concept equal in category java

This is an excerpt from Manning's book OCA Java SE 8 Programmer I Certification Guide. Login to get full access to this book.

Purpose: The switch construct uses the equals method to compare the value of its argument with the case values. It doesn’t compare the variable references.

The correct way to compare two String values for equality is to use the equals method defined in the String class. This method returns a true value if the object being compared to it isn’t null, is a String object, and represents the same sequence of characters as the object to which it’s being compared.

The following listing shows the method definitions of the equals method defined in class String in the Java API.

Listing 4.1. Method definition of the equals method from the class String

In listing 4.1, the equals method accepts a method parameter of type Object and returns a boolean value. Let’s walk through the equals method defined by the class String:

The Java API defines a contract for the equals method, which should be taken care of when you implement it in any of your classes. I’ve pulled the following contract explanation directly from the Java API documentation:^[1]

¹ The Java API documentation for equals can be found on the Oracle site: http://docs.oracle.com/javase/8/docs/api/java/lang/Object.html#equals(java.lang.Object).

The equals method implements an equivalence relation on non-null object references:

As per the contract, the definition of the equals method that we defined for the class BankAccount in an earlier example violates the contract for the equals method. Take a look at the definition again:

public boolean equals(Object anObject) {
    return true;
}

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This code returns true, even for null values passed to this method. According to the contract of the method equals, if a null value is passed to the equals method, the method should return false.