Java (J2SE 5.0) and C# Comparison-优快云博客

本文对比了Java和C#两种编程语言的关键语法差异，包括程序结构、注释、数据类型、常量、枚举、运算符等内容。适用于希望了解这两种语言特性和用法的开发者。

摘要生成于 C知道，由 DeepSeek-R1 满血版支持，前往体验 >

Java (J2SE 5.0) and C# Comparison
This is a quick reference guide to highlight some key syntactical differences between Java and C#.
This is by no means a complete overview of either language. Hope you find this useful!
Also see VB.NET and C# Comparison.

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 + "!");
}
}
}

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

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

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 is3 (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 is3 (truncates decimal)

// 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;

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

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 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;

// 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

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

// String comparison
Stringmascot = "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
stringmascot = "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 methodthat 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; // Importsingle class

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

namespace Harding.Compsci.Graphics {
...
}

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 callparent'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.

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.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());

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();