Java add method to object

Defining Methods

The only required elements of a method declaration are the method’s return type, name, a pair of parentheses, () , and a body between braces, <> .

More generally, method declarations have six components, in order:

1. Modifiers—such as public , private , and others you will learn about later.
2. The return type—the data type of the value returned by the method, or void if the method does not return a value.
3. The method name—the rules for field names apply to method names as well, but the convention is a little different.
4. The parameter list in parenthesis—a comma-delimited list of input parameters, preceded by their data types, enclosed by parentheses, () . If there are no parameters, you must use empty parentheses.
5. An exception list—to be discussed later.
6. The method body, enclosed between braces—the method’s code, including the declaration of local variables, goes here.

Modifiers, return types, and parameters will be discussed later in this lesson. Exceptions are discussed in a later lesson.

Definition: Two of the components of a method declaration comprise the method signature—the method’s name and the parameter types.

The signature of the method declared above is:

calculateAnswer(double, int, double, double)

Naming a Method

Although a method name can be any legal identifier, code conventions restrict method names. By convention, method names should be a verb in lowercase or a multi-word name that begins with a verb in lowercase, followed by adjectives, nouns, etc. In multi-word names, the first letter of each of the second and following words should be capitalized. Here are some examples:

run runFast getBackground getFinalData compareTo setX isEmpty

Typically, a method has a unique name within its class. However, a method might have the same name as other methods due to method overloading.

Overloading Methods

The Java programming language supports overloading methods, and Java can distinguish between methods with different method signatures. This means that methods within a class can have the same name if they have different parameter lists (there are some qualifications to this that will be discussed in the lesson titled «Interfaces and Inheritance»).

Suppose that you have a class that can use calligraphy to draw various types of data (strings, integers, and so on) and that contains a method for drawing each data type. It is cumbersome to use a new name for each method—for example, drawString , drawInteger , drawFloat , and so on. In the Java programming language, you can use the same name for all the drawing methods but pass a different argument list to each method. Thus, the data drawing class might declare four methods named draw , each of which has a different parameter list.

public class DataArtist < . public void draw(String s) < . >public void draw(int i) < . >public void draw(double f) < . >public void draw(int i, double f) < . >>

Overloaded methods are differentiated by the number and the type of the arguments passed into the method. In the code sample, draw(String s) and draw(int i) are distinct and unique methods because they require different argument types.

You cannot declare more than one method with the same name and the same number and type of arguments, because the compiler cannot tell them apart.

The compiler does not consider return type when differentiating methods, so you cannot declare two methods with the same signature even if they have a different return type.

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Java Class Methods

You learned from the Java Methods chapter that methods are declared within a class, and that they are used to perform certain actions:

Example

Create a method named myMethod() in Main:

myMethod() prints a text (the action), when it is called. To call a method, write the method’s name followed by two parentheses () and a semicolon;

Example

Inside main , call myMethod() :

public class Main < static void myMethod() < System.out.println("Hello World!"); >public static void main(String[] args) < myMethod(); >> // Outputs "Hello World!" 

public class MyClass <
static void myMethod(int x) <
System.out.println(x);
>

public static void main(String[] args) myMethod(10);
>
>

Static vs. Public

You will often see Java programs that have either static or public attributes and methods.

In the example above, we created a static method, which means that it can be accessed without creating an object of the class, unlike public , which can only be accessed by objects:

Example

An example to demonstrate the differences between static and public methods:

public class Main < // Static method static void myStaticMethod() < System.out.println("Static methods can be called without creating objects"); >// Public method public void myPublicMethod() < System.out.println("Public methods must be called by creating objects"); >// Main method public static void main(String[] args) < myStaticMethod(); // Call the static method // myPublicMethod(); This would compile an error Main myObj = new Main(); // Create an object of Main myObj.myPublicMethod(); // Call the public method on the object >> 

Note: You will learn more about these keywords (called modifiers) in the Java Modifiers chapter.

Access Methods With an Object

Example

Create a Car object named myCar . Call the fullThrottle() and speed() methods on the myCar object, and run the program:

// Create a Main class public class Main < // Create a fullThrottle() method public void fullThrottle() < System.out.println("The car is going as fast as it can!"); >// Create a speed() method and add a parameter public void speed(int maxSpeed) < System.out.println("Max speed is: " + maxSpeed); >// Inside main, call the methods on the myCar object public static void main(String[] args) < Main myCar = new Main(); // Create a myCar object myCar.fullThrottle(); // Call the fullThrottle() method myCar.speed(200); // Call the speed() method >> // The car is going as fast as it can! // Max speed is: 200 

Example explained

1) We created a custom Main class with the class keyword.

2) We created the fullThrottle() and speed() methods in the Main class.

3) The fullThrottle() method and the speed() method will print out some text, when they are called.

4) The speed() method accepts an int parameter called maxSpeed — we will use this in 8).

5) In order to use the Main class and its methods, we need to create an object of the Main Class.

6) Then, go to the main() method, which you know by now is a built-in Java method that runs your program (any code inside main is executed).

7) By using the new keyword we created an object with the name myCar .

8) Then, we call the fullThrottle() and speed() methods on the myCar object, and run the program using the name of the object ( myCar ), followed by a dot ( . ), followed by the name of the method ( fullThrottle(); and speed(200); ). Notice that we add an int parameter of 200 inside the speed() method.

Remember that..

The dot ( . ) is used to access the object’s attributes and methods.

To call a method in Java, write the method name followed by a set of parentheses (), followed by a semicolon ( ; ).

A class must have a matching filename ( Main and Main.java).

Using Multiple Classes

Like we specified in the Classes chapter, it is a good practice to create an object of a class and access it in another class.

Remember that the name of the java file should match the class name. In this example, we have created two files in the same directory:

Main.java

public class Main < public void fullThrottle() < System.out.println("The car is going as fast as it can!"); >public void speed(int maxSpeed) < System.out.println("Max speed is: " + maxSpeed); >> 

Second.java

When both files have been compiled:

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Using Objects

Once you’ve created an object, you probably want to use it for something. You may need to use the value of one of its fields, change one of its fields, or call one of its methods to perform an action.

Referencing an Object’s Fields

Object fields are accessed by their name. You must use a name that is unambiguous.

You may use a simple name for a field within its own class. For example, we can add a statement within the Rectangle class that prints the width and height :

System.out.println("Width and height are: " + width + ", " + height);

In this case, width and height are simple names.

Code that is outside the object’s class must use an object reference or expression, followed by the dot (.) operator, followed by a simple field name, as in:

For example, the code in the CreateObjectDemo class is outside the code for the Rectangle class. So to refer to the origin, width, and height fields within the Rectangle object named rectOne, the CreateObjectDemo class must use the names rectOne.origin, rectOne.width, and rectOne.height, respectively. The program uses two of these names to display the width and the height of rectOne:

System.out.println("Width of rectOne: " + rectOne.width); System.out.println("Height of rectOne: " + rectOne.height);

Attempting to use the simple names width and height from the code in the CreateObjectDemo class doesn’t make sense — those fields exist only within an object — and results in a compiler error.

Later, the program uses similar code to display information about rectTwo. Objects of the same type have their own copy of the same instance fields. Thus, each Rectangle object has fields named origin, width, and height. When you access an instance field through an object reference, you reference that particular object’s field. The two objects rectOne and rectTwo in the CreateObjectDemo program have different origin, width, and height fields.

To access a field, you can use a named reference to an object, as in the previous examples, or you can use any expression that returns an object reference. Recall that the new operator returns a reference to an object. So you could use the value returned from new to access a new object’s fields:

int height = new Rectangle().height;

This statement creates a new Rectangle object and immediately gets its height. In essence, the statement calculates the default height of a Rectangle. Note that after this statement has been executed, the program no longer has a reference to the created Rectangle, because the program never stored the reference anywhere. The object is unreferenced, and its resources are free to be recycled by the Java Virtual Machine.

Calling an Object’s Methods

You also use an object reference to invoke an object’s method. You append the method’s simple name to the object reference, with an intervening dot operator (.). Also, you provide, within enclosing parentheses, any arguments to the method. If the method does not require any arguments, use empty parentheses.

objectReference.methodName(argumentList);

objectReference.methodName();

The Rectangle class has two methods: getArea() to compute the rectangle’s area and move() to change the rectangle’s origin. Here’s the CreateObjectDemo code that invokes these two methods:

System.out.println("Area of rectOne: " + rectOne.getArea()); . rectTwo.move(40, 72);

The first statement invokes rectOne‘s getArea() method and displays the results. The second line moves rectTwo because the move() method assigns new values to the object’s origin.x and origin.y.

As with instance fields, objectReference must be a reference to an object. You can use a variable name, but you also can use any expression that returns an object reference. The new operator returns an object reference, so you can use the value returned from new to invoke a new object’s methods:

new Rectangle(100, 50).getArea()

The expression new Rectangle(100, 50) returns an object reference that refers to a Rectangle object. As shown, you can use the dot notation to invoke the new Rectangle‘s getArea() method to compute the area of the new rectangle.

Some methods, such as getArea(), return a value. For methods that return a value, you can use the method invocation in expressions. You can assign the return value to a variable, use it to make decisions, or control a loop. This code assigns the value returned by getArea() to the variable areaOfRectangle :

int areaOfRectangle = new Rectangle(100, 50).getArea();

Remember, invoking a method on a particular object is the same as sending a message to that object. In this case, the object that getArea() is invoked on is the rectangle returned by the constructor.

The Garbage Collector

Some object-oriented languages require that you keep track of all the objects you create and that you explicitly destroy them when they are no longer needed. Managing memory explicitly is tedious and error-prone. The Java platform allows you to create as many objects as you want (limited, of course, by what your system can handle), and you don’t have to worry about destroying them. The Java runtime environment deletes objects when it determines that they are no longer being used. This process is called garbage collection.

An object is eligible for garbage collection when there are no more references to that object. References that are held in a variable are usually dropped when the variable goes out of scope. Or, you can explicitly drop an object reference by setting the variable to the special value null. Remember that a program can have multiple references to the same object; all references to an object must be dropped before the object is eligible for garbage collection.

The Java runtime environment has a garbage collector that periodically frees the memory used by objects that are no longer referenced. The garbage collector does its job automatically when it determines that the time is right.

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