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Java

C#

Primitive Types
boolean
byte
char
short, int, long
float, double

Reference Types
Object   (superclass of all other classes)
String
arrays, classes, interfaces

Conversions

// int to String
int x = 123;
String y = Integer.toString(x);  // y is "123"

// String to int
y = "456";
x = Integer.parseInt(y);   // x is 456

// double to int
double z = 3.5;
x = (int) z;   // x is 3  (truncates decimal)

Value Types
bool
byte, sbyte
char
short, ushort, int, uint, long, ulong
float, double, decimal
structures, enumerations

Reference Types
object    (superclass of all other classes)
string
arrays, classes, interfaces, delegates

Convertions

// int to string
int x = 123;
String y = x.ToString();  // y is "123"

// string to int
y = "456";
x = int.Parse(y);   // or x = Convert.ToInt32(y);

// double to int
double z = 3.5;
x = (int) z;   // x is 3  (truncates decimal)

// Can be set to a const or a variable. May be initialized in a constructor.
readonly int MAX_HEIGHT = 9;

enum Action {Start, Stop, Rewind, Forward};

// Special type of class
enum Status {
  Flunk(50), Pass(70), Excel(90);
  private final int value;
  Status(int value) { this.value = value; }
  public int value() { return value; }
};

Action a = Action.Stop;
if (a != Action.Start)
  System.out.println(a);               // Prints "Stop"

Status s = Status.Pass;
System.out.println(s.value());      // Prints "70"

enum Action {Start, Stop, Rewind, Forward};

enum Status {Flunk = 50, Pass = 70, Excel = 90};

No equivalent.





Action a = Action.Stop;
if (a != Action.Start)
  Console.WriteLine(a);             // Prints "Stop"

Status s = Status.Pass;
Console.WriteLine((int) s);       // Prints "70"

Comparison
==  <  >  <=  >=  !=

Arithmetic
+  -  *  /
%  (mod)
/   (integer division if both operands are ints)
Math.Pow(x, y)

Assignment
=  +=  -=  *=  /=   %=   &=  |=  ^=  <<=  >>=  >>>=  ++  --

Bitwise
&  |  ^   ~  <<  >>  >>>

Logical
&&  ||  &  |   !

Note: && and || perform short-circuit logical evaluations

String Concatenation
+

Comparison
==  <  >  <=  >=  !=

Arithmetic
+  -  *  /
%  (mod)
/   (integer division if both operands are ints)
Math.Pow(x, y)

Assignment
=  +=  -=  *=  /=   %=  &=  |=  ^=  <<=  >>=  ++  --

Bitwise
&  |  ^   ~  <<  >>

Logical
&&  ||   !

Note: && and || perform short-circuit logical evaluations, no & and | equivalents

String Concatenation
+

greeting = age < 20 ? "What's up?" : "Hello";

if (x < y)
  System.out.println("greater");

if (x != 100) {   
  x *= 5;
  y *= 2;
}
else
  z *= 6;

int selection = 2;
switch (selection) {     // Must be byte, short, int, char, or enum
  case 1: x++;            // Falls through to next case if no break
  case 2: y++;   break;
  case 3: z++;   break;
  default: other++;
}

greeting = age < 20 ? "What's up?" : "Hello";

if (x < y) 
  Console.WriteLine("greater");

if (x != 100) {   
  x *= 5;
  y *= 2;
}
else
  z *= 6;

string color = "red";
switch (color) {                          // Can be any predefined type
  case "red":    r++;    break;       // break is mandatory; no fall-through
  case "blue":   b++;   break;
  case "green": g++;   break;
  default: other++;     break;       // break necessary on default
}

while (i < 10)
  i++;

for (i = 2; i <= 10; i += 2)
  System.out.println(i);

do
  i++;
while (i < 10);

for (int i : numArray)  // foreach construct 
  sum += i;

// for loop can be used to iterate through any Collection
import java.util.ArrayList;
ArrayList<Object> list = new ArrayList<Object>();
list.add(10);    // boxing converts to instance of Integer
list.add("Bisons");
list.add(2.3);    // boxing converts to instance of Double

for (Object o : list)
  System.out.println(o);

while (i < 10)
  i++;

for (i = 2; i <= 10; i += 2)
  Console.WriteLine(i);

do
  i++;
while (i < 10);

foreach (int i in numArray) 
  sum += i;

// foreach can be used to iterate through any collection 
using System.Collections;
ArrayList list = new ArrayList();
list.Add(10);
list.Add("Bisons");
list.Add(2.3);

foreach (Object o in list)
  Console.WriteLine(o);

int[][] jagged = new int[5][];
jagged[0] = new int[5];
jagged[1] = new int[2];
jagged[2] = new int[3];
jagged[0][4] = 5;

int[][] jagged = new int[3][] {
    new int[5], new int[2], new int[3] };
jagged[0][4] = 5;

// Primitive types and references are always passed by value
void TestFunc(int x, Point p) {
   x++;
   p.x++;       // Modifying property of the object
   p = null;    // Remove local reference to object
}

class Point {
   public int x, y;
}

Point p = new Point();
p.x = 2;
int a = 1;
TestFunc(a, p);
System.out.println(a + " " + p.x + " " + (p == null) );  // 1 3 false

// Accept variable number of arguments
int Sum(int ... nums) {
  int sum = 0;
  for (int i : nums)
    sum += i;
  return sum;
}

int total = Sum(4, 3, 2, 1);   // returns 10

// Pass by value (default), in/out-reference (ref), and out-reference (out)
void TestFunc(int x, ref int y, out int z, Point p1, ref Point p2) {
   x++;  y++;  z = 5;
   p1.x++;       // Modifying property of the object     
   p1 = null;    // Remove local reference to object
   p2 = null;   // Free the object
}

class Point {
   public int x, y;
}

Point p1 = new Point();
Point p2 = new Point();
p1.x = 2;
int a = 1, b = 1, c;   // Output param doesn't need initializing
TestFunc(a, ref b, out c, p1, ref p2);
Console.WriteLine("{0} {1} {2} {3} {4}",
   a, b, c, p1.x, p2 == null);   // 1 2 5 3 True

// Accept variable number of arguments
int Sum(params int[] nums) {
  int sum = 0;
  foreach (int i in nums)
    sum += i;
  return sum;
}

int total = Sum(4, 3, 2, 1);   // returns 10

// String concatenation
String school = "Harding ";
school = school + "University";   // school is "Harding University"

// String comparison
String mascot = "Bisons";
if (mascot == "Bisons")    // Not the correct way to do string comparisons
if (mascot.equals("Bisons"))   // true
if (mascot.equalsIgnoreCase("BISONS"))   // true
if (mascot.compareTo("Bisons") == 0)   // true

System.out.println(mascot.substring(2, 5));   // Prints "son"

// My birthday: Oct 12, 1973
java.util.Calendar c = new java.util.GregorianCalendar(1973, 10, 12);
String s = String.format("My birthday: %1$tb %1$te, %1$tY", c);

// Mutable string
StringBuffer buffer = new StringBuffer("two ");
buffer.append("three ");
buffer.insert(0, "one ");
buffer.replace(4, 7, "TWO");
System.out.println(buffer);     // Prints "one TWO three"

// String concatenation
string school = "Harding ";
school = school + "University";   // school is "Harding University"

// String comparison
string mascot = "Bisons";
if (mascot == "Bisons")    // true
if (mascot.Equals("Bisons"))   // true
if (mascot.ToUpper().Equals("BISONS"))   // true
if (mascot.CompareTo("Bisons") == 0)    // true

Console.WriteLine(mascot.Substring(2, 3));    // Prints "son"

// My birthday: Oct 12, 1973
DateTime dt = new DateTime(1973, 10, 12);
string s = "My birthday: " + dt.ToString("MMM dd, yyyy");

// Mutable string
System.Text.StringBuilder buffer = new System.Text.StringBuilder("two ");
buffer.Append("three ");
buffer.Insert(0, "one ");
buffer.Replace("two", "TWO");
Console.WriteLine(buffer);     // Prints "one TWO three"

// Must be in a method that is declared to throw this exception
Exception ex = new Exception("Something is really wrong.");
throw ex;  

try {
  y = 0;
  x = 10 / y;
} catch (Exception ex) {
  System.out.println(ex.getMessage());
} finally {
  // Code that always gets executed
}

Exception up = new Exception("Something is really wrong.");
throw up;  // ha ha

try {
  y = 0;
  x = 10 / y;
} catch (Exception ex) {      // Variable "ex" is optional
  Console.WriteLine(ex.Message);
} finally {
  // Code that always gets executed
}

package harding.compsci.graphics;

import harding.compsci.graphics.Rectangle;  // Import single class

import harding.compsci.graphics.*;   // Import all classes

namespace Harding.Compsci.Graphics {
  ...
}

or

namespace Harding {
  namespace Compsci {
    namespace Graphics {
      ...
    }
  }
}

// Import all class. Can't import single class.
using Harding.Compsci.Graphics;

Accessibility keywords
public
private
protected
static

// Inheritance
class FootballGame extends Competition {
  ...
}

// Interface definition
interface IAlarmClock {
  ...
}

// Extending an interface
interface IAlarmClock extends IClock {
  ...
}

// Interface implementation
class WristWatch implements IAlarmClock, ITimer {
   ...
}

Accessibility keywords
public
private
internal
protected
protected internal
static

// Inheritance
class FootballGame : Competition {
  ...
}

// Interface definition
interface IAlarmClock {
  ...
}

// Extending an interface
interface IAlarmClock : IClock {
  ...
}

// Interface implementation
class WristWatch : IAlarmClock, ITimer {
   ...
}

class SuperHero {
  private int mPowerLevel;

  public SuperHero() {
    mPowerLevel = 0;
  }

  public SuperHero(int powerLevel) {
    this.mPowerLevel= powerLevel;
  }

  // No destructors, just override the finalize method
  protected void finalize() throws Throwable {
    super.finalize();   // Always call parent's finalizer  
  }
}

class SuperHero {
  private int mPowerLevel;

  public SuperHero() {
     mPowerLevel = 0;
  }

  public SuperHero(int powerLevel) {
    this.mPowerLevel= powerLevel;
  }

  ~SuperHero() {
    // Destructor code to free unmanaged resources.
    // Implicitly creates a Finalize method.
  }
}

SuperHero hero = new SuperHero();

hero.setName("SpamMan");
hero.setPowerLevel(3);

hero.Defend("Laura Jones");
SuperHero.Rest();  // Calling static method

SuperHero hero2 = hero;   // Both refer to same object
hero2.setName("WormWoman");
System.out.println(hero.getName());  // Prints WormWoman

hero = null;   // Free the object

if (hero == null)
  hero = new SuperHero();

Object obj = new SuperHero();
System.out.println("object's type: " + obj.getClass().toString());
if (obj instanceof SuperHero)
  System.out.println("Is a SuperHero object.");

SuperHero hero = new SuperHero();

hero.Name = "SpamMan";
hero.PowerLevel = 3;

hero.Defend("Laura Jones");
SuperHero.Rest();   // Calling static method

SuperHero hero2 = hero;   // Both refer to same object
hero2.Name = "WormWoman";
Console.WriteLine(hero.Name);   // Prints WormWoman

hero = null ;   // Free the object

if (hero == null)
  hero = new SuperHero();

Object obj = new SuperHero();
Console.WriteLine("object's type: " + obj.GetType().ToString());
if (obj is SuperHero)
  Console.WriteLine("Is a SuperHero object.");

private int mSize;

public int getSize() { return mSize; }
public void setSize(int value) {
  if (value < 0)
    mSize = 0;
  else
    mSize = value;
}

int s = shoe.getSize();
shoe.setSize(s+1);

private int mSize;

public int Size {
  get { return mSize; }
  set {
    if (value < 0)
      mSize = 0;
    else
      mSize = value;
  }
}

shoe.Size++;

No structs in Java.

int c = System.in.read();   // Read single char
System.out.println(c);      // Prints 65 if user enters "A"

// The studio costs $499.00 for 3 months.
System.out.printf("The %s costs $%.2f for %d months.%n", "studio", 499.0, 3);

// Today is 06/25/04
System.out.printf("Today is %tD%n", new java.util.Date());

int c = Console.Read();  // Read single char
Console.WriteLine(c);    // Prints 65 if user enters "A"

// The studio costs $499.00 for 3 months.
Console.WriteLine("The {0} costs {1:C} for {2} months.\n", "studio", 499.0, 3);

// Today is 06/25/2004
Console.WriteLine("Today is " + DateTime.Now.ToShortDateString());

import java.io.*;

// Character stream writing
FileWriter writer = new FileWriter("c:\\myfile.txt");
writer.write("Out to file.");
writer.close();

// Character stream reading
FileReader reader = new FileReader("c:\\myfile.txt");
BufferedReader br = new BufferedReader(reader);
String line = br.readLine();
while (line != null) {
  System.out.println(line);
  line = br.readLine();
}
reader.close();

// Binary stream writing
FileOutputStream out = new FileOutputStream("c:\\myfile.dat");
out.write("Text data".getBytes());
out.write(123);
out.close();

// Binary stream reading
FileInputStream in = new FileInputStream("c:\\myfile.dat");
byte buff[] = new byte[9];
in.read(buff, 0, 9);   // Read first 9 bytes into buff
String s = new String(buff);
int num = in.read();   // Next is 123
in.close();

using System.IO;

// Character stream writing
StreamWriter writer = File.CreateText("c:\\myfile.txt");
writer.WriteLine("Out to file.");
writer.Close();

// Character stream reading
StreamReader reader = File.OpenText("c:\\myfile.txt");
string line = reader.ReadLine();
while (line != null) {
  Console.WriteLine(line);
  line = reader.ReadLine();
}
reader.Close();

// Binary stream writing
BinaryWriter out = new BinaryWriter(File.OpenWrite("c:\\myfile.dat"));
out.Write("Text data");
out.Write(123);
out.Close();

// Binary stream reading
BinaryReader in = new BinaryReader(File.OpenRead("c:\\myfile.dat"));
string s = in.ReadString();
int num = in.ReadInt32();
in.Close();