Collections class


Collections is a class of handy static utility methods for Lists, Sets, and Maps.


■ More utility methods for Collections are in the Collection Interface.



int binarySearch( List, lookforObj ) method


Note that the collection must be sorted before any seasrching here!


If the item is found, it returns a positive number which is its index. It returns a negative number if not found. The not found return is one less than the index valuewhere the not-found value would be inserted into the list if it were present, minus one more, and as a negative number. i.e. If the not-found element would go at index 3 if present (4th position), you will get back -4. i.e. This snippet returns -4 because the 33 is not found.


String[ ] myArray = {"55", "22", "11", "44", "xx", "77","66","99","00","88"};

ArrayList myList = new ArrayList (Arrays.asList(myArray) );

Collections.sort(myList); // Must sort list before searching!

System.out.println("List now contains: " + myList);

System.out.println(Collections.binarySearch(myList, "33"));



Object max( Coll ) and min( Coll ) methods


These find and return the maximum or minimum values in the collection. i.e. This code will find and return the 99.


String[ ] myArray = {"55", "22", "11", "44", "33", "77","66","99","00","88"};

ArrayList myList = new ArrayList (Arrays.asList(myArray) );




void shuffle( List ) method


Shuffling a List. Performs random shuffling. The same shuffle(...), method, used with asList(...), can shuffle a Strings contents. i.e

import java.util.*;


String[ ] myArray = {"00", "11", "22", "33", "44", "55","66","77","88","99"};

ArrayList myList = new ArrayList (Arrays.asList(myArray) );

System.out.println("Unshuffled: " + myList);


System.out.println("Shuffled: " + myList);



void copy( tolist , fromlist ) method


■ Copies in all elements from the source list.


■ Throws IndexOutOfBoundsException if the target List is too small


■ Ignores any remaining existing elements in the target, if the target List is too big. i.e Here the last five entries in the destination list are unaffected. The first five are overlaid.


String[ ] s1 = {"00", "11", "22", "33", "44", "55","66","77","88","99"};

String[ ] s2 = {"AA", "BB", "CC", "DD", "EE"};

ArrayList l1 = new ArrayList (Arrays.asList(s1) );

ArrayList l2 = new ArrayList (Arrays.asList(s2) );

Collections.copy(l1, l2);

System.out.println("After copy: " + l1);



void fill( List, Obj ) method


■ Fills an existing List with elements of the specified object. Thus every element becomes the same. i.e. This example forces aa into all elements.


String[ ] s1 = {"00", "11", "22", "33", "44", "55","66","77","88","99"};

ArrayList l1 = new ArrayList (Arrays.asList(s1) );

Collections.fill(l1, "aa");

System.out.println("Ater fill: " + l1);



List ncopies( ) method


■ Just for Lists, you can create entire Lists with multiple duplicate elements. i.e.


List lotsofAs = Collections.nCopies(10, A);


List lotsofNulls = Collections.nCopies(10, null);


■ Note that results are immutable, so be careful with your constuction. i.e.


List l = new ArrayList(Collections.nCopies(10, "aa")); // This one is not immutable, because of the new creating another object

System.out.println("Before adds: " + l);


System.out.println("After adds: " + l);


List l = new ArrayList( );

l = (Collections.nCopies(10, "aa")); // This one is immutable

System.out.println("Before adds: " + l);


System.out.println("After add: " + l);