Java for Beginner - 3 - Java from C# cheats sheet

Doing C# long enough, you get yourself familiar with C# concepts and syntax. That’s fine, but sometime when you need “the same” concepts in Java, it’s quite difficult. This cheat sheet here to tackle that problem.

You can view other posts in the series here:

1. Introduction
2. Hello World
3. Cheats sheet (this post)

Table of Content

• 1. Classes
  • 1.1. Some keywords
  • 1.2. Generic Class
  • 1.3. Interface Implement
• 2. Variables
• 3. Methods
  • 3.1. Extension Method
  • 3.2. Safely use resource
  • 3.3. Class Properties

1. Classes

1.1. Some keywords

C#

Java

Explanation for Java

: (to extend the class)

extends

Java use the keyword extends to extend a class

base

super

To access the base class methods

namespace

package

Java class belongs to a package, which is the name of the folder contains the .java file

internal

Java use the concept of package. More complicated convertion is here

using

import

Java Example

// File Animal.java, inside folder animals
package main.java.animals;

import RandomPackage.*;
import RandomPackage.RandomSubPackage.*;

public class Animal {
… other methods and properties …
public Double getWeight() {
return weight;
}
… other methods and properties …
}

// File Pig.java, inside folder animals
package main.java.animals;

public class Pig extends Animal {
… other methods and properties …

// override the getWeight method
public Double getWeight() {
    // do your override stuffs
    return super.getWeight();
}
... other methods and properties ...

}

1.2. Generic Class

Generic class with constraint

C# Example

public class GenericClass where T: SomeBase
{
}

Java Example

public class GenericClass
{
}

1.3. Interface Implement

Java Example

public class Pig implements IAnimal {
public void Run() {
…your code to make the pig run…
}
}

2. Variables

C#

Java

Explanation for Java

string

java.lang.String

string in both is a immutable class (cannot change instance of object after create)

object

java.lang.Object

decimal

java.math.BigDecimal

Java Example

String hello = “Hello World!”;
System.out.print(hello);

3. Methods

3.1. Extension Method

In C#, you can write extension to an object, there is no equivalent to this in Java.

C#

public static class StringExtension
{
public static string GetFirstThreeLetters(this string targetString)
{
// your code here to do the job
// for example:
// string result = targetString.SubString(0,3);
return result;
}
}

public class TestClass
{
public void TestMethod()
{
string randomString = “This is so random”;
Console.WriteLine(randomString.GetFirstThreeLetters);
}
}

// Console Output
// Thi

But in Java, you need to have the full static class

public final class StringUtils {
public static String GetFirstThreeLetters(String stringToGet) {
// your code here to do the job
// for example:
// String result = stringToGet.substring(0,3);
return result;
}
}

public class TestClass
{
private void TestMethod()
{
String randomString = “This is so random”;
StringUtils.GetFirstThreeLetters(randomString);
}
}

// Console Output
// Thi

3.2. Safely use resource

In C#, you can use a resource safely with the keyword using

C# Example

using(MyResource myResource = new MyResource())
{
myResource.DoSomething();
}

In Java, you will use a different keyword: try

Java Example

try(MyResource myResource = new MyResource()) {
myResource.DoSomething();
}

3.3. Class Properties

It is very convenient for C# developers to create and use property with auto-property, property with private backing fields.

C# Example

public class Animal
{
// Auto-property
public int Height { get; set; }

// Property with private backing field

// {rivate field
private int \_weight;

// Exposed with public property
public int Weight
{
    get
    {
        return \_weight;
    }
    set
    {
        // do some check to validate the value
        if(value > 0)
        {
            \_weight = value;
        }
    }
}

}

// using in other class
public void RandomMethod()
{
Animal pig = new Animal();
pig.Weight = 500;

Console.WriteLine("Pig weight: " + pig.Weight);

}

There is no equivalent to this in Java, you need to create 2 normal methods for get and set value of a private field

Java Example

public class Animal {
private int _weight;

public void setWeight(int weight) {
    if(weight > 0) {
        \_weight = weight;
    }
}

public int getWeight() {
    return \_weight;
}

}

// using in other class
public void RandomMethod() {
Animal pig = new Animal();
pig.setWeight(500);

System.out.print("Pig weight: " + pig.getWeight());

}

Other different is just one google search result away ;) See you next post