There are two "same" in java.

== (identity)

The == operator determines that they are the same. That is, it is determined that a plurality of variables refer to the same instance.


A a = new A(); //Create an instance called type A
A b = a; //Copy each instance of a to b as it is
System.out.println(a == b);

true

example


public class Sample {
	private int num;
	public Sample(int num) {
		this.num = num;
	}
}

public class Main {
	public static void main(String[] args) {
		Sample s0 = new Sample(5); //Sample type instantiation
		Sample s1 = s0; //Copy to s1
		s0 = new Sample(5); //Create a new Sample type instance
		System.out.println(s0 == s1);
	}
}

false

At the time of copying with Sample s1 = s0;, it had the same instance, but on the next line, s0 created a new instance, and s0 and s1 became different instances.

equals method (equivalence)

The equals method determines that they are equivalent. That is, the instances are different, but it is determined whether they have the same value.

The equals method defined in the default Object class is

public boolean equals(Object obj){
  return (this == obj);
}

Because it is defined as, and it is a specification to confirm the identity

*** It is assumed to be used by overriding. *** ***

further! !!

*** When overriding the equals method, you must also override the hashCode in the set. *** ***

HashCode is a method that can be confirmed by converting the value of the instance into a hash function.

Object(Java SE 11 & JDK 11)

equals Keep in mind that in general, when overriding this method, you should always override the hashCode method and follow the general convention of the hashCode method that equivalent objects must hold the equivalent hash code.

example


public class Sample {
	private int num;
	private String name;
	public Sample(int num, String name) {
		this.num = num;
		this.name = name;
	}

	@Override
	public int hashCode() {
		final int prime = 31;
		int result = 1;
		result = prime * result + ((name == null) ? 0 : name.hashCode());
		result = prime * result + num;
		return result;
	}

	@Override
	public boolean equals(Object obj) {
		if (this == obj)
			return true;
		if (obj == null)
			return false;
		if (getClass() != obj.getClass())
			return false;
		Sample other = (Sample) obj;
		if (name == null) {
			if (other.name != null)
				return false;
		} else if (!name.equals(other.name))
			return false;
		if (num != other.num)
			return false;
		return true;
	}


}


public class Main {
	public static void main(String[] args) {
        //argument(num, name)Is the same
		Sample a = new Sample(5, "xxx"); 
		Sample b = new Sample(5, "xxx");
		System.out.println(a.equals(b));
        //Output each hashCode
        System.out.println(a.hashCode());
        System.out.println(b.hashCode());
	}
}

true
3694926
3694926

Overriding the equals and hashCode methods.

If even one num or name is different, it will be as follows.


public class Main {
	public static void main(String[] args) {
        //argument(name)Is different
		Sample a = new Sample(5, "xxx");
		Sample b = new Sample(5, "yyy"); 
		System.out.println(a.equals(b));
        //Output each hashCode
        System.out.println(a.hashCode());
        System.out.println(b.hashCode());
	}
}