C for Java programmers I

Countdown in three programming languages

Java

// Count in Java

import java.io.* ;
import java.util.Scanner ;

public class Count {

  public static void main(int argc, String[] argv) {
    Scanner stdin = new Scanner(System.in) ;
    int count = stdin.nextInt() ;
    PrintStream stdout = new PrintStream(System.out) ;
    while (count >= 0) {
      stdout.format("%3d\n", count) ;
      --count ;
    }
    stdout.println("Hello World!") ;
  }
}

C

/* CountDown in ANSI C -- and consequently C++ */

#include <stdio.h>
#include <stdlib.h>

int main(int argc, char *argv[]) {
  int count ;
  (void) scanf("%d", &count) ;
  while (count &gt;= 0) {
    printf("%3d\n", count) ;
    --count ;
  }
  puts("Hello World!") ;
  return (EXIT_SUCCESS) ;
  
}

C++

// CountDown in C++

#include <iostream>
#include <iomanip>
using namespace std ;

int main(int argc, char *argv[]) {
  int count ;
  cin >> count ;
  while (count >= 0) {
    cout << setw(3) << count << endl ;
    --count ;
  }
  cout << "Hello world!" << endl ;
  return (EXIT_SUCCESS) ;
}

Compilation and execution

• C
  • pre-processor
  • compilation
  • linking
  • execution in machine code
• Java
  • compilation
  • interpretation of java byte code

Comments

ANSI C does not allow // as a way to start comments. Only /* and */ pairs are allowed.

Primitive Data types

• C
  • integers
    • several different sizes
      • char
      • short
      • int
      • long
    • no standard sizes
    • no common representation (until C99)
  • floating point
    • float
    • double
    • long double
  • no boolean
    • false is 0
    • true is any non-zero integer
• Java
  • integers
    • several different sizes
      • byte
      • char
        • 16-bit Unicode
      • short
      • int
      • long
    • standard bit sizes for all types
    • twos-complement represenation
  • floating point
    • float
    • double
  • boolean

Operators

C and Java have the same set of arithmetic, relational, logical, and bit operators except for Java's >>> logical right shift which isn't needed in C because right shifts of unsigned integer are always logical.

In C logical and relational operators always return integers, either 0 or 1. Logical operators can also be applied to values other than 0 and 1. For example, 5 || 0 is 1, 5 && 0 is 0, and 5 > 4 > 3 is 0.

Control structures

For the most part, the major control structures if, if-then, while, do-while, and for are the same in Java and C. Even the switch construct has the same need for break.

However, in ANSI C, all variables must be declared at the beginning of statment blocks (statements enclosed by { and }). This means that the following section of code are not legal ANSI C. However, since they are both allowed in C++, many C compilers will accept them.

if (x != 3) {
  int n = 2*x ;
  ++x ;
  int m = 3*b ;                 /* Can't declare m here in C */
  y = n+m ;
}

for (int i=0; i<10; ++i) {     /* Can't declare i here in C */
  sum = sum + i ;
}

Classes

C is not an object oriented programming language. It does not have classes. It does not have methods. It does not have inheritence. It does not have interfaces.

C++ has classes and inheritence. It supports multiple inheritence rather than interfaces.