Lesson 2: Recap methods

Goals

• Intro Git and GitHub
• Recap method signature, arguments and return value
• Code, code, code 🤩

Java Method

in computer programming, a function is a block of code that performs a specific task. In object-oriented programming, the method is just another name used for function. Methods are bound to a class and they define the behavior of a class. In Java, all function definitions must be inside classes. Let’s look at an example of the main method

public class Main {
    public static void main(String[] args){
      //code to execute
    }
}

Here, we have created a method named main in the class Main. We can see that we have used the public, static and void keywords before the method name. We will look at their role in Java in this lesson. For the aforementioned keywords, they mean this:

• public serves as a access modifier. It means the method can be accessed from anywhere.
• static here means this method belongs to the general class of Main and not to a specific instance of Main. We can therefore call the method from Main without creating an actual instance of Main. That would look like this: Main.foo().
• void means this method doesn’t return a value. Methods can return a single value in Java and it has to be defined in the method declaration. However, you can use return by itself to exit the method.
• This method takes as argument an array of String.

A method is structured as follow:

public class Main {
    <modifiers> <return type> methodName(<arguments list>){
      //code to execute
    }
}

To use a method, it should be called. If a method returns a value, it can be immediately assigned to a variable when called. A method that returns nothing i.e. void cannot be assigned to a variable when called.

The main advantage is code reusability. We can write a method once, and use it multiple times. We do not have to rewrite the entire code each time. Think of it as, “write once, reuse multiple times”. Methods also make code more readable and easier to debug. For example, a method named getSquare() is so readable that we can know that this method will be calculating the square of a number whenever it is called.

Method Visibility with Access Modifiers

• private: Can only be accessed within the owning class
• protected: Can only be accessed within the owning class, classes within the same package and any sub-class of the owning class
• public: Can be accessed from anywhere.
• No modifier (default): Can only be accessed within the class and classes within the same package

Static Methods

Static methods like the main method belongs to the class and you do not need an instance of the class to invoke it. If you define a static method call myMethod in a class called MyClass. You can invoke that method by using the class in the following way:

MyClass.myMethod(...)

Example 1:

public class Main {

    public static void print(String name) {
        System.out.println(name);
    }

    public static void main(String[] args){
        print("Java");
    }
}

Example 2:

Here, we have mentioned the return type of the method as int. Hence, the method should always return an integer value. Also observe that when we called sum the first time, we assigned the return value to a variable value, however we did assign the return value to anything at the second invocation.

public class Main {

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

    public static void main(String[] args){
        int value = sum(5, 1);
        int anotherValue = Main.sum(5, 1);
        sum(5, 1);
    }
}

Non-static Methods

A non-static method belongs to an instance of the class. To invoke or call a non-static method, you must have and use an instance of the class. What is an instance of a class?

If you define a non-static method call myMethod in a class called MyClass you can invoke that method in the following way:

new MyClass().myMethod(...);

Here, new MyClass() is a new instance of the class MyClass. You will learn more about this and Object Oriented Programming later in the course.

OR in the following way

MyClass myClass = new MyClass();
myClass.myMethod(...)

Example 1:

public class Main {

    public void print(String name) {
        System.out.println(name);
    }

    public static void main(String[] args){
        print("Java"); // This is wrong and will not compile
        new Main().print("Java"); // This is correct and will compile
    }
}

Example 2:

public class Main {

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

    public static void main(String[] args){
        Main main = new Main();
        int value = main.sum(5, 1);
    }
}

More exercises

Follow the link and accept the assignment to be able to see the exercies and submit your solution.

Homework Assignment

Accept the assignment and solve it on your own. Reach out whenever you are stuck and good luck!

Table of contents

• Assignment Solutions