Primitive Types:

• Integers (all signed)
  • byte (8 bits)
  • short (16 bits)
  • int (32 bits)
  • long (64 bits)
• Floating Point
  • float (32 bits, IEEE 754)
  • double (64 bits, IEEE 754)
• Characters
  • char (16-bit Unicode)
    The first 128 of these are ASCII}
• Boolean
  • boolean
    values true and false
    Integers may not be used in place of logical variables
    May not be cast to an integer

Type Promotion in Expressions

class Test1 {
   public static void main(String args[]) {
      byte b = 25;
      b = b*4;
      System.out.println("Hello World! b = " + b);
   }
}
 
make Test1.class
javac Test1.java
Test1.java:4: Incompatible type for =.
Explicit cast needed to convert int to byte.
      b = b*4;
        ^
1 error
*** Error code 1
make: Fatal error: Command failed for target `Test1.class'

class Test2 {
   public static void main(String args[]) {
      int b = 25;
      b = b*4;
      System.out.println("Hello World! b = " + b);
   }
}
 
make Test2.class
javac Test2.java
 
java Test2
Hello World! b = 100

Assignment Conversion:

A narrowing conversion is only allowed if:

• the expression is of type int
• the variable is of type byte, short, or char
• the value of the expression is known at compile time and fits in the variable.

class Test3 {
   public static void main(String args[]) {
      byte b = 25;
      b = 100;
      System.out.println("Hello World! b = " + b);
   }
}

class Test4 {
   public static void main(String args[]) {
      byte b = 25;
      b = 25*4;
      System.out.println("Hello World! b = " + b);
   }
}
 
make Test4.class
javac Test4.java
Test4.java:4: Incompatible type for =.
Explicit cast needed to convert int to byte.
      b = 25*4;
        ^
1 error
*** Error code 1
make: Fatal error: Command failed for target `Test4.class'

Type Casting

Anything can be cast to a String

boolean values cannot be cast to anything else.

You can cast between any arithmetic types (integer, floating point, character)

Basically for objects, you can only cast if one is a subclass of the other.

Java Types and Default Values

All variables are given a value when they are created.

There are two classifications of types in Java.

• Primitive Types
  • integer types: byte, short, int, long
  • floating point types: float, double
  • character type: char
  • boolean type: boolean
• Reference Types
  • class types
  • interface types
  • array types

All variables of reference type have default value null.

Arrays

Unlike in C, arrays are created dynamically.
A declaration does not automatically create the array.
Arrays are created by new.

Array declarations which do not create an array:

   int num[];

   int[] num;

   int num[] = new int[10];

   int[] num = new int[10];

   int num[] = {2,4,6,8};

   int num[];
   int size;
 
   size = 20;
   num = new int[size];

Bounds checking is done on all array references.

A program can determine the size of an array:

class Test5 {
   public static void main(String args[]) {
      int num[];
      int size;
      int i;
 
      size = 3;
      num = new int[size];
      for (i=0; i < num.length; i++)
         num[i] = i*i;
      for (i=0; i < num.length; i++)
         System.out.println("num[" + i + "] = " + num[i]);
   }
}
 
java Test5
num[0] = 0
num[1] = 1
num[2] = 4

Arrays with Multiple Dimensions

class Test6 {
   public static void main(String args[]) {
      double   matrix[][];
      byte[][] bytematrix = {{2,3,4},{6,8,10}};
 
      matrix = new double[3][12];
      int i,j;
 
      System.out.println("matrix has lengths " +
         matrix.length + " and " + matrix[0].length);
      System.out.println("bytematrix has lengths " +
         bytematrix.length + " and " +
         bytematrix[0].length);
      for (i=0; i<2; i++)
         for (j=0; j<3; j++)
         System.out.println(
            "bytematrix[" + i + "][" + j + "] = " +
            bytematrix[i][j]);
   }
}
 
java Test6
matrix has lengths 3 and 12
bytematrix has lengths 2 and 3
bytematrix[0][0] = 2
bytematrix[0][1] = 3
bytematrix[0][2] = 4
bytematrix[1][0] = 6
bytematrix[1][1] = 8
bytematrix[1][2] = 10

Strings

• once created, a string is read-only
• you cannot add characters to a string
• strings are not terminated by a terminator
• constant strings can be defined using double quotes as in C
• the length method returns the length of the string
• the + operator can concatenate strings
• if only one argument of + is a string, the other will be converted to a string
• the charAt method returns individual characters of the string indexed from 0 to length()-1
• the equals() method can be used to compare two strings

A String Example

class Test7 {
   public static void main(String args[]) {
      String str1 = "This is a string";
      String str2;
      String str3;
      String str4;
      int i = 5;
      char fourth;
 
      System.out.println("str1: " + str1);
      System.out.println("str1 has length " + str1.length());
      str2 = str1 + i;
      System.out.println("str2: " + str2);
      str3 = "This is" + " a string";
      System.out.println("str3: " + str3);
      if (str1.equals(str3))
          System.out.println("str1 equals str3");
      else
          System.out.println("str1 and str3 are different");
 
      str1 = "This is a new string 1";
      System.out.println("str1: " + str1);
      if (str1.equals(str3))
          System.out.println("str1 equals str3");
      else
          System.out.println("str1 and str3 are different");
      fourth = str1.charAt(3);
      System.out.println(
         "The fourth character in str1 is " + fourth);
   }
}
 
java Test7
str1: This is a string
str1 has length 16
str2: This is a string5
str3: This is a string
str1 equals str3
str1: This is a new string 1
str1 and str3 are different
The fourth character in str1 is s

A Command Line Arguments Example

class Test8 {
   public static void main(String args[]) {
      int numargs;
      int i;
 
      numargs = args.length;
      System.out.println(
         "Number of command line arguments: " + numargs);
      for (i=0; i< numargs; i++)
         System.out.println("   " + i + ": " + args[i]);
   }
}
 
java Test8
Number of command line arguments: 0
 
java Test8 abc def ghij
Number of command line arguments: 3
   0: abc
   1: def
   2: ghij
 
java Test8 abc "def ghij"
Number of command line arguments: 2
   0: abc
   1: def ghij

Arrays of Characters

class Test9 {
   public static void main(String args[]) {
      String str1 = "A string";
      String str2;
      char arr[];
      int strlen;
      int i;
 
      strlen = str1.length();
      System.out.println("str1: " + str1);
      System.out.println("str1 has length " + strlen);
      arr = new char[strlen];
      str1.getChars(0,strlen,arr,0);
      for (i=0; i< strlen; i++)
         System.out.println("   " + i + ": " + arr[i]);
      arr[2] = 'S';
      str2 = String.copyValueOf(arr);
      System.out.println("str2: " + str2);
   }
}
 
java Test9
str1: A string
str1 has length 8
   0: A
   1:
   2: s
   3: t
   4: r
   5: i
   6: n
   7: g
str2: A String

String Buffers

class Test10 {
   public static void main(String args[]) {
      StringBuffer mystrbuf = new StringBuffer();
      String str1 = "A string of any size";
      String str2 = " Another string of any length";
      String str3;
 
      System.out.println("str1: " + str1);
      System.out.println("str2: " + str2);
      mystrbuf.append(str1);
      mystrbuf.append(str2);
      str3 = mystrbuf.toString();
      System.out.println("str3: " + str3);
   }
}
 
java Test10
str1: A string of any size
str2:  Another string of any length
str3: A string of any size Another string of any length

Operators

Here are a few of the differences.

• No unary & * operators
• No -> or sizeof operators
• The comma operator can only be used in the initialization and continuation sections of a for loop.
• If one argument of the binary + operator is a string the other argument will be converted to a string and the strings will be concatenated.
• The >>> operator is an unsigned shift right.

Flow Control

• if-else
• switch
• while and do-while
• for
• break
• continue
• return

double arr[][];
 
outer:
   for (i=0; i < arr.length; i++) {
      for (j=0; i < arr[0].length; j++) {
         if (arr[i][j] == val) {
            foundit = true;
            break outer;
         }
      }  
   }