When a collection contains object of user defined class

When a collection contains object of user defined class:-

contains() of collection classes internally used equals() method to compare objects provided by Object class, equals()’s method of Object class compare value of reference variable not content of object. So, must override equals() method in own class to provide content by comparison.

1. Using ArrayList or LinkedList simply override equals() method

public class Emp {

String name;

String job;

int sal;

    public Emp(String n, String j, int s) {

        name = n;

        job = j;

        sal = s;

    }

    public String getJob() {

        return job;

    }

    public void setJob(String job) {

        this.job = job;

    }

    public String getName() {

        return name;

    }

    public void setName(String name) {

        this.name = name;

    }

    public int getSal() {

        return sal;

    }

    public void setSal(int sal) {

        this.sal = sal;

    }

  public boolean equals(Object o){

    Emp e=(Emp)o;

    return (name.equals(e.name))&&(job.equals(e.job))&& sal==e.sal;

  }

}

public class Main {

    /**

     * @param args the command line arguments

     */

    public static void main(String[] args) {

        ArrayList list=new ArrayList();

        list.add(new Emp("naveen","SE",18000));

        list.add(new Emp("raj","SE",15000));

        list.add(new Emp("dinesh","Analyst",18000));

        Emp e=new Emp("raj","SE",15000);

        System.out.println(list.contains(e));

    }

}

2. Using HahSet or HashMap must override hashCode() and equals() both.

To store object of user define class in hashing based collection:

1. Must override hashcode() method in such a manner that two objects with same content has same has code.
2. Must override equals() method in such a manner compare content of the object

public class Emp {

String name;

String job;

int sal;

    public Emp(String n, String j, int s) {

        name = n;

        job = j;

        sal = s;

    }

    public String getJob() {

        return job;

    }

    public void setJob(String job) {

        this.job = job;

    }

    public String getName() {

        return name;

    }

    public void setName(String name) {

        this.name = name;

    }

    public int getSal() {

        return sal;

    }

    public void setSal(int sal) {

        this.sal = sal;

    }

    @Override

public boolean equals(Object o){

    Emp e=(Emp)o;

    return (name.equals(e.name))&&(job.equals(e.job))&& sal==e.sal;

}

    @Override

    public int hashCode() {

        return name.hashCode()+job.hashCode()+sal;

    }

}

public class Main {

    /**

     * @param args the command line arguments

     */

    public static void main(String[] args) {

        HashSet list=new HashSet();

        list.add(new Emp("naveen","SE",18000));

        list.add(new Emp("raj","SE",15000));

        list.add(new Emp("dinesh","Analyst",18000));

        Emp e=new Emp("raj","SE",15000);

        System.out.println(list.contains(e));

    }

}

3. Using TreeSet or TreeMap

Tree Base collection internally uses some method to comparing element for sorting. This comparison logic is provided either using java.lang.Comparable interface (default) or java.util.Comparator interface (by specifying in constructor e.g. TreeMap(Comparator)).

So, must override this method.(discuss in detail in earlier session)