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Kunal Dawn

Java Collections

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1. Java Collections

A Java collection is a data structure which contains and processes a set of data. The data stored in the collection is encapsulated and the access to the data is only possible via predefined methods.

For example if your application saves data in an object of type People, you can store several Peopleobjects in a collection.

While arrays are of a fixed size, collections have a dynamic size, e.g. a collection can contain a flexible number of objects.

Typical collections are: stacks, queues, deques, lists and trees.

As of Java 5 collections should get parameterized with an object declaration to enable the compiler to check if objects which are added to the collection have the correct type. This is based on Generics. Generics allow a type or method to operate on objects of various types while providing compile-time type safety.

The following code shows an example how to create a Collection of type List which is parameterized with <String> to indicate to the Java compiler that only Strings are allowed in this list. .

 

package collections;

import java.util.ArrayList;

public class MyArrayList {

    public static void main(String[] args) {

        // Declare the List concrete type is ArrayList
        List<String> var = new ArrayList<String>();

        // add a few Strings to it
        var.add("Lars");
        var.add("Tom");

        // Loop over it and print the result to the console
        for (String s : var) {
            System.out.println(s);
        }
    }
}

 

If you try to put a non String into this list, you would receive a compiler error.

List is only an interface, a common implementation is the ArrayList class, hence you need to call new ArrayList().

2. Important implementations

2.1. Map and HashMap

The Map interface defines an object that maps keys to values. A map cannot contain duplicate keys; each key can map to at most one value.

The HashMap class is an efficient implementation of the Map interface. The following code demonstrates its usage.

 

package com.vogella.java.collections.map;

import java.util.HashMap;
import java.util.Iterator;
import java.util.Map;

public class MapTester {
  public static void main(String[] args) {
    // Keys are Strings
    // Objects are also Strings

    Map<String, String> mMap = new HashMap<String, String>();
    mMap.put("Android", "Mobile");
    mMap.put("Eclipse", "IDE");
    mMap.put("Git", "Version control system");

    // Output 
    for (String key : mMap.keySet()) {
      System.out.println(key +" "+ mMap.get(key));
    }

    System.out.println("Changing the data");
    // adding to the map
    mMap.put("iPhone", "Created by Apple");

    // Delete from map

    mMap.remove("Android");

    System.out.println("New output:");
    // Output
    for (String key : mMap.keySet()) {
      System.out.println(key +" "+ mMap.get(key));
    }
  }
}

 

2.2. List, ArrayList and LinkedList

List is the interface which allows to store objects in a resizable container.

ArrayList is implemented as a resizable array. If more elements are added to ArrayList than its initial size, its size is increased dynamically. The elements in an ArrayList can be accessed directly and efficiently by using the get() and get() methods, since ArrayList is implemented based on an array.

LinkedList is implemented as a double linked list. Its performance on add() and remove() is better than the performance of Arraylist. The get() and get() methods have worse performance than the ArrayList, as the LinkedList does not provide direct access.

The following code demonstrates the usage of List and ArrayList.

 

package com.vogella.java.collections.list;

import java.util.ArrayList;
import java.util.List;

public class ListExample {
  public static void main(String[] args) {
    List<Integer> list = new ArrayList<Integer>();
    list.add(3);
    list.add(2);
    list.add(1);
    list.add(4);
    list.add(5);
    list.add(6);
    list.add(6);
    for (Integer integer : list) {
      System.out.println(integer);
    }
  }

}

 

3. Useful collection methods

The java.util.Collections class provides useful functionalities for working with collections.

Table 1. Collections

Method Description
Collections.copy(list, list) Copy a collection to another
Collections.reverse(list) Reverse the order of the list
Collections.shuffle(list) Shuffle the list
Collections.sort(list) Sort the list

4. Using Collections.sort and Comparator in Java

Sorting a collection in Java is easy, just use the Collections.sort(Collection) to sort your values. The following code shows an example for this.

 

package de.vogella.algorithms.sort.standardjava;

import java.util.ArrayList;
import java.util.Collections;
import java.util.List;

public class Simple {
    public static void main(String[] args) {
        List list = new ArrayList();
        list.add(5);
        list.add(4);
        list.add(3);
        list.add(7);
        list.add(2);
        list.add(1);
        Collections.sort(list);
        for (Integer integer : list) {
            System.out.println(integer);
        }
    }
}

 

This is possible because Integer implements the Comparable interface. This interface defines the method compare which performs pairwise comparison of the elements and returns -1 if the element is smaller then the compared element, 0 if it is equal and 1 if it is larger.

If what to sort differently you can define your own implementation based on the Comparator interface.

 

package de.vogella.algorithms.sort.standardjava;

import java.util.Comparator;

public class MyIntComparable implements Comparator<Integer>{

    @Override
    public int compare(Integer o1, Integer o2) {
        return (o1>o2 ? -1 : (o1==o2 ? 0 : 1));
    }
}

 

 

package de.vogella.algorithms.sort.standardjava;

import java.util.ArrayList;
import java.util.Collections;
import java.util.List;

public class Simple2 {
  public static void main(String[] args) {
    List<Integer> list = new ArrayList<Integer>();
    list.add(5);
    list.add(4);
    list.add(3);
    list.add(7);
    list.add(2);
    list.add(1);
    Collections.sort(list, new MyIntComparable());
    for (Integer integer : list) {
      System.out.println(integer);
    }
  }

}

 

Note

For the above you could also have used the Collection.reverse() method call.

This approach is that you then sort any object by any attribute or even a combination of attributes. For example if you have objects of type Person with an attribute income and dataOfBirth you could define different implementations of Comparator and sort the objects according to your needs.

5. Exercise: Use Java Collections

Create a new Java project called com.vogella.java.collections. Also add a package with the same name.

Create a Java class called Server with one String attribute called url.

 

package com.vogella.java.collections;

public class Server {
    private String url;
}

 

Create getter and setter methods for this attribute using code generation capabilities of Eclipse. For this select Source → Generate Getters and Setters from the Eclipse menu.

Create via Eclipse a constructor which gets a url as parameter. For this select Source → Generate Constructor using Fields… from the Eclipse menu.

Type main in the class body and use code completion (Ctrl+Space) to generate a main method.

 

Code completion

 

In your main method create a List of type ArrayList and add 3 objects of type Server objects to this list.

 

public static void main(String[] args) {
    List<Server> list = new ArrayList<Server>();
    list.add(new Server("http://www.vogella.com"));
    list.add(new Server("http://www.google.com"));
    list.add(new Server("http://www.heise.de"));
}

 

Use code completion to create a foreach loop and write the toString method to the console. Use code completion based on syso for that.

Run your program.

Use Eclipse to create a toString method based on the url parameter and re-run your program again.

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