In Java, a function (commonly referred to as a method) is a block of code that performs a specific task. It can take input (called parameters), process that input, and return a result. Methods allow for code reuse, better organization, and modularity.

Key Characteristics of Functions (Methods) in Java:

1. Modifiers (optional): Keywords like public, private, static, etc., control the accessibility and behavior of the method.
2. Return Type: Specifies the type of value the method will return. If the method doesn’t return anything, the return type is void.
3. Method Name: A unique name that identifies the method.
4. Parameters (optional): Values that you pass into the method for processing. These are optional.
5. Body: The block of code that defines the actions of the method.
6. Return Statement (optional): Specifies what value the method will return if it’s not void.

Syntax of a Method in Java:

modifier returnType methodName(parameterList) {
    // Method body
    // Optional return statement
}

Example:

Method without Return Type (void):

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

1. Method with Return Type (int):

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

Calling a Method:

• If the method is static, it can be called without creating an object.
  
• For example:
• public static void main(String[] args) {
• System.out.println(addNumbers(5, 3));
• // Outputs: 8
• }

Why Use Functions?

• Reusability: Avoid duplicating code by writing reusable methods.
• Modularity: Break complex programs into smaller, manageable chunks.
• Abstraction: Hide details from the user by using methods to abstract operations.

1. Function to Add Two Numbers

In Java, a method (function) is defined using the returnType methodName(parameters) syntax. Here is a simple example of adding two numbers:

public class Main {
    // Function to add two numbers
    public static int addTwoNumbers(int num1, int num2) {
        return num1 + num2;
    }

    public static void main(String[] args) {
        int result = addTwoNumbers(10, 20);
        System.out.println("Sum: " + result);  // Output: Sum: 30
    }
}

2. Function to Concatenate Two Strings

Similarly, here’s a function to concatenate two strings:

public class Main {
    // Function to concatenate two strings
    public static String concatenateStrings(String str1, String str2) {
        return str1 + str2;
    }

public static void main(String[] args) {
        String result = concatenateStrings("Hello, ", "World!");
        System.out.println("Concatenated String: " + result);  
        // Output: Concatenated String: Hello, World!
    }
}

Key Points:

• public static: Used to define a method accessible from the main method without needing an object of the class.
• int/String: Specifies the return type of the method.
• Method parameters are passed inside parentheses.

You can run the examples in any Java IDE or environment.