66 lines
3.2 KiB
Markdown
66 lines
3.2 KiB
Markdown
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---
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title: Checking for Equality
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---
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# Checking for Equality
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In Java, there are two ways to check if two variables are the "same": `==` and `.equals()`. These two methods do not work the same, however.
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## The `==` Operator
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The basic equality operation in Java, `==` as in `var1 == var2`, checks whether `var1` and `var2` point to the same *object reference*.
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That is, if `var1` is the same *instance* of a class in memory as `var2`, then `var1 == var2` is true.
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However, if `var1` and `var2` were created as two separate instances of a class (i.e. with the `new` keyword), then `var1 == var2` will be false. Even if both objects happen to contain the exact same properties and values, the `==` comparison would not pass because they are not pointing to the same object in memory.
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For primitive variable types, such as `int` and `double`, the `==` operator can always be used to check for equality, as their values are stored directly with the variable (rather than as a reference to another slot in memory).
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```java
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int var1 = 1;
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int var2 = 1;
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System.out.println(var1 == var2) // true
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MyObject obj1 = new MyObject();
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MyObject obj2 = obj1;
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MyObject obj3 = new MyObject();
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System.out.println(obj1 == obj2) // true
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System.out.println(obj1 == obj3) // false
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System.out.println(obj2 == obj3) // false
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```
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## The `.equals()` Method
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The built-in `Object` class in Java, which all other classes automatically extend, contains a number of helpful built-in methods. One such method is `equals()`, which takes another object as its argument and returns whether the two objects should be considered "equal" according to the relevant logic for that class.
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The 'String' class is one of the most common examples of a class that overrides the 'equals()' method. When comparing two 'String's for equality, you need to use the 'equals()' method, as '==' won't work as you expect.
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```java
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String s1 = "Bob";
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String s2 = "ob";
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s2 = "B" + s2; //s2 now is also "Bob"
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System.out.println(s1 == s2); //false
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System.out.println(s1.equals(s2)); //true
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```
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When you create a new class in Java, you will often want to override the `equals()` method in order to provide a more meaningful way to compare two objects of the same class. How this method is implemented is completely up to the developer's judgement.
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For example, you may decide that two `Person`s should be considered "equal" if their `name` and `dateOfBirth` are the same. This logic would be implemented in your `Person` class's `equals()` method:
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```java
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public class Person {
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public String name;
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public Date dateOfBirth;
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public boolean equals(Person person) {
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return this.name.equals(person.name) && this.dateOfBirth.equals(person.dateOfBirth);
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}
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}
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```
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Most of the built-in classes in Java, as well as classes provided by popular libraries, will implement the `equals()` method in a meaningful way.
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For example, the `java.util.Set` interface specifies that a `Set`'s `equals()` method will return true if "the specified object is also a set, the two sets have the same size, and every member of the specified set is contained in this set".
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However, if a class does not override the default `equals()` implementation, the default implementation will apply, which simply uses the `==` operator to compare the two objects.
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