c# 与 java 语法异同

Java and C# Comparison
This is a quick reference guide to highlight some key syntactical differences between Java and C#. 
This is not a complete overview of either language. Hope you find this useful! 
Also see VB.NET and C# Comparison.

Program Structure  Comments  Data Types  Constants  Enumerations  Operators

Choices  Loops  Arrays  Functions  Strings

Exception Handling  Namespaces  Classes / Interfaces  Constructors / Destructors  Objects

Properties  Structs  Console I/O  File I/O

Java

Program Structure

C#

package hello;

public class HelloWorld {
   public static void main(String[] args) {
      String name = "Java";

// See if an argument was passed from the command line
      if (args.length == 1)
         name = args[0];

System.out.println("Hello, " + name + "!");
    }
}

using System;

namespace Hello {
   public class HelloWorld {
      public static void Main(string[] args) {
         string name = "C#";

// See if an argument was passed from the command line
         if (args.Length == 1)
            name = args[0];

Console.WriteLine("Hello, " + name + "!");
      }
   }
}

Java

Comments

C#

// Single line
/* Multiple
    line  */
/** Javadoc documentation comments */

// Single line
/* Multiple
    line  */
/// XML comments on a single line
/** XML comments on multiple lines */

Java

Data Types

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)

Java

Constants

C#

// May be initialized in a constructor 
final double PI = 3.14;

const double PI = 3.14;

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

Java

Enumerations

C#

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"

Java

Operators

C#

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

String Concatenation
+

Java

Choices

C#

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
}

Java

Loops

C#

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);

Java

Arrays

C#

int nums[] = {1, 2, 3};   or   int[] nums = {1, 2, 3};
for (int i = 0; i < nums.length; i++)
  System.out.println(nums[i]);

String names[] = new String[5];
names[0] = "David";

float twoD[][] = new float[rows][cols];
twoD[2][0] = 4.5;

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[] nums = {1, 2, 3};
for (int i = 0; i < nums.Length; i++)
  Console.WriteLine(nums[i]);

string[] names = new string[5];
names[0] = "David";

float[,] twoD = new float[rows, cols];
twoD[2,0] = 4.5f;

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

Java

Functions

C#

// Return single value int Add(int x, int y) {     return x + y;  } int sum = Add(2, 3);

// Return no value void PrintSum(int x, int y) {     System.out.println(x + y);  } PrintSum(2, 3);

// 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

// Return single value int Add(int x, int y) {     return x + y;  } int sum = Add(2, 3);

// Return no value void PrintSum(int x, int y) {     Console.WriteLine(x + y);  } PrintSum(2, 3);

// 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

Java

Strings

C#

// 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"

Java

Exception Handling

C#

// 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
}

Java

Namespaces

C#

package harding.compsci.graphics;

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

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

namespace Harding.Compsci.Graphics {
  ...
}

or

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

// Import single class
using Rectangle = Harding.CompSci.Graphics.Rectangle;

// Import all class
using Harding.Compsci.Graphics;

Java

Classes / Interfaces

C#

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 {
   ...
}

Java

Constructors / Destructors

C#

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.
  }
}

Java

Objects

C#

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.");

Java

Properties

C#

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++;

Java

Structs

C#

No structs in Java.

struct StudentRecord {
  public string name;
  public float gpa;

public StudentRecord(string name, float gpa) {
    this.name = name;
    this.gpa = gpa;
  }
}

StudentRecord stu = new StudentRecord("Bob", 3.5f);
StudentRecord stu2 = stu;

stu2.name = "Sue";
Console.WriteLine(stu.name);    // Prints "Bob"
Console.WriteLine(stu2.name);   // Prints "Sue"

Java

Console I/O

C#

java.io.DataInput in = new java.io.DataInputStream(System.in);
System.out.print("What is your name? ");
String name = in.readLine();
System.out.print("How old are you? ");
int age = Integer.parseInt(in.readLine());
System.out.println(name + " is " + age + " years old.");

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());

Console.Write("What's your name? ");
string name = Console.ReadLine();
Console.Write("How old are you? ");
int age = Convert.ToInt32(Console.ReadLine());
Console.WriteLine("{0} is {1} years old.", name, age);
// or
Console.WriteLine(name + " is " + age + " years old.");

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());

Java

File I/O

C#

import java.io.*;

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

Page last modified: 02/01/2011 06:45:24

This work is licensed under a Creative Commons License.

Please send any corrections or comments to fmccown@harding.edu.

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