Consider the following Java program:
The reason for printing “Not Equal” is simple: when we compare c1 and c2, it is checked whether both c1 and c2 refer to same object or not (object variables are always references in Java). c1 and c2 refer to two different objects, hence the value (c1 == c2) is false. If we create another reference say c3 like following, then (c1 == c3) will give true.
So, how do we check for equality of values inside the objects? All classes in Java inherit from the Object class, directly or indirectly (See point 1 of this). The Object class has some basic methods like clone(), toString(), equals(),.. etc. We can override the equals method in our class to check whether two objects have same data or not.
As a side note, when we override equals(), it is recommended to also override the hashCode() method. If we don’t do so, equal objects may get different hash-values; and hash based collections, including HashMap, HashSet, and Hashtable do not work properly (see this for more details). We will be coverig more about hashCode() in a separate post.
Effective Java Second Edition
Please write comments if you find anything incorrect, or you want to share more information about the topic discussed above.
- Why Java is not a purely Object-Oriented Language?
- Why to Override equals(Object) and hashCode() method ?
- Object class in Java
- Clone() method in Java
- Static vs Dynamic Binding in Java
- Instance Variable Hiding in Java
- Reflection in Java
- Flexible nature of java.lang.Object
- The Initializer Block in Java
- Private and final methods in Java
- Overriding toString() in Java
- Output of Java Program | Set 3
- Comparison of Inheritance in C++ and Java
- Function overloading and return type
- How are Java objects stored in memory?