Java basic

1. in one .java file, there could be as many class as you wish, but there should be at most 1 public class, and this class name should be the same with the file name.

// B.java

class A

{
}

public class B

{
}

class C

{
}

2. for boolean, there are only true and false, no -1, 0,1
in C++, you can write like this:

while(1)

{
}

but in Java, while(1) is wrong, and you can only use:

while (true)

{
}

3. define array

//-1

int num[];

num=new int[3]; //ok;

int num[]=new int[3];//ok

int num[]={1,2,3};//ok

int num[]=new int[]{1,2,3};//ok

int num[]=new int[3]{1,2,3};//error

int num[3]; //error



//-2

int [][] num;

num=new int[2][];

num[0]=new int[3]; //3 elements

num[1]=new int[4]; //4 elements

4. for loop

for(int i=0; i<5; i++)

{
   int a=i; //ok

}

int b=i;// error, i is not defined out of the for loop

5. print hex number

  int hx=0xffffffff;
  int hc=hx<<2;
  System.out.println(Integer.toHexString(hc));

6. default value

int   0;

char '\0'

boolean false

object  null

7.  same name for parameter and member

class Point

{
    int x,y;

    void func1(int x, int y)

   {

      x=x; //parameter to parameter, not what we want

      y=y; //parameter to parameter, not what we want

   }

  

    void func2(int x, int y)

   {

      this.x=x; //parameter x to member x

      this.y=y; //parameter y to member y

   }



}

8. call another construction method

 class Point

{
    int x,y;

    Point(int a, int b)

  {

     x=a;

     y=b;

  }



  Point()

 {

    this(3,4);// call another construct method, and should be the first statement

 }

}

9. static
static methods could call static methods and access the static variable, could not call the non-static methods or access variable;
while  non-static methods  could call static methods and access to static variable.
10.. final variable

In most cases, the final variable should be initialized when it is defined.

class Point

{

    final double PI=3.1415926;

}



one exception:

class Point

{

    final double PI;

    Point()

  {

     PI=3.1415926; 

  }

}

however, if the final variable is also static, you must initialize it when defining it:

static final double PI=3.1415926;

11. call parent method

public class MyAnimal
{
   MyAnimal()
  {
       System.out.println("I am Animal! without parameter");
  }

     MyAnimal(int param)
  {
       System.out.println("I am Animal! with parameter");
  }

}


 class MyFish extends MyAnimal
{

      public MyFish()
   {
      // in this place, super()is called even if you do not write it;

     // so if there is no MyAnimal() at all, error would happen.

     // in that case, you could use  super(0) if  MyAnimal(int param) is defined

     // or you could redifine the MyAnimal() 

      System.out.println("I am fish! without parameter");
   }

    public MyFish(int param)
   {
      //in this place, still super(),not super(0), is called even if you do not write it;

     // to call MyAnimal(int param), should write super(0) in this place

     // 0 represents the parameter, you can use anyone you like
      System.out.println("I am fish! with parameter!");
 
 } 

    public void breathe()
    {
          super. breathe(); // use this call  parent  breathe();

          System.out.println("Fish breathe!");

    }

}

12. polymorphic

class Parent

{

    void fun1()

  {

     System.out.println(" Parent 1");

  }

   void fun2()

  {

     System.out.println(" Parent 2");

  }

}



class Child extends Parent

{

  

   void fun1()

  {

     System.out.println(" Child 1");

  }  

  public static void main(String[] args)

 {

       Child cd=new Child();

       Parent pt=cd;//Ok

       pt.fun1();    // result: Child 1, because of new Child();

       pt.fun2();    // result: Parent 2, because Child dose not override fun2()



       Parent pt1=new Parent();//Ok

       Child cd1=pt1;//error, can not convert from Parent to Child

 }

}