Java Class Methods
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 MyClass:
public class MyClass {
static void myMethod() {
System.out.println("Hello World!");
}
}
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 MyClass {
static void myMethod() {
System.out.println("Hello World!");
}
public static void main(String[] args) {
myMethod();
}
}
// Outputs "Hello World!"
Static vs. Non-Static
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 MyClass {
// 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
MyClass myObj = new MyClass(); // Create an object of MyClass
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 Car class
public class Car {
// 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) {
Car myCar = new Car(); // 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 Car class with the class keyword.
2) We created the fullThrottle() and
speed()
methods in the Car 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 Car class and its
methods, we need to create an object of the Car 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 a Car 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 (Car and
Car.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:
- Car.java
- OtherClass.java
Car.java
public class Car {
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);
}
}
OtherClass.java
class OtherClass {
public static void main(String[] args) {
Car myCar = new Car(); // Create a myCar object
myCar.fullThrottle(); // Call the fullThrottle() method
myCar.speed(200); // Call the speed() method
}
}
When both files have been compiled:
C:\Users\Your Name>javac Car.java
C:\Users\Your Name>javac OtherClass.java
Run the OtherClass.java file:
C:\Users\Your Name>java OtherClass
And the output will be:
The car is going as fast as it can!
Max speed is: 200
