equals Method

The == operator and the equals() method serve different purposes:

• == works with both primitive and reference types.

  • For primitives, it compares actual values (e.g., int i = 10; double d = 10.0; → i == d is true).
  • For references, it checks if both variables point to the same memory address (i.e., the same object).

• equals() is only applicable to reference types. By default, it behaves like ==, comparing object identity. However, many classes overrride it to compare logical equality based on content.

Example from String:

public boolean equals(Object anObject) {
    if (this == anObject) {
        return true;
    }
    if (anObject instanceof String) {
        String other = (String) anObject;
        if (coder() == other.coder()) {
            return isLatin1() 
                ? StringLatin1.equals(value, other.value)
                : StringUTF16.equals(value, other.value);
        }
    }
    return false;
}

Example from Integer:

public boolean equals(Object obj) {
    if (obj instanceof Integer) {
        return value == ((Integer) obj).intValue();
    }
    return false;
}

hashCode Method

Key points about hashCode():

1. It enhances performance in hash-based collections like HashMap or HashSet.
2. If two references point to the same object, their hash code are identical.
3. The converse isn’t guaranteed: equal hash codes don’t imply the same object.
4. The hash code is derived from the object’s internal address but is not the address itself.

public class HashCodeDemo {
    public static void main(String[] args) {
        AA x = new AA();
        AA y = new AA();
        AA z = x;
        System.out.println(x.hashCode()); // e.g., 1967205423
        System.out.println(y.hashCode()); // e.g., 42121758
        System.out.println(z.hashCode()); // same as x: 1967205423
    }
}
class AA {}

toString Method

The default toString() implementation in Object returns a string composed of the fully qualified class name and the hexadecimal representation of the object’s hash code:

public String toString() {
    return getClass().getName() + "@" + Integer.toHexString(hashCode());
}

When an object is printed or concatenated in a string context, toString() is automatically invoked. Classes often override this method to provide meaningful output:

public class ToStringDemo {
    public static void main(String[] args) {
        Monster creature = new Monster("小猪妖", "巡山");
        System.out.println(creature); // Uses overridden toString()
    }
}

class Monster {
    private String name;
    private String role;

    public Monster(String name, String role) {
        this.name = name;
        this.role = role;
    }

    @Override
    public String toString() {
        return "Monster{name='" + name + "', job='" + role + "'}";
    }
}