The relationship and use of equals(Object) method and hashCode() method in Java

The equals(Object) method and hashCode() method in Java are two important methods for comparing the equality of objects. There is a close relationship and interdependence between them, and they play an important role in actual development.

First, let’s analyze the role of the equals(Object) method. The equals(Object) method is a method defined in the Object class, and all Java classes inherit from the Object class. The equals(Object) method is used to compare whether two objects are equal. Its default implementation is to compare the memory addresses of objects. If the equals(Object) method is not overridden, the object references will be compared.

However, for most cases, we need to compare according to the actual content of the object. For example, we define a Person class, which contains two attributes: name and age. We want two Person objects to be considered equal when their names and ages are equal. At this time, you need to rewrite the equals(Object) method and customize the comparison rules as needed.

When overriding the equals(Object) method, you need to follow the following principles:

1. Symmetry: If a.equals(b) returns true, then b.equals( a) should also return true.
2. Reflexivity: For any non-null reference value x, x.equals(x) should return true.
3. Transitivity: If a.equals(b) returns true, and b.equals(c) returns true, then a.equals(c) should also return true.
4. Consistency: For any non-null reference values ​​x and y, if x.equals(y) is called multiple times without modification of the object, it always returns true or always returns false, then x.equals( y) Always return true or always return false in all these calls.
5. equals(null) should return false.

It should be noted that rewriting the equals(Object) method also requires rewriting the hashCode() method to ensure the consistency of the hashCode() method. The hashCode() method returns the hash code of the object, which is a value of type int. The hash code of an object is calculated by a certain algorithm from the object's attribute values. The hashCode() method has many uses in Java, such as in collection classes to improve search efficiency.

The consistency requirements of the hashCode() method and the consistency requirements of the equals(Object) method are interdependent. If two objects call the equals(Object) method and return true, then their hashCode() methods must return equal values. In other words, if two objects are equal, then their hash codes must be equal. Therefore, when overriding the equals(Object) method, you also need to override the hashCode() method.

Overriding the hashCode() method follows the following principles:

1. If the equals(Object) method of two objects returns true, then their hashCode() method must return equal values.
2. If the equals(Object) method of two objects returns false, then their hashCode() method does not necessarily return different values, but returning different values ​​can improve the performance of the hash table.

In actual development, we can use IDE tools to automatically generate rewritten codes for the equals(Object) method and hashCode() method to improve development efficiency. At the same time, we can also use tool classes such as EqualsBuilder and HashCodeBuilder in Apache Commons Lang to simplify the rewriting process.

In summary, the equals(Object) method and hashCode() method are two important methods for object comparison and hash code calculation in Java. There is a close relationship and interdependence between them, and they play an important role in actual development. When overriding the equals(Object) method, you need to override the hashCode() method at the same time to ensure consistency and performance. By understanding and using these two methods correctly, we can better perform object comparisons and set operations.

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