Java: Objects and Object References

What is an Object? ,[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object]

Software objects ,[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],Variables balance interest rate owner Methods deposit() withdraw() charge() Bank account Chunk of memory

Using a Class to make Objects ,[object Object],[object Object],[object Object],[object Object]

[object Object],[object Object],[object Object],[object Object],[object Object],Example Program class StringTester { public static void main ( String [] args ) { String str1 ; // str1 is a variable that refers to an object, // but the object does not exist yet. int len; // len is a primitive variable of type int str1 = new String ( "Salam Shabaab" ); //create an object of type String len = str1 . length(); // invoke the object's method length() System . out . println( "The string is " + len + " characters long" ); } }

Constructors ,[object Object],[object Object],[object Object],[object Object],[object Object]

Primitive data types & Objects ,[object Object],[object Object],[object Object],Primitive Data Objects int long float double char boolean String Applet Graphics

Object References ,[object Object],[object Object],[object Object],Kind of Variable Information it Contains When on the left of "=" primitive variable Contains actual data. Previous data is replaced with new data. reference variable Contains information on how to find an object. Old reference is replaced with a new reference

Object References - Examples ,[object Object],[object Object],String strA = new String (“Java”); String strB = new String (“C++”); String strA = new String (“Java”); String strB = strA strB JAVA strA C++ strB JAVA strA

Equality of Reference Variable Contents ,[object Object],strA == strB is true strA == strB is false srtA.equals(strB) is true strA.equals(strB) is true

The Class Point ,[object Object],[object Object],[object Object]

The Class Point Description ,[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object]

Using the Class Point ,[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],a a 0, 0 move() equals() toString() // Constructors Point(); Point(int x, int y); Point( Point pt );

toString() method ,[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],This is a request to run a method of an object. The object referenced by a contains a method toString() which, when called, returns the contents as a String. The reference variable str is used to refer to String Object.

Equivalent Data and Alias ,[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object]

move() Method ,[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],java.awt.Point[x=12,y=7] java.awt.Point[x=10,y=13]

Type casting ,[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],int long float double loss of information ,[object Object],[object Object]

Static Methods ,[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object]

String literals ,[object Object],[object Object],[object Object]

String References as Parameters ,[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object]

The null Value ,[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object]

Empty String ,[object Object],[object Object],[object Object],[object Object]

[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],String Concatenation public String concat (String str)

Some String methods ,[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],public boolean startsWith (String prefix) public int length ();

Some String methods ,[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],String trim (); Removes leading and trailing white spaces. String toLowerCase (); Returns the lower case equivalent of this string. String toUpperCase (); Returns the lower case equivalent of this string.

Strings are Immutable ,[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object]

Temporary Objects ,[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],String d = “JAVA COURSE”.toLowerCase(). String answer = “ Nothing ”; if ( answer.trim().toUpperCase().startsWith(“NO”) ) System.out.println(“Thank you.”);

Java: Objects and Object References

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Java: Objects and Object References