Getting Started

Commonly Used Java Objects

1. 
   int iVal = 1;  
   
2. 
   Integer integ1= new Integer(iVal);  
   
3. 
   float f = integ1.floatValue();  
   

1. 
   String strHello = new String("Hello World");  
   

For more complex data structures like lists, queues, stacks, dates, and times, appropriate classes are in the java.util package.

Finally, Android applications rely on a number of helpful classes that define the commonly used application components, like Activity, Application, Dialog, and Service. These classes can be found in the android.app package.

Class Permissions and Access

For example, the following SillySensor class definition defines several variables and methods with different access levels:

• 
  A class variable called sensorData, which is only visible within the class
  
• 
  A public constructor that can be called outside the class
  
• 
  A private method called calibrate(), which can only be called from within the class itself
  
• 
  A protected method called seedCalibration(), which can be called from within the class itself, or by a subclass
  
• 
  A public method called getSensorData(), which can be called from anywhere, allowing for “read-only” access to the sensorData variable
  

1. 
   public class SillySensor  
   
2. 
   {  
   
3. 
       private int sensorData;    
   
4. 
       public SillySensor()  
   
5. 
       {  
   
6. 
           sensorData=0;  
   
7. 
       }  
   
8. 
       private void calibrate(int iSeed)  
   
9. 
       {  
   
10. 
            // Do some calibration here  
    
11. 
        }  
    
12. 
        protected void seedCalibration(int iSeed)  
    
13. 
        {  
    
14. 
            calibrate(iSeed);  
    
15. 
        }  
    
16. 
        public int getSensorData()  
    
17. 
        {  
    
18. 
            // Check sensor here  
    
19. 
            return sensorData;  
    
20. 
        }  
    
21. 
    }  
    

Conditionals

Java has all the typical conditional operators, such as:

• 
  == equal to, as in (a == b)
  
• 
  != not equal to, as in (x != y)
  
• 
  > greater than, as in (z > y)
  
• 
  >= greater than or equal to, as in (q >= z)
  
• 
  < less than, as in (b < a)
  
• 
  <= less than or equal to, as in (a <= z)
  

• 
  ((a==b)&& (a==c))     // true only if A is equal to B and equal to C
  
• 
  ((a==b) || (a==c))         // true only if A is equal to B or equal to C
  

Now that you know how to craft a conditional statement, you can create conditional code segments. The simplest form of a conditional code statement is the if() statement:

1. 
   boolean condition = true;  
   
2. 
   if(condition==true)  
   
3. 
   {      // Execute this code only if  condition variable is true  }  
   

1. 
   boolean condition = true;  
   
2. 
   if(condition==true)  
   
3. 
   {  
   
4. 
       // Execute this code only if condition variable value is true  
   
5. 
   } else {  
   
6. 
       // Execute this code only if condition variable value is false  
   
7. 
   }  
   

1. 
   if(iVar==0) {  
   
2. 
       // variable is zero  
   
3. 
   } else if (iVar > 0) {  
   
4. 
       // variable is a positive number  
   
5. 
   } else {  
   
6. 
       // variable is a negative number  
   
7. 
   }  
   

Switch Case Statements

1. 
   char singleChar = 'z';  
   
2. 
   switch(singleChar) {  
   
3. 
   case 'a':  
   
4. 
   case 'e':  
   
5. 
   case 'i':  
   
6. 
   case 'o':  
   
7. 
   case 'u':  
   
8. 
       // singleChar is a vowel! Execute this code!  
   
9. 
       break;  
   
10. 
    default:  
    
11. 
         // singleChar is a consonant! Execute this code instead!  
    
12. 
         break;  
    
13. 
    }  
    

Loops

To loop continuously provided that a statement is true, use a while() loop:

1. 
   int numItemsToProcess = 3;  
   
2. 
   while(numItemsToProcess > 0)  
   
3. 
   {  
   
4. 
       // process an item  
   
5. 
       numItemsToProcess--;  
   
6. 
   }  
   

1. 
   do  
   
2. 
   {  
   
3. 
       // check for items to process, update numItemsToProcess as required  
   
4. 
       // process item, updated numItemsToProcess  
   
5. 
   } while (numItemsToProcess > 0);  
   

1. 
   for(int i = 1; i <=100; i++)  
   
2. 
   {  
   
3. 
       // print i  
   
4. 
   }