    }
    return num / denom;
}

/**
 * divides two randomly generated numbers
 */
public double divideRandom()
{
    Random rand = Random.generator();
    double numerator = 5.5 * rand.nextInt(5);
    double denominator = 5.5 * rand.nextInt(5);

    double quotient = div(numerator, denominator);
    return quotient;
}

Here, we generate two random doubles then run the div method. However, recall what we know about the code: rand.nextInt(5). This code will generate a random integer between 0 and 4. This means there is a chance either variable could end up being set to the result of 5.5 * 0 or 0. Setting numerator to 0 is no big deal, but if denominator is 0, we’ll see that exception get thrown.

What if we want to gracefully get out of this potential problem without an error getting thrown? We can employ a “try/catch” block.

/**
 * divides two randomly generated numbers
 */
public double divideRandom()
{
    Random r = Random.generator();
    double numerator = 5.5 * r.nextInt(5);
    double denominator = 5.5 * r.nextInt(5);

    double quotient = 0.0;
    try
    {
        quotient = div(numerator, denominator);
    }
    catch (IllegalArgumentException err)
    {
        quotient = div(numerator, 2.0);
    }
    return quotient;
}

The handling of exceptions in Java takes place in two parts. First we try to execute some statements that may or may not lead to an exception. These are enclosed within the try clause.

try
{
    quotient = div(numerator, denominator);
}

Second, we provide one or more catch clauses to handle particular types of exceptions that we expect may occur. In this case, we are only handling one IllegalArgumentException, so we just provide one catch (although you could provide multiple if needed, each for a different type of exception).

catch (IllegalArgumentException err)
{
    quotient = div(numerator, 2.0);
}

The way that this code works is that the code in the try block is run. If it works fine, great–there is nothing extra to do. However, if an IllegalArgumentException is thrown inside the try part, the catch block catches the exception without causing the whole program to grind to a halt. Instead, if that exception is thrown, the variable quotient is assigned to the result of running div(numerator, 2.0).

As we said earlier, throwing an exception is like pulling a fire alarm. The throw occurs somewhere within the scope of the try block. The “fire department” in this case is the code contained in the catch clause that immediately follows the try.

try
{
    // block of statements at least one of which may throw an exception
}
catch (ExceptionClassName parameterName)
{
    // block of statements to be executed if ExceptionClassName is thrown
}
catch (DifferentExceptionClassName otherParameterName)
{
    // block of statements to be executed if DifferentExceptionClassName is thrown
}

try
{
    // block of statements at least one of which may throw an exception
}
catch (ExceptionClassName parameterName)
{
    // block of statements to be executed if ExceptionClassName is thrown
}
catch (DifferentExceptionClassName otherParameterName)
{
    // block of statements to be executed if DifferentExceptionClassName is thrown
}
finally
{
    // optional block of statements that is executed
    // whether or not an exception is thrown
    // this code is always run
}

try
{
    // block of statements at least one of which may throw an exception
}
catch (Exception exc)
{
    // This will catch any possible exception thrown as Exception
    // is the general parent class.
}
catch (IllegalArgumentException illegal)
{
    // This code is not reachable in this format.
}

11. Check Your Understanding: Try/Catch Blocks¶

12. Syntax Practice: Try-Catch Blocks¶