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:

public class Main {
  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: Calling `myMethod()` Inside `main`

public class Main {
  static void myMethod() {
    System.out.println("Hello World!");
  }
 
  public static void main(String[] args) {
    myMethod();
  }
}

Output:

Hello World!

Static vs. Public Methods

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: Demonstrating 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: Creating and Calling Methods on an Object

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
  }
}

Output:

The car is going as fast as it can!
Max speed is: 200

Explanation:

We created a custom Main class with the class keyword.
We created the fullThrottle() and speed() methods in the Main class.
The fullThrottle() and speed() methods print out some text when called.
The speed() method accepts an int parameter called maxSpeed (used in step 8).
To use the Main class and its methods, we need to create an object of the Main class.
In the main() method, which is a built-in Java method, we execute our program.
Using the new keyword, we created an object named myCar.
We then call the fullThrottle() and speed(200) methods on the myCar object.

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.

Example: Using Multiple Classes

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

class Second {
  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
  }
}

Compiling and Running

When both files have been compiled:

C:\Users\Your Name>javac Main.java
C:\Users\Your Name>javac Second.java

Run the Second.java file:

C:\Users\Your Name>java Second

Output:

The car is going as fast as it can!
Max speed is: 200

Key Takeaways

The dot (.) is used to access the object's attributes and methods.
To call a method in Java, write the method name followed by parentheses (), then a semicolon ;.
A class must have a matching filename (e.g., Main and Main.java).

Java Class Attributes Java Constructors