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In this lesson we look at the Utilities hierarchy and examine the java.util.Arrays and java.util.Collections classes that contains a lot of static utility methods we can use with our arrays and collections.

Utilities Hierarchy DiagramTop

The diagram below is a representation of the Utilities hierarchy and covers the classes we will study in this lesson.

java.util.Arrays ClassTop

The java.util.Arrays class gives us methods to search, compare, fill, get a hashcode, sort and give a readable string representation of arrays. All the methods accept Object and most are overloaded to accept a primitive type, appropriate to the method in question. The class also includes a static factory that allows arrays to be viewed as lists.

Arrays Method OverviewTop

The table below shows the declarations of the methods for the java.util.Arrays class used in the example below:

For more information on the other methods in the java.util.Arrays class the following link will take you to the online version of documentation for the JavaTM 2 Platform Standard Edition 5.0 API Specification. Take a look at the documentation for the java.util.Arrays class which you can find by scrolling down the lower left pane and clicking on Arrays.

Arrays ExampleTop

Lets write a simple class that creates some arrays and uses some of the methods from the java.util.Arrays class on them:

/*
  Simple class to create some array and use several methods from the java.util.Arrays class 
*/
import java.util.*; // Import the java.util package

public class UsingJavaUtilArrays {

    public static void main (String[] args) {
        Integer[][] intArray1 = { {5, 33}, {0}, {5, 4, 71} };
 
        List<Integer[]> l = Arrays.asList(intArray1);
        System.out.println(l.toString());
 
        System.out.println("intArray1 is " + Arrays.deepToString(intArray1));
        System.out.println("intArray1[0] is " + Arrays.deepToString(intArray1[0]));
        System.out.println("intArray1[1] is " + Arrays.deepToString(intArray1[1]));
        System.out.println("intArray1[2] is " + Arrays.deepToString(intArray1[2]));

        int intArray2[] = {0, 1, 5, 8, 6, 3, 4, 9, 2, 7};
        System.out.println("intArray2 hashcode = " + Arrays.hashCode(intArray2)); 
        System.out.println("Binary search of intArray2 for a value of 4 = " 
                           + Arrays.binarySearch(intArray2, 4)); 
 
        int intArray3[] = {0, 1, 2, 3, 4, 5, 6, 7, 8, 9};
        System.out.println("intArray3 hashcode = " + Arrays.hashCode(intArray3)); 
        System.out.println("intArray2 = intArray3 ? " + Arrays.equals(intArray2, intArray3)); 
        Arrays.sort(intArray2);
        System.out.println("intArray2 hashcode = " + Arrays.hashCode(intArray2)); 
        System.out.println("intArray2 = intArray3 ? " + Arrays.equals(intArray2, intArray3)); 
    }
}

Save, compile and run the UsingJavaUtilArrays class in directory   c:\_Collections in the usual way.

The above screenshot shows the results of creating and running our UsingJavaUtilArrays class. A few points of interest are the asList() method which converts an array to a list. This method acts as a bridge between the array-based and collection-based APIs, when used in combination with java.util.Collection.toArray(). The returned list is serializable and implements RandomAccess much as an ArrayList would. The other point to notice is how the hashcode() and equals methods are performed. Even though the contents of intArray2 and intArray3 are the same (before the sort) they are in a different order and so have different hashcodes and therefore cannot be equal. After the sort the arrays exactly match in contents and ordering and so are equal.

java.util.Collections ClassTop

The java.util.Collections class static methods give us ways to search, typesafe, empty, synchronize, immute, sort and several other ways to work with our collections. As a reminder collections only work with reference variables, so if you are using primitive variables then these need to be boxed before adding to a collection, or use an array instead.

Collections Method OverviewTop

The table below shows the declarations of the methods for the java.util.Collections class used in the example below:

For more information on the other methods in the java.util.Collections class the following link will take you to the online version of documentation for the JavaTM 2 Platform Standard Edition 5.0 API Specification. Take a look at the documentation for the java.util.Collections class which you can find by scrolling down the lower left pane and clicking on Collections.

Collections ExampleTop

Lets write a simple class that creates some collections and uses some of the methods from the java.util.Collections class on them:

/*
  Simple class to create some collections and use several methods from the java.util.Collections class 
*/
import java.util.*; // Import the java.util package

public class UsingJavaUtilCollections {

    public static void main (String[] args) {
        List<String> al = new ArrayList<String>();
        List<String> al2 = new ArrayList<String>();
        String s = "hello";
        al.add(s);
        al.add(s);
        al.add("goodbye");
        al.add(s+s);
        al.add("goodbye2");
        al2.add(s);
        al2.add(s);
        al2.add("goodbye");
        al2.add(s+s);
        al2.add("goodbye2");
        al2.add(s);
        System.out.println("Binary search of al for a value of goodbye = " + 
                           Collections.binarySearch(al, "goodbye"));  
        System.out.println("The content of al, before sort: " + al);
        Collections.reverse(al);
        System.out.println("The content of al, after sort: " + al);
        Collections.shuffle(al);
        System.out.println("The content of al, after shuffle: " + al);
        System.out.println("The content of al2, before copy: " + al2);
        Collections.copy(al2, al);
        System.out.println("The content of al2, after copy: " + al2);
        Collections.sort(al2);
        System.out.println("The content of al2, after sort: " + al2);
    }
}

Save, compile and run the UsingJavaUtilCollections class in directory   c:\_Collections in the usual way.

The above screenshot shows the results of creating and running our UsingJavaUtilCollections class. The names of the methods used are fairly self explanatory so no need to go into details about the above code.

Lesson 7 Complete

In this lesson we looked at the Utilities hierarchy and examine the java.util.Arrays and java.util.Collections classes that contains a lot of static utility methods we can use with our collections.

What's Next?

In our final lesson in the Collections section we look at sorting our collections using the Comparable and Comparator interfaces.

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