Removing elements from an ArrayList object

  • Methods used to update and remove an element in an ArrayList object: 

      set(int index, E e):
         Replace the element at the position index with e
	 Returns the old value that was stored in that element. 


    
  remove(int index):
         Removes the element at the position index 
	 from this list (and returns it)
	 Returns the old value that was stored in that element.

  remove(E e):
         Removes the first occurrence of the element e
	 from this list. 
	 Returns true if successful; otherwise, returns false

  set( index , elem )  

  • set( index, elem ) replaces the element stored at the index "index" with element "elem" and returns the old value.

    Before set(1, "AAA"):   

    After set(1, "AAA"):     

  set( index , elem )  

  • Syntax of set( index , elem ): 

            myList.set(index , elem);

  • Example:   replace the element at index 1 with "AAA" 

        public static void main(String[] args)
    {
        ArrayList<String> myList = new ArrayList<>();
        
        myList.add("john"); 
	myList.add("peter"); // Index 1
        myList.add("mary"); 
	myList.add("peter");

        System.out.println( myList.set(1, "AAA") ); // peter
        System.out.println( myList ); // [john,AAA,mary,peter]
    }

DEMO: emo/11-arrayList/04-update/Set.java

  remove( index )  

  • remove( index ) removes the element stored at the index "index" and returns it.

    Before remove(1):   

    After remove(1):     

  remove( index )  

  • Syntax of remove( index ): 

            myList.remove(index);

  • Example:   romove the element at index 1 from an ArrayList<String> object: 

        public static void main(String[] args)
    {
        ArrayList<String> myList = new ArrayList<>();
        
        myList.add("john"); 
	myList.add("peter"); // Index 1
        myList.add("mary"); 
	myList.add("peter");

        System.out.println( myList.remove(1) ); // peter
        System.out.println( myList ); // [john,mary,peter]
    }

DEMO: emo/11-arrayList/04-update/Remove1.java

Using remove(index) to remove all occurences in an ArrayList   (a frequent question in AP CS exams)

  • Problem description: remove all occurences of "peter" in an ArrayList: 

        public static void main(String[] args)
    {
        ArrayList<String> myList = new ArrayList<>();
        
        myList.add("john"); 
	myList.add("peter"); // occurence 1
        myList.add("mary"); 
	myList.add("peter"); // occurence 2

        // Remove all occurence of "peter" in myList
	int indexPeter = myList.indexOf("peter");    // Find "peter"

	while ( indexPeter != -1 )
	{
	     myList.remove( indexPeter );           // If found, remove
	     indexPeter = myList.indexOf("peter");  // Repeat
	}   
	System.out.println(myList);
    }

Using remove(index) to remove all occurences in an ArrayList   (a frequent question in AP CS exams)

  • Find the first occurence: 

        public static void main(String[] args)
    {
        ArrayList<String> myList = new ArrayList<>();
        
        myList.add("john"); 
	myList.add("peter"); // occurence 1
        myList.add("mary"); 
	myList.add("peter"); // occurence 2

        // Remove all occurence of "peter" in myList
	int indexPeter = myList.indexOf("peter");    // Find "peter"

	while ( indexPeter != -1 )
	{
	     myList.remove( indexPeter );           // If found, remove
	     indexPeter = myList.indexOf("peter");  // Repeat
	}   
	System.out.println(myList);
    }

Using remove(index) to remove all occurences in an ArrayList   (a frequent question in AP CS exams)

  • As long as we can still find an occurence: 

        public static void main(String[] args)
    {
        ArrayList<String> myList = new ArrayList<>();
        
        myList.add("john"); 
	myList.add("peter"); // occurence 1
        myList.add("mary"); 
	myList.add("peter"); // occurence 2

        // Remove all occurence of "peter" in myList
	int indexPeter = myList.indexOf("peter");    // Find "peter"

	while ( indexPeter != -1 )  // "peter" was found in myList
	{
	     
	     
	}   
	System.out.println(myList);
    }

Using remove(index) to remove all occurences in an ArrayList   (a frequent question in AP CS exams)

  • Delete that occurence: 

        public static void main(String[] args)
    {
        ArrayList<String> myList = new ArrayList<>();
        
        myList.add("john"); 
	myList.add("peter"); // occurence 1
        myList.add("mary"); 
	myList.add("peter"); // occurence 2

        // Remove all occurence of "peter" in myList
	int indexPeter = myList.indexOf("peter");    // Find "peter"

	while ( indexPeter != -1 )  // "peter" was found in myList
	{
	     myList.remove( indexPeter );          // If found, remove 
	     
	}   
	System.out.println(myList);
    }

Using remove(index) to remove all occurences in an ArrayList   (a frequent question in AP CS exams)

  • Check if there are any more occurences: 

        public static void main(String[] args)
    {
        ArrayList<String> myList = new ArrayList<>();
        
        myList.add("john"); 
	myList.add("peter"); // occurence 1
        myList.add("mary"); 
	myList.add("peter"); // occurence 2

        // Remove all occurence of "peter" in myList
	int indexPeter = myList.indexOf("peter");    // Find "peter"

	while ( indexPeter != -1 )  // "peter" was found in myList
	{
	     myList.remove( indexPeter );          // If found, remove 
	     indexPeter = myList.indexOf("peter"); // Repeat
	}   
	System.out.println(myList);
    }

DEMO: demo/11-arrayList/04-update/Remove1All.java

Using remove(index) to remove all items > "ppp" in an ArrayList   (a frequent question in AP CS exams)

  • Problem description: remove all elements that are > "ppp" in an ArrayList: 

        public static void main(String[] args)
    {
        ArrayList<String> myList = new ArrayList<>();
        
        myList.add("aaa"); myList.add("qqq");
	myList.add("ccc"); myList.add("zzz"); 

        // Remove all elements > "ppp" in myList
	int i = 0; 

	while ( i < myList.size() )
	{
	    if ( myList.get(i).compareTo("ppp") > 0 )
	        myList.remove( i ); 
		// Do not increment i, because another element 
		// will occupy slot i in myList !
            else
	        i++; // Do not remove element i
	}   
	System.out.println(myList);
    }

 

Using remove(index) to remove all items > "ppp" in an ArrayList   (a frequent question in AP CS exams)

  • We use index i to run through the indexes from   0   to   myList.size() - 1: 

        public static void main(String[] args)
    {
        ArrayList<String> myList = new ArrayList<>();
        
        myList.add("aaa"); myList.add("qqq");
	myList.add("ccc"); myList.add("zzz"); 

        // Remove all elements > "ppp" in myList
	int i = 0;  // Go through all elements in myList 

	while ( i < myList.size() )
	{
	    if ( myList.get(i).compareTo("ppp") > 0 )
	        myList.remove( i ); 
		// Do not increment i, because another element 
		// will occupy slot i in myList !
            else
	        i++; // Do not remove element i
	}   
	System.out.println(myList);
    }

 

Using remove(index) to remove all items > "ppp" in an ArrayList   (a frequent question in AP CS exams)

  • If element i in array list is > "ppp", we remove it (and re-check the element i): 

        public static void main(String[] args)
    {
        ArrayList<String> myList = new ArrayList<>();
        
        myList.add("aaa"); myList.add("qqq");
	myList.add("ccc"); myList.add("zzz"); 

        // Remove all elements > "ppp" in myList
	int i = 0;  // Go through all elements in myList 

	while ( i < myList.size() )
	{
	    if ( myList.get(i).compareTo("ppp") > 0 )
	        myList.remove( i ); 
		// Do not increment i, because another element 
		// will occupy slot i in myList after remove()!
            else
	        i++; // Do not remove element i
	}   
	System.out.println(myList);
    }

 

Using remove(index) to remove all items > "ppp" in an ArrayList   (a frequent question in AP CS exams)

  • Otherwise, we move to the next element i+1: 

        public static void main(String[] args)
    {
        ArrayList<String> myList = new ArrayList<>();
        
        myList.add("aaa"); myList.add("qqq");
	myList.add("ccc"); myList.add("zzz"); 

        // Remove all elements > "ppp" in myList
	int i = 0;  // Go through all elements in myList 

	while ( i < myList.size() )
	{
	    if ( myList.get(i).compareTo("ppp") > 0 )
	        myList.remove( i ); 
		// Do not increment i, because another element 
		// will occupy slot i in myList after remove()!
            else
	        i++; // Skip element i (do not remove)
	}   
	System.out.println(myList);
    }

DEMO: demo/11-arrayList/04-update/Remove1All2.java

  remove( element )  

  • remove( element ) removes the first occurence of element and returns true if removal was successful and returns false otherwise

    Before remove("peter"):   

    After remove("peter"):     

  remove( element )  

  • Syntax of remove( element ): 

            myList.remove(element);

  • Example:   remove the first occurence of "peter" 

        public static void main(String[] args)
    {
        ArrayList<String> myList = new ArrayList<>();
        
        myList.add("john"); 
	myList.add("peter"); // first occurence
        myList.add("mary"); 
	myList.add("peter");

        System.out.println( myList.remove("peter") ); // true
        System.out.println( myList ); // [john,mary,peter]
    }

DEMO: demo/11-arrayList/04-update/Remove2.java

Using remove(element) to remove all occurences in an ArrayList   (a frequent question in AP CS exams)

  • Problem description: remove all occurences of "peter" in an ArrayList: 

        public static void main(String[] args)
    {
        ArrayList<String> myList = new ArrayList<>();
        
        myList.add("john"); 
	myList.add("peter"); // occurence 1
        myList.add("mary"); 
	myList.add("peter"); // occurence 2

        // Remove all occurence of "peter" in myList

	while ( myList.remove( "peter" ) ) ;
	  
	System.out.println(myList);
    }

    Alternative solution:   use   remove( element )   instead of   remove( index )

Using remove(element) to remove all occurences in an ArrayList   (a frequent question in AP CS exams)

  • Remove it if a (first) occurence can be found: 

        public static void main(String[] args)
    {
        ArrayList<String> myList = new ArrayList<>();
        
        myList.add("john"); 
	myList.add("peter"); // occurence 1
        myList.add("mary"); 
	myList.add("peter"); // occurence 2

        // Remove all occurence of "peter" in myList

	while ( myList.remove( "peter" ) ) ; // returns true if found
	  
	System.out.println(myList);
    }

    Alternative solution:   use   remove( element )   instead of   remove( index )

Using remove(element) to remove all occurences in an ArrayList   (a frequent question in AP CS exams)

  • Keep on removing as long as a (first) occurence can be found to remove all: 

        public static void main(String[] args)
    {
        ArrayList<String> myList = new ArrayList<>();
        
        myList.add("john"); 
	myList.add("peter"); // occurence 1
        myList.add("mary"); 
	myList.add("peter"); // occurence 2

        // Remove all occurence of "peter" in myList

	while ( myList.remove( "peter" ) ) ; // returns true if found
	  
	System.out.println(myList);
    }

    Alternative solution:   use   remove( element )   instead of   remove( index )

DEMO: demo/11-arrayList/04-update/Remove2All.java

Other useful methods in ArrayList

  • Obtaining information about an ArrayList object: 

      size():
       Returns the number of elements in this ArrayList.

  isEmpty():
       Returns true if this ArrayList contains no elements.
       Otherwise return false

  • Resetting an ArrayList object: 

     clear():
       Removes all of the elements from this ArrayList.

(1) Number of elements, (2) check if an ArrayList is empty and (3) clear an ArrayList 

public class Info
{
    public static void main(String[] args)
    {
        ArrayList<String> myList = new ArrayList<>();
        
        System.out.println( myList.size() );    // 0
        System.out.println( myList.isEmpty() ); // true

        myList.add("john"); 
	myList.add("peter"); 
        myList.add("mary"); 
	myList.add("peter");

        System.out.println( myList.size() );    // 4
        System.out.println( myList.isEmpty() ); // false

	myList.clear();

        System.out.println( myList.size() );    // 0
        System.out.println( myList.isEmpty() ); // true
    }
}

DEMO: demo/11-arrayList/04-update/Info.java