Utility methods

1. hashCode() Method

Definition

• The hashCode() method returns an integer (hash code) representing the object.
• It is used in hash-based collections like HashMap, HashSet, and HashTable.

Characteristics

• If two objects are equal (equals() method returns true), their hash codes must be the same.
• However, two objects having the same hash code do not necessarily mean they are equal.

Example

class Student {
    int id;
 
    Student(int id) {
        this.id = id;
    }
 
    @Override
    public int hashCode() {
        return id; // Simple hash function
    }
}

Advantages

• Improves performance in hash-based collections.
• Reduces the number of comparisons in collections like HashMap.

Disadvantages

• Poorly implemented hashCode() can lead to hash collisions, reducing efficiency.

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2. equals() Method

Definition

• The equals() method compares the contents of two objects for logical equality.

Characteristics

• By default, equals() in Object class compares memory addresses (== operator).
• It should be overridden when objects have meaningful equality.

Example

class Student {
    int id;
    String name;
 
    Student(int id, String name) {
        this.id = id;
        this.name = name;
    }
 
    @Override
    public boolean equals(Object obj) {
        if (this == obj) return true;
        if (obj == null || getClass() != obj.getClass()) return false;
        Student student = (Student) obj;
        return id == student.id && name.equals(student.name);
    }
}

Advantages

• Ensures correct equality checks in collections like HashSet.
• Prevents duplicate elements in hash-based collections.

Disadvantages

• Needs to be implemented carefully to avoid incorrect behavior.

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3. Comparator (Custom Sorting)

Definition

• Comparator is an interface used to define custom sorting logic for objects.

Characteristics

• It has a compare() method, which compares two objects.
• It is used when the natural ordering (from Comparable) is not sufficient.

Example

import java.util.*;
 
class Student {
    int id;
    String name;
 
    Student(int id, String name) {
        this.id = id;
        this.name = name;
    }
}
 
// Custom Comparator to sort by name
class NameComparator implements Comparator<Student> {
    public int compare(Student s1, Student s2) {
        return s1.name.compareTo(s2.name);
    }
}
 
public class ComparatorExample {
    public static void main(String[] args) {
        List<Student> list = new ArrayList<>();
        list.add(new Student(1, "Alice"));
        list.add(new Student(2, "Bob"));
 
        Collections.sort(list, new NameComparator()); // Sorting by name
 
        for (Student s : list) {
            System.out.println(s.name);
        }
    }
}

Advantages

• Allows multiple sorting criteria.
• Can be used without modifying the original class.

Disadvantages

• More verbose than Comparable (which is built-in).

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4. Iterator (Traversing Collections)

Definition

• Iterator is an interface used to iterate over collections like ArrayList, HashSet, etc.

Characteristics

• Provides hasNext() and next() methods.
• Prevents ConcurrentModificationException when removing elements.

Example

import java.util.*;
 
public class IteratorExample {
    public static void main(String[] args) {
        List<String> list = new ArrayList<>();
        list.add("Apple");
        list.add("Banana");
 
        Iterator<String> it = list.iterator();
        while (it.hasNext()) {
            System.out.println(it.next());
        }
    }
}

Advantages

• Works on all collection types.
• Allows safe removal of elements during iteration.

Disadvantages

• Slower than indexed access (for loop in ArrayList).

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5. toString() Method

Definition

• The toString() method provides a string representation of an object.

Characteristics

• By default, Object.toString() returns the class name + hash code.
• Should be overridden to provide meaningful output.

Example

class Student {
    int id;
    String name;
 
    Student(int id, String name) {
        this.id = id;
        this.name = name;
    }
 
    @Override
    public String toString() {
        return "Student{id=" + id + ", name='" + name + "'}";
    }
}
 
public class ToStringExample {
    public static void main(String[] args) {
        Student s = new Student(1, "Alice");
        System.out.println(s); // Calls toString() automatically
    }
}

Advantages

• Improves debugging.
• Makes objects readable when printed.

Disadvantages

• Needs manual implementation for meaningful output.

Concept	Purpose	Key Method(s)	Used In
hashCode()	Generates hash value	hashCode()	HashMap, HashSet
equals()	Compares objects	equals(Object obj)	Checking equality
Comparator	Custom sorting	compare(T o1, T o2)	Sorting collections
Iterator	Traverses collections	hasNext(), next(), remove()	ArrayList, HashSet
toString()	Converts object to string	toString()	Debugging, Logging