In this post we will talk about going from C into Java knowing all that I told in previous posts.
Statements:
Most statements like if-else, while, for, switch-case are the same as in C and are predefined and don't need to be "imported". The way you define variable and functions is also mostly the same, except we need to use words in front now like public, static, private that are not so present in non-object-oriented programming.
Objects and Classes:
In object-oriented programming like Java or C# you use Objects that are similar to the Structs we talked about previously. The only difference is that a Struct now will also have functions or methods that can be called via the "." modifier the same way as we got or changed the values of specific member's of the Struct. This Objects are mostly made in Classes, a specific subprogramm of the whole Programm in which we split our programm. So, using objects and classes you put the code where it needs to be and the code is easier to read and parts of it reusable. For example, you don't have to make an char array now for String's but there is a String Class or Object predefined in Java that has all the methods and stuff you need to get going. There are many of those Object's included in Java Libraries and you can find all of them in the Java API: https://docs.oracle.com/javase/7/docs/api/
User Input:
For user input I like to use the Object Scanner, cause it's easy to use and can be used for all datatypes needed for basic stuff. You define it like that:
Scanner s = new Scanner(System.in); //where System.in means Keyboard Inputthen using s.nextInt() you can get Integer Input, s.next() String Input, s.nextLine() Line Input (Like an whole text) and s.next().CharAt(0) Character Input, where you call the String Input and that call a String function that returns the first Character. There are also other ways of doing it, but I think for starting out Scanner is the best way. For closing the Scanner you use s.close(). I do it to get rid of the Error, but sometimes it may come handy.
Printing at Console:
For printing we use System.out.print() or System.out.println() that puts a new line also. You could also use System.out.printf() if you like the way C does things. For Printing an Variable with it you can just put it inside alone, but when you also have some message you will have to concatenate the String's using the + sign.
For example:
when you want to print the text "Hello, World!" you do System.out.println("Hello, World!");
when you want to print "value : 5" where 5 is the value of some integer a you do:
System.out.println("value : " + value);
Arrays:
An Array is an Object in Java and when using objects you use the new modifier to create and allocate the object, cause else it's like an pointer in C and is null and this will cause null pointer exceptions. So for creating an Integer Array of size "size" you do: int[] A = new int[size]; You can also split those if you want to put the size afterwards like this:
int[]A; //Other CodeA = new int[size];Code:
Now that we know some basic stuff about Java let's create an C-Like programm in Java. Suppose we want to create a program that does some calculations on an integer array. The array will be of size "size" a variable that we get as user input (pseudo-dynamic) and size needs to be between 1...100.
We will create a menu that gets a string user input(switch case) and does the following:
“fill”: fill Array with rand numbers between a...b where a, b integers and a < b
“sum” : sum Array
“max” : find max
“min” : find min
“sort” : sort Array
“search” : search number
“avg” : average
“exit” : Exit
You don't need functions for them cause we don't spoke about them yet.
package steemit; //the packet in the IDE I save the code
import java.util.Scanner; //library for Scanner object
public class clike { // you have to call your .java file also clike.java
public static void main(String[] args) { // this is the way we define main
Scanner in = new Scanner(System.in); // java object for user input / int[] A; // define array without size int size; // variable for the size int a, b; // values from user int sum = -1, max, min; int search, num, temp; // user input double avg = -1; char sort; // for sort choice String choice; // string for switch case choice
// size of array as user input do { System.out.print("Give size for array: "); // printing to console size = in.nextInt(); // getting user integer input if (size < 1 || size > 100) System.out.println("The size must be in the range of 1...100"); } while (size < 1 || size > 100); A = new int[size]; // "allocate" the array of size
do { // infinite loop // print menu System.out.println("“fill”: fill Array with rand numbers"); System.out.println("“sum” : sum Array"); System.out.println("“max” : find max"); System.out.println("“min” : find min"); System.out.println("“sort” : sort Array"); System.out.println("“search” : search number"); System.out.println("“avg” : average"); System.out.println("“Q” : implementation of question Q"); System.out.println("“exit” : Exit"); // getting choice System.out.print("choice: "); choice = in.next(); // getting string input switch (choice) { // switch case of String (couldn't be done in C // that easily cause in Java String is a Object case "fill": // read numbers a, b for range do { System.out.println("Give 2 ints to fill array with random numbers between them"); System.out.print("a: "); a = in.nextInt(); // getting user integer input System.out.print("b: "); b = in.nextInt(); // getting user integer input if (!(a < b)) System.out.println("a must be less then b"); } while (!(a < b));
// fill array with number in range [a,b] for (int i = 0; i < A.length; i++) { // getting random number between a...b A[i] = a + (int) ((b - a + 1) * Math.random()); } break; case "sum": // calculating sum sum = 0; for (int i = 0; i < A.length; i++) { sum += A[i]; } System.out.println("sum of array is: " + sum); break; case "max": // finding max max = 0; for (int i = 1; i < A.length; i++) { if (A[i] > A[max]) max = i; } System.out.println("max is " + A[max]); break; case "min": // finding min min = 0; for (int i = 1; i < A.length; i++) { if (A[i] < A[min]) min = i; } System.out.println("min is " + A[min]); break; case "sort": // sorting array System.out.println("Do you want ascending (a) or descending (d) order?"); do { System.out.print("Choice: "); sort = in.next().charAt(0); // getting char input } while (sort != 'a' && sort != 'd'); if (sort == 'a') { // ascending sort for (int i = 0; i < A.length; i++) { for (int j = 0; j < A.length; j++) { if (A[i] < A[j]) { temp = A[i]; A[i] = A[j]; A[j] = temp; } } } } else if (sort == 'd') { // descending sort for (int i = 0; i < A.length; i++) { for (int j = 0; j < A.length; j++) { if (A[i] > A[j]) { temp = A[i]; A[i] = A[j]; A[j] = temp; } } } } break; case "search": // search for a number System.out.print("Give number to search for: "); num = in.nextInt(); search = -1; for (int i = 0; i < A.length; i++) { if (A[i] == num) { search = i; break; } } if (search != -1) System.out.println("Found at pos: " + search); else System.out.println(num + " was not found!"); break; case "avg": // find average if (sum == -1) { // if sum was not called: error System.out.println("Choose “sum” first"); break; } avg = (double) sum / A.length; System.out.println("avg of array is: " + avg); break; case "exit": // terminate programm in.close(); // close the Scanner System.exit(0); default: // wrong choice System.out.println("Wrong choice! Give another choice!"); }
} while (true);
}
}