Simple array usage demonstration

I just finished this code for my own practice purpose. This code works just fine but I understand there still a lot insidious problems. Any advice for improvement would be appreciated. So here it goes:

Package DS;

import java.util.*; 

public class basic_Array {

public static void main(String[] args) {

    int option;

    System.out.println("Welcome to basic array test, please choose your testing subject: ");

    Scanner user_option = new Scanner(System.in);

    do{

        System.out.println("Option 1: Simple one dimensional array.");
        System.out.println("Option 2: Simple two dimensional array. (under construction)");
        System.out.println("Option 0: Exit program");

        option = user_option.nextInt();
        System.out.println("current option is: " + option);

        switch(option){

        case 1:
            //simple array input method 
            simple_a1();
            break;
        case 2:
            simple_a2();
            break;
        }

    }while(option != 0);

    System.out.println("Program terminated...");
    user_option.close();

}

private static void simple_a1() {

    int[] a1 = new int[5];

    int in_num;                                                             //redundant but clear.

    Scanner user_input = new Scanner(System.in);

    for(int i = 0; i < 5; i++){

        System.out.println("Request input for item " + i);
        in_num = user_input.nextInt();
        a1[i] = in_num;
        System.out.println("Got it, next one ");
    }

    System.out.println("The numbers in a1 are: ");

    for(int i2: a1){
        System.out.print(i2 + " ");
    }

    System.out.println("\nEnd of testing 1, back to main menu...");

}

private static void simple_a2() {

    System.out.println("This function still under construction, back to main menu...");
}

}

Edit in 2/26/2017:

Thanks for EBrown and others input, I've modified my program structure and added a few new functions.

I also have a question o

Let's talk about SRP for the moment (I'm not going to cover anything else but this). SRP (or Single-Responsibility Principle) dictates that each and every component of code should have exactly one responsibility.

So we're going to look at all your code top-to-bottom and determine what responsibilities we have in each method.

We'll start with main:

• Output information regarding programme definition to user;

• Accept and process user input;

• Call appropriate method(s) based on user input;

So that's more than one, but that's fine, we need to finish our analysis to make appropriate decisions on what to do next. Let's look at simple_a1:

• Process items in an array;

• Accept and process user input;

• Add user input to array;

• Print array information;

• Inform the caller we're going back to the main menu;

That's a lot, and even simple_a2 has more than one responsibility:

• Inform the user the feature is not implemented;

• Inform the user we are returning to the main menu;

What happens when we're not going back to the main menu again from either of those methods? Now you have multiple places that you have to modify, and in a larger, production application that would mean very possible breakages.

So we're going to eliminate responsibilities until each and every method only has one left.

Let's start with simple_a1:

We can take the following block and extract it to a new method:

System.out.println("Request input for item " + i);
in_num = user_input.nextInt();
a1[i] = in_num;
System.out.println("Got it, next one ");

This does exactly one thing: prompt the user for a number and store it in the array. We won't store it in the array in our sub-method, but we'll extract the prompting out. Let's create a new method:

private static int getNumber(Scanner input, int index) {
    System.out.println("Request input for item " + index);
    int result = input.nextInt();
    System.out.print("Got it");
    return result;
}

Perfect, so this has one responsibility. It's not the prettiest, but it will do. Next we have to implement this in our other method:

for (int i = 0; i < 5; i++) {
    a1[i] = getNumber(user_input, i);

    if (i + 1 < 5) {
        System.out.println(", next one");
    }
}

So now our input prompting is separated from our processing, that's good but we can do better.

The block is now something along the lines of:

int[] a1 = new int[5];
Scanner input = new Scanner(System.in);

for (int i = 0; i < 5; i++) {
    a1[i] = getNumber(input, i);

    if (i + 1 < 5) {
        System.out.println(", next one");
    }
}

Well this whole block has one responsibility: build an array. So we can extract that out further:

private static int[] buildArray(int size) {
    int[] result = new int[5];
    Scanner input = new Scanner(System.in);

    for (int i = 0; i < 5; i++) {
        result[i] = getNumber(input, i);

        if (i + 1 < 5) {
            System.out.println(", next one");
        }
    }

    input.close();

    return result;
}

Good, so now simple_a1 looks like:

private static void simple_a1() {
    int[] a1 = buildArray(5);

    System.out.println("The numbers in a1 are: ");

    for(int i2: a1){
        System.out.print(i2 + " ");
    }

    System.out.println("\nEnd of testing 1, back to main menu...");
}

But we still have three remaining responsibilities:

• Build an array;

• Print the array;

• Inform the user we're going back to the main menu;

Well we can make a printArray method:

private static void printArray(int[] printArray) {
    for (int val : printArray) {
        System.out.print(val + " ");
    }
}

Then we finally finish simple_a1:

private static void simple_a1() {
    int[] a1 = buildArray(5);
    printArray(a1);
    System.out.print("\nEnd of testing 1");
}

So we're down to one responsibility: process an array. It's made up of steps, but the implementation for those steps is not part of simple_a1.

Next we'll fix simple_a2:

private static void simple_a2() {
    System.out.print("This function still under construction");
}

So we don't care where we return to, we only care that we're returning.

Finally, fixing main is easy:

```
public static void main(String[] args) {
System.out.println("Welcome to basic array test, please choose your testing subject: ");
Scanner input = new Scanner(System.in);

mainMenu(input);

System.out.println("Program terminated...");
input.close();
}

private static void mainMenu(Scanner input) {
int option = 0;

do {
System.out.println("Option 1: Simple one dimensional array.");
System.out.println("Option 2: Simple two dimensional array. (under construction)");
System.out.println("Option 0: Exit program");

option = user_option.nextInt();
System.out.println("current option is: " + option);

switch(option) {
case 1:
//simple array input method

StackExchange Code Review Q#156261, answer score: 10