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Learning Java for Python Programmers

User-Defined Functions and Libraries

Understand how to define and use user-defined functions in Java compared to Python, including method structure, return types, and function calls. Learn to create and import user libraries, manage modular code, and handle user input with Java's Scanner and Python's input() function for practical programming solutions.

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In Python, a user-defined function is a block of statements that does a specific task, and once defined, it can be used throughout the program. Functions are named and parametrized, and a def keyword comes ahead, followed by the brackets containing the function name and the parameters. The function body contains the code that will be executed when the function is called.

Python 3.5
# Function in Python
def sum_numbers(n):
  total = 0
  for i in range(1, n+1):
    total += i
  return total
# Calling the function
result = sum_numbers(5)
print("Sum of numbers:", result)


In Java, a user-defined method/function is similar to a function in Python. It is a section of code within a class that performs a specific task. Functions are defined within a class using the public static keyword (for static methods), followed by the return type, the function name, and the parameters within the parentheses. The function body is the code that will run when the function is called.

Java
public class MyJavaApp {
  // Function in Java
  public static int sumNumbers(int n) {
    int total = 0;
    for (int i = 1; i <= n; i++) {
      total += i;
    }
    return total;
  }
  // Main method to call the function
  public static void main(String[] args) {
    int result = sumNumbers(5);
    System.out.println("Sum of numbers: " + result);
  }
}

Note: In both examples, the function print_numbers() (Python) and printNumbers() (Java) takes an integer n as input and prints numbers from 1 to n using a for loop. This demonstrates how a function can encapsulate repetitive tasks, enhancing code reusability and readability.

Let’s understand the structure of a function with the help of an illustration:

Return type and function name
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Return type and function name

  • Function name: It is a unique identifier that represents the function and is used to call the function from other parts of the program.

  • Arguments: Arguments are also termed parameters and are the values passed to the function when called. These values can be used ...