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Primitive Types

Primitive Types. Java offers a number of primitive types eg.) int, short, long double, float char A variable which is declared as a primitive type stores any value assigned to it. A variable which is declared as a class type

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Primitive Types

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  1. Primitive Types • Java offers a number of primitive types eg.) int, short, long double, float char • A variable which is declared as a primitive type stores any value assigned to it. • A variable which is declared as a class type can store the reference to an object of that type.

  2. chars • The char type can store one character value 'A', '\n', '\u00E9‘ • char represented by a 16 bit unsigned integer * A total of 216 different Unicode chars can be represented • int value; char letter; value = 5; letter = ‘5’; value = letter; //stores a 53 letter = value; //error ……. Information can be lost // when converting 32 bits to 16

  3. A String is NOT a primitive type.. • String is a class provided by the Java API (A object of this type stores a ordered group of chars) • Strings ARE OBJECTS!! • Look at Java API specs………….

  4. The String class provides constructors String word; word = new String(“hello”); //constructing a //String object from a String literal Seems redundant …. String is the ONLY class that lets you create a String object by assigning a value…. word = “hello”;

  5. String constructors are useful, however String word, word2; word = JOptionPane.showInputDialog(“Enter info:”); The showInputDIalog method calls one of the String constructors to create a String object. This String object is then returned to the caller. word2 = new String(word); You would call a constructor to make a copy of an existing String.

  6. int length() • length() method returns the number of chars in a String object • System.out.println(“size” + “hello”.length()) ; The length of a String is not always obvious… • String in = JOptionPane.showInputDialog(“Enter”); int size = in.length();

  7. String characters are indexed………… The first character in a String is at index 0. Note: last char is at index: length()-1

  8. char charAt(int) • charAt method returns character from a string • the String object is unchanged by this call String in = JOptionPane.showInputDialog(“Enter”); JOptionPane.showMessageDialog(null,”first char is” + in.charAt(0) ); JOptionPane.showMessageDialog(null,”last char is” + in.charAt(in.length()-1 ) );

  9. String substring(int,int) • This method returns a new String object, which contains a portion of the characters of the invoking object • Parameters supply start and “past the end” position String in, apiece; in = JOptionPane.showInputDialog(“Enter”); //user types hello world apiece = in.substring(1,8); // String in is unchanged // String apiece contains characters “ello wo”

  10. A String is an IMMUTABLE object………… Note that we changed the String object that inwas referring to by creating a new object and assigning the new object to in . It is not possible to change the data (chars) stored in a String object. If you wish to do this, you must create a new String object. This is because the String class has no mutator methods (methods which change its instance data). A class without mutator methods is called IMMUTABLE.

  11. int indexOf(char) • indexOf method returns the position of the first occurrence of parameter (-1 if parameter does not occur) String in = JOptionPane.showInputDialog(“Enter”); int blk = in.indexOf(“ “); //where is first blank? If user had entered hello world, blk now contains 5 in = in.substring(blk+1, in.length() ); //in is now referencing an object without the first word

  12. String substring(int) • Many String methods are OVERLOADED … • This call has an implied 2nd parameter – which is the length of the String String in = JOptionPane.showInputDialog(“Enter”); int blk = in.indexOf(“ “); //where is first blank? If user had entered hello world, blk now contains 5 in = in.substring(blk+1 ); //in is now referencing an object without the first word

  13. + is the String concatenation operator • + is an overloaded operator in Java • String fname = "Harry";String lname = "Hacker";String name = fname + lname; • name is "HarryHacker"   • If one operand of + is a string, the other is converted to a string:String a = "Agent";String name = a + 7; • name is "Agent7"  • You commonly use this for output: • System.out.println(“The value is “ + 7 );     

  14. Converting between Strings and Numbers • Convert to number:int n = Integer.parseInt(str);double x = Double.parseDouble(x); • Convert to string: String str = Integer.toString(n); • More efficient, less readable • str = "" + n; • System.out.println(“” + n ); //explicit conversion needed here

  15. Practice • Convert String word to pig latin (assume word stores 1 word) • Ask user for a a sentence If the word COW is contained in the sentence, replace the first occurrence of it with MOOSE • Replace ALL occurrances of COW with MOOSE

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