Defining array variables in Java

  • Syntax to define an array (reference) variable: 

     dataType[] arrayRefVar;   // Defines an array reference variable

  • Syntax to create an array object: 

     new dataType[N]; // Create an array of N elements

  • Properties of an array in Java:

    • All elements in the array have the same data type.

    • Array elements are stored consecutively in memory

  • Example: define an array of 10 int variables 

       int[] A; 
   A = new int[10];

What happens inside the computer when you define an array

The following diagrams show what happens inside the computer system: 

     int[] A;  // A is a reference variable !
     A = new int[10];

"int[] A" will allocate (reserve memory) a reference variable A:

What happens inside the computer when you define an array

The following diagrams show what happens inside the computer system: 

     int[] A;  // A is a reference variable !
     A = new int[10];

"new int[10]" will allocate (reserve memory) for an int[10] array (= 40 bytes) and return its base address:

What happens inside the computer when you define an array

The following diagrams show what happens inside the computer system: 

     int[] A;  // A is a reference variable !
     A = new int[10];

"A = " will assign the return value to the variable A:

What happens inside the computer when you define an array

Summary of the effect of: 

     int[] A;  // A is a reference variable !
     A = new int[10];

The reference variable A will point to a newly created int[10] (array of 10 integers) object:

Copying an array

  • Copy an array means:

      • Make a duplicate of an array where the duplicate contains the same data as the original

      • Updating array elements in the duplicate must not affect the data in the original array

  • Schematically:   situation before making a copy of the array myList

Copying an array

  • Copy an array means:

      • Make a duplicate of an array where the duplicate contains the same data as the original

      • Updating array elements in the duplicate must not affect the data in the original array

  • Schematically:   situtaion after making a copy of the array myList

How to copy an array in Java    step-by-step instructions

  • Initial state: 

        public static void main(String[] args) 
    {
        double[] myList = {34, 15, 66, 7};
       
        double[] myListCopy = new double[ myList.length ];
        
        for ( int i = 0; i < myList.length; i++ )
            myListCopy[i] = myList[i];
    }

How to copy an array in Java    step-by-step instructions

  • Step 1: create an array to store the copy 

        public static void main(String[] args) 
    {
        double[] myList = {34, 15, 66, 7};
      
        double[] myListCopy = new double[ myList.length ];
 
        for ( int i = 0; i < myList.length; i++ )
            myListCopy[i] = myList[i];
    }

How to copy an array in Java    step-by-step instructions

  • Step 2: copy the array elements to the copy array 

        public static void main(String[] args) 
    {
        double[] myList = {34, 15, 66, 7};
      
        double[] myListCopy = new double[ myList.length ];
 
        for ( int i = 0; i < myList.length; i++ )
            myListCopy[i] = myList[i];
    }

DEMO: demo/02-review/CopyArray.java

A common error in array copy

  • Variables of primitive data types (such as int, double, etc) can be copied with an assignment: 

        int x = 4;
    int xCopy;

    xCopy = x;   // Copies x to xCopy

  • However, in Java, the assignment operation will not copy objects of non-primitive data types: 

        double[] myList = {34, 15, 66, 7};

    double[] myListCopy;

    myListCopy = myList;   // Does not copy an array object 
                           // ** This copies the reference
			   // ** in myList to myListCopy

The effect of assignment with a reference variable

  • Initially we have the array myList:                                                                                     

        double[] myList = {34, 15, 66, 7};

    Schematically:

     

The effect of assignment with a reference variable

  • The definition double[] myListCopy defines another reference variable:                                         

        double[] myList = {34, 15, 66, 7};

    double[] myListCopy;

    Schematically:

     

The effect of assignment with a reference variable

  • The assignment myListCopy = myList will copy the reference in myList to the myListCopy variable: 

        double[] myList = {34, 15, 66, 7};

    double[] myListCopy;

    myListCopy = myList;   // Copies an reference

    Schematically:

    Now myList and myListCopy both reference to the same array object

The effect of assignment with a reference variable

  • Because myListCopy and myList reference the same array object, updates made with myListCopy[i] will also affect myList[i] and vice verse: 

    public class Alias
{
    public static void main(String[] args) 
    {
        double[] myList = {34, 15, 66, 7};

        double[] myListCopy;

        myListCopy = myList; // Does not copy an array object 

	myListCopy[1] = 999; // Update will also affect myList[1]

	for ( int i = 0; i < myList.length; i++ )
	    System.out.print( myList[i] + " " );
    }  
}

Demo: demo/02-review/Alias.java

  Alias  

  • We saw in the last example that: myListCopy[1] and myList[1] were the different names for the same (array) variable: 

        public static void main(String[] args) 
    {
        double[] myList = {34, 15, 66, 7};

        double[] myListCopy;

        myListCopy = myList; // Does not copy an array object 

	myListCopy[1] = 999; // Update will also affect myList[1]
    }

  • Alias:

      • When a different variable names can be used to reference the same variable, they are called aliases in computer science